Performance Gains and Adaptations from Fasted Training, with Dr. Brian Carson  

We try to decipher if there are any advantages or disadvantages to occasionally exercising in a fasted state.

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Have you ever woken up early in the morning to go for a ride, skipped breakfast, and thought, “I wonder if that was bad for my training? Or maybe it was good?!” In today’s episode, we try to decipher if there are any advantages or disadvantages to occasionally riding, or exercising in general, in a fasted state. What does “fasted” actually mean, in this context? How’s that help, or hurt, my training? How conclusive is the evidence? And how often should I do it? Those are just some of the questions we’ll try to answer today.

Fasting is one of those subjects that many of you have likely heard mentioned in passing, but whether or not it can lead to true performance gains is another matter. To fast or not to fast, that is the question. Today, we go particularly deep into the details of fasting, from the different types of fasting you can use for both health and performance benefits, to the genetic and cellular mechanisms which could play a role in adaptations.

In essence, there are two overriding questions: Does fasting have health benefits? And does it help in training and performance? The science is pointing towards clear health benefits, but performance and training are less clear. We’ll explore all of that and much more today on Fast Talk.

Our primary guest is someone who has spent his research career looking into these very questions. Dr. Brian Carson, of the University of Limerick, in Ireland, is a leading expert on the effects of exercising in a fasted state, as will become patently clear when we dive into the science.

We’ll also hear today from longtime USA Cycling coach Jim Miller, pro roadie Petr Vakoc and pro mountain biker Payson McElveen, leading physiologist Dr. Iñigo San Millan, and neurologist Dr. Dale Bredesen.

Put down that cookie. Let’s make you fast.


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  • Burke, L. M., Ross, M. L., Garvican‐Lewis, L. A., Welvaert, M., Heikura, I. A., Forbes, S. G., … Hawley, J. A. (2017). Low carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers. The Journal of Physiology, 595(9), 2785–2807. Retrieved from
  • Hawley, J. A., & Burke, L. M. (2010). Carbohydrate Availability and Training Adaptation. Exercise and Sport Sciences Reviews, 38(4), 152–160. Retrieved from
  • Hawley, J. A., & Leckey, J. J. (2015). Carbohydrate Dependence During Prolonged, Intense Endurance Exercise. Sports Medicine, 45(Suppl 1), 5–12. Retrieved from
  • Jeukendrup, A. E. (2017). Periodized Nutrition for Athletes. Sports Medicine, 47(Suppl 1), 51–63. Retrieved from
  • Lane, S. C., Camera, D. M., Lassiter, D. G., Areta, J. L., Bird, S. R., Yeo, W. K., … Hawley, J. A. (2015). Effects of sleeping with reduced carbohydrate availability on acute training responses. Journal of Applied Physiology, 119(6), 643–655. Retrieved from
  • Mattson, M. P., Allison, D. B., Fontana, L., Harvie, M., Longo, V. D., Malaisse, W. J., … Panda, S. (2014). Meal frequency and timing in health and disease. Proceedings of the National Academy of Sciences, 111(47), 16647–16653. Retrieved from
  • Volek, J. S., Noakes, T., & Phinney, S. D. (2014). Rethinking fat as a fuel for endurance exercise. European Journal of Sport Science, 15(1), 13–20. Retrieved from
  • Zouhal, H., Saeidi, A., Salhi, A., Li, H., Essop, M. F., Laher, I., … Abderrahman, A. B. (2020). Exercise Training and Fasting: Current Insights. Open Access Journal of Sports Medicine, 11, 1–28. Retrieved from

Episode Transcript

Chris Case  00:00

Hello, and welcome to Fast Talk, your source for the science of cycling performance. I’m your host Chris Case.


Introduction to Intermittent Fasting

Chris Case  00:16

Have you ever woken up early in the morning, going for a ride after skipping breakfast, and thought, I wonder if that was bad for my training? Or maybe it was good? Well, in today’s episode, we try to decipher if there are advantages or disadvantages to occasionally riding or exercising in general, in a fasted state. What does fasted actually mean in this context? How does that help? How’s that hurt my training? How conclusive is the evidence? How often should I do it? Those are just some of the questions we’ll try to answer today. Fasting is one of those subjects that many of you have likely heard, mentioned in passing, or whether or not it can lead to true performance gains, that’s another matter that many of us probably don’t have the answer to. To fast or not to fast, that is the question. Today, we go particularly deep into the details of fasting from the different types of fasting you can use for both health and performance benefits, to the genetic and cellular mechanisms which could play a role in adaptations. There are two overriding questions. Does fasting have health benefits? Does it help in training and performance? As I mentioned, the science is pointing towards clear health benefits, but performance and training are less there. We’ll explore all that and much more today on Fast Talk. Our primary guest is someone who has spent his research career looking into these very questions. Dr. Brian Carson, of the University of Limerick in Ireland, is a leading expert on the effects of exercise in a fasted state, as will become patently clear when we dive into the science. We’ll also hear today from longtime USA cycling coach Jim Miller, pro Petr Vakoc, pro mountain biker Payson McElveen, leading physiologist, Dr. Inigo San Millan, and neurologist, Dr. Dale Bredesen. Put down that cookie, let’s make you fast.


Chris Case  02:33

This episode of Fast Talk is brought to you by WHOOP. Trevor, I know you’re a fan of looking at the big picture when it comes to training, sounds like WHOOP has introduced a feature that really allows you to look at that big picture with your WHOOP data.


Trevor Connor  02:47

I don’t remember if they had this back when I was using the 2.0 strap, but they have it now, and I actually really liked this journal feature. There are a whole variety of metrics you can track from diet to sleep, whether you’re sleeping at altitude, supplements, a whole bunch of other things I’m not going to go into, but you could pick all these metrics and then you track them every day. So, you get up in the morning you put in your results for those metrics, and first I was putting and going okay, so what does this do? And then saw the monthly assessment. So, at the end of every month, they give you an analysis, and I love looking for trends in data, so this is just a goldmine for me because it will match up these different things that you’re tracking in your journal, with your recovery scores, with your sleep scores, and so for example, something that I noticed, I have been taking a magnesium supplement at night to help, I get migraines, and it seemed to help with my migraines and everybody, says we’ll take back usually and before you go to bed, it helps you to sleep. Well interestingly that magnesium supplement is matching up with poor sleep. So, when I take it I don’t sleep as well, which surprised me, but there’s a truth to it. Since I saw that in my monthly report, I’ve been watching it, and yeah, it’s there. I don’t sleep as well when I take it.


Chris Case  04:09

WHOOP is offering 15% off with the code “fasttalk,” at checkout. Go to WHOOP and enter “fasttalk” at checkout to save 15%. Sleep better, recover faster, and train smarter. Optimize your performance with WHOOP.


Chris Case  04:32

Welcome back, everybody to another episode of Fast Talk. We’ve got a very exciting episode in store for you today, a topic that Trevor and I have spoken about internally, at least a lot, we’ve received a lot of questions from listeners out there about this. This topic of exercising while in a fasted state, does it lead to performance gains? Does it lead to adaptations? What’s going on? Why should you do it? How do you do it? And we’ve got a fantastic guest on today, Dr. Brian Carson, sitting in Limerick, Ireland. Welcome to the program, Dr. Carson.


Dr. Brian Carson  05:08

Hi, guys. Thanks very much for having me on, I was flattered to get the invitation and delighted to come and speak to you today.


Trevor Connor  05:14

We are excited to have you on the show. So, thank you.


Chris Case  05:18

Yeah, you know, this is a field you’ve researched for quite some time, I imagine. We want to cover it from a few different angles. Could you give us a one or two sentence or one or two minute, we’ll give you a little bit longer than a sentence, background on your research in this field.


Dr. Brian Carson Background in Fasting

Dr. Brian Carson  05:38

Principally, I was trained as a sports scientist originally, and I went on to do a Ph.D. in exercise physiology, and my specialism was in muscle metabolism, mainly metabolic adaptation to exercise at the molecular level in the muscle. So, I’ve always been interested in exercise, and then more recently became interested in the interaction between exercise and nutrition to drive adaptation in the muscle. So, always been interested in muscle metabolic genes and how they adapt in response to an acute amount of exercise, how that can be manipulated by nutrition, and then chronically over time. So, repeated amount of exercise combined with nutrition strategies to augment and, I guess, optimize the adaptation at the level of the muscle in terms of metabolism. So, that’s been my research focus for the last number of years since I’ve moved to Limerick. I work with a number of industry partners who come in from the nutrient side, and then we combine some of their products with some of the exercise regimes that I’m interested in. So, one topic that’s always been of interest to me was the influence of carbohydrate, I guess on that adaptation. What we’re going to talk a little bit about today, sometimes how that adaptation is blunted by the presence of carbohydrate, or it’s augmented in its absence, and that’s always been fascinating to me. I guess we’ll talk a little bit more about that as the afternoon progresses.


Trevor Connor  07:15

We asked Jim Miller, Chief of sports performance for USA Cycling, his thoughts on fasting. He didn’t mince his words.


Chris Case  07:21

Curious to know if you have experimented or if you are a believer in intermittent fasting, and its effect on performance.


Jim Miller: Thoughts on Intermittent Fasting and Performance

Jim Miller  07:33

Yeah, I have experimented with it. I have actually gone, I don’t use it, I tried it, I didn’t find it very effective. I found what it did was put athletes into a low-energy state, and we spent more time trying to recover than we’ve benefited from it.


Chris Case  07:58

Yeah, so that’s a pretty short way of saying your experiences that it’s not effective at all?


Jim Miller  08:05

Correct. I know the science for it, and I don’t disagree with the science for it, just in my experience, it hasn’t been successful.


Trevor Connor  08:13

Yeah, you’re finding that it’s just shutting down, glycolysis shutting down, your ability to use sugars and you just kind of get lethargic, is that what I am hearing?


Jim Miller  08:23

Yep. Yep. That training session sucks, and then the subsequent training session sucked if they don’t reveal properly, and with athletes, the chances of them not refueling properly from something like that is high.


Trevor Connor  08:40

A question I’ve got asked you based on the focus of your research, how many muscle biopsies have you done now? Have you lost count?


Dr. Brian Carson  08:50

I have lost count. I actually had a count there may be a couple of years ago, I’m just trying to think back to that. I think we’ve probably done about 3 to 400 muscle biopsies, in terms of muscle biopsies on myself, it’s nothing many. It’s about 16.


Trevor Connor  09:01

That was going to be my second question. 16?


Dr. Brian Carson  09:04

It’s about 16, yeah, about 16, tend to be the grad students now, and the postdocs and, and the undergrads, that we’re proud of these days, and I’m starting to fall outside the age range for recruitment for most of the studies, so I’m somewhat protected at this point.


Trevor Connor  09:19

My old adviser, when I was doing my Masters, his research was very similar to yours, and he used to brag about the fact that his one leg was a pin cushion. He had done so many muscle biopsies on himself.


Dr. Brian Carson  09:31

Yeah, I haven’t been too bad. I’ve been lucky enough. Most of the biopsies I’ve had have been with the Bergstrom needle, but we’ve actually used, or we’ve moved in our lab, at least to the microbiology technique, which is like a punch biopsy, and doesn’t require an incision beforehand, and it’s much less invasive. So, you know, we can still brag to the new students that we used to use the old school technique where we take about 200 megs a muscle, whereas now we’re taking 25-50. So, we still have something over them at least.


Chris Case  10:03

The good ol’ days.


Dr. Brian Carson  10:05



Chris Case  10:06

Well, Trevor, I know, as we’ve spoken about this topic, there’s a lot of different types of fasting to this conversation. Probably helpful to start off with a review of what those different fasting types are, give a little explanation, so that we are clear about what we’re going to speak about today.


Different Types of Fasting

Trevor Connor  10:31

Fasting is becoming a very popular term. There’s a lot of people who are giving it a try. I don’t think that a lot of people realize that when you’re talking about fasting, there’s a whole lot of different things you’re talking about. So, whenever somebody says, I want to try intermittent fasting, either just for health or for performance reasons, my first question is always well, what are you trying? I’m just going to give you my bias, because I have had this question more times than I would like on the nutrition side of things, where I’ve had people say, well, I’ve heard intermittent fasting is good, what do you think of five or seven-day fasts? To which I respond, that is not fasting, that’s intentionally starving yourself. Let’s not talk about that. So, we’re talking about intermittent fasting, which is more short-term. We’re not going to go into this too deep in this show, but in the nutrition world, you’re starting to see evidence that there are potential health benefits to this, though, bear in mind, they still haven’t done a lot of human studies. It’s been mostly animal studies, but on the animal side, they’ve seen a lot of positive benefits. We’re talking about sports, so we will discuss a bit of the health side. We’re also going to talk about the performance side. You know this is my list, I’m sure other people divide this up differently, but I really see it as when you’re talking about fasting and sports there’s four potential things you’re talking about. The first and the simplest, which is kind of old school is just that not eating or drinking anything except water when you ride. So, we used to call this the coffee ride when I lived up in British Columbia, because there was this whole group that liked to go out and do six-hour rides, and they called it the coffee ride, because they would drink a cup of coffee before the ride, and then go ride. That’s kind of old school, and there can be some issues with that. Another one, which we’ve talked about on this show that isn’t really fasting, but it’s kind of fasting-mimicking is carbohydrate manipulation. So, we’ve had Dr. Noakes on the show, we’ve had Dr. Holley on the show, and they both talked about this. This is the idea that you severely restrict carbohydrate intake. So, you’re still eating a fairly normal quantity of calories throughout the day, but it’s high fat, little higher protein, either very little are almost no carbohydrates, and that puts your body in a fasting-mimicking state. like I said, we’ve already talked about that a lot on the show, so we’re not really going to go into that today. The third group I would have is talking about intermittent fasting, but intermittent fasting, where you’re doing it for health reasons and it just also happens that you’re an athlete who exercises, so we’re not getting yet into the timing it with exercise. So, you might be doing it for health reasons, you might be doing it for religious reasons, as a matter of fact, a lot of the research on intermittent fasting looks at Ramadan, where that’s religious fasting. But when you’re talking about intermittent fasting, even there, there’s three types of intermittent fasting. I’ve heard a lot of different names for this. The ones that I’m going to use are, there’s intermittent caloric restriction, which is the most basic, it’s basically you just don’t eat for a day, for at least 24-hours, sometimes longer, and you might do that once or twice per week. The next type is alternate day fasting, which is very similar, this is two or three times a week, you severely restrict your caloric intake, you eat about 25% of what you expand, which is for most people about 4 to 600 calories, and then on your other days, you eat what’s called ad libitum, which is just don’t count calories, just eat what you’d like. The last type is called time-restricted feeding, and this is this idea of you have an extended period of time through the day where you don’t eat, and then you have a short window where you eat normally, and again, don’t really restrict yourself, so typical might be something like a 16-hour fast with eight hours of eating, or an 18-hour fast, six hours of eating, and this is something that many people who do time-restricted feeding will do this every single day. So, they have a period through the day where they don’t eat, they often combine it with sleep, because sleep remember is a time-restricted fast. So, they might stop eating four or five hours before they go to bed, or they might eat until they go to sleep, then do their fasting, then when they wake up in the morning, wait until sometime in the afternoon to start eating. The last type is what we’re really going to be focusing on today, which is somewhat of a hybrid of many of what we’re talking about, which is this exercising in a fasted state. Dr. Carson, very interested in you kind of jumping with this, but if you eat until you exercise and then don’t eat while exercising, that’s not actually exercising in a fasted state, because it takes your body time to go into a fasted mode. So, in order to do this, you have to both have kind of apply what we’re talking about with this time restriction, where you have an extended period where you don’t eat before you exercise. Then you go and exercise, and you also don’t eat while you’re exercising. So, it’s taking elements of everything that we just spoke about.


Trevor Connor  16:24

We just described a variety of ways to do intermittent fasting and fasting while training. The question is, what are athletes actually doing? We asked Petr Vakoc, with Alpecin–Fenix, to share his thoughts. He had some personal experience to share.


Petr Vakoc: Experience With Intermittent Fasting

Chris Case  16:40

One of the things that you mentioned in your email was about intermittent fasting that you do. So, you want to talk a little bit about how much you’ve tried it, your experiences with that, what you find to be advantageous, what you find to be sort of detrimental when it comes to Intermittent fasting in training.


Petr Vakoc  17:02

Yeah, I’ve been always really interested in nutrition, and obviously, recently there is a lot of talk about intermittent fasting. I was trying to, you know, find a way how to put it in the training. But I found that it’s very difficult when you have a hard training program or races to fit it in, more than maybe let’s say once a month or even two months. So, originally, I started in the offseason, where the training load is lower, almost none, and yeah, I just wanted to see if my body can adapt a bit more on using fat as fuel, and from what I found this should be a good strategy. So, I started pretty easy with baby steps, I would say just like a 24-hour fast in the offseason, then I tried 36-hours or so. And then I tried even few times during the season, maybe if I had a stage race or something that was hard, but not extremely hard, I would take, I would take the first day I would eat normally, just fill up my body, and the second day, I would take completely off, no training at all, not even recovery, right? And then I would use this 24-hour fast maybe until the next morning, so some 36-hours, and in that way, give my body a little bit of break from all the things that we have to do during the races.


Trevor Connor  18:52

Now, did you find it had at least in the short run, any sort of impact on your training or your racing? Did you notice that effect?


Effect on Training

Petr Vakoc  19:01

I think the most effective I noticed like on my stomach, the gut issues that I often after the race feel not so comfortable, or even like longer period during the season just like eating so much, and even like what happens to me when I race, although we burn so many calories, I tend to put on weight and even like, you know, the days after the race, I still tend to be really hungry. So, that’s also a way how to help me to correct this overeating. The strategy to use it, only the second day after the race I find out so good because then I make sure that nobody gets enough energy to recover after the race, and then maybe even give it some extra race that the digestive system doesn’t have to work, and in that way, it helps me really well to maintain my weight and to feel better.


Why Exercise in a Fasted State?

Dr. Brian Carson  20:19

I’ll actually pick up just on some of what you finished on, in terms of the intermittent fasting type protocols. I think there is some potential for, you mentioned the maybe the modified alternate-day fasting, and whereby you have maybe one meal per day between maybe 12 and 2 o’clock, then the next day you’re feeding more normally, and then back to another modified fasting day, but you’re getting that sort of 18-20 hours between meals, and that seems to be potentially very important. In terms of having an additional effect to normal caloric restriction, based on the literature that I’ve read so far, what you’re seeing is, you know, things like the five-two, where two of the days are interspersed and throughout the week, you’re having the same effect as a normal caloric restriction, but if you combine two of the days together, and you extend that period of fasting, then you seem to have a greater impact on some of the metabolic markers, not during fasting, but actually during the postprandial period, and the same is true for that alternate day fasting model. I’m really interested in some of the time-restricted eating data that’s coming out, in all of these models we have very limited data in humans, but that’s starting to be rectified at this point in time. I know you’ve had Dr. John Holley on the show before and I attended a webinar that he gave during lockdown actually, it was really, really interesting some of the preliminary data they have in terms of that time-restricted eating, to an eight-hour window within the day, some of the metabolic effects are really quite staggering, and similar to what you’d see with exercise training, actually. That’s without the exercise. So, it is a good potential model. But again, it needs a greater evidence base at this time. That kind of comes down to that final point that you made is that when we’re talking about fasting, we’re not talking about something here to just three or four hours, you’re talking about something which is extended, and then lasts into the actual exercise period, which is where we’re going to focus on today. So, I kind of almost start with a definition here. Fasted exercise is the absence of food and/or an energy beverage prior to exercise. So, you mentioned a few moments ago, the coffee ride, maybe just about be acceptable, there’s a nutrient in there in terms of caffeine, but it’s not an energy source, as the amount of energy that will be in that particular drink will be negligible. But typically, what we’re talking about here is just maybe water, ad libitum in or around the exercise session. I guess maybe we just kind of start from the top, why would we even consider exercising in a fasted state? The goal of most people when they’re doing this and in the science around it is to try and manipulate substrate utilization during exercise. So, manipulating the contribution to your energy provision from both carbohydrate and liquid sources. So, what you’re looking to probably do or what most athletes are looking to do, or most of your listeners are trying to do at this point, is to utilize liquid sources more predominantly, and where possible, spare muscle glycogen, spare carbohydrate sources. The other thing that people are trying to do is to try and drive the endurance phenotype in the adaptive response. So, to move to a model where we probably have greater metabolic flexibility, and at lower intensities, again, we can shift that substrate utilization more predominantly to the liquid rather than the carbohydrate stores, and increase things like markers of our mitochondrial content and function, which provide ATP, the energy currency for the cell, so that we can, I guess, ride at higher speeds or greater intensities for longer, and increase some of the markers and enzymes that are involved in both liquid and carbohydrate oxidation within these mitochondria, and to provide that ATP more readily. Then a couple of the other reasons why people are doing this is probably to manipulate energy balance in some way, with a view to maybe improving body composition, decreasing fat, and maintaining a lean tissue mass or muscle mass. The final reason that certain people actually undertake fasted exercise generally is because they experience gut or GI discomfort when exercising So, particularly people who exercise in the morning and they’re getting up after fasting overnight, trying to cram in a meal, and then their exercise is difficult in terms of timing without encountering some GI discomfort. So, they find it easier and more convenient to actually exercise in the fastest state. locally. Just in advance of the call today, just going over the most recent literature pre-published online from a group in Auckland, and they’ve surveyed that almost 2000 athletes to establish their practice around fasted exercise and establish their reasons for exercising in the fasted state. What they found was interesting, and that was quite popular, quite common, but 62% of the athletes they surveyed were using fasted training somewhere in their preparation for their events. A couple of other things were interesting with that, it’s more predominant in males, non-professional athletes are more likely to use fasted exercise. Those who are manipulating their diet or following a specific diet, such as perhaps a low carbohydrate, high-fat diet are more likely to engage in fasted training. The reasons are for, as I outlined a couple of moments ago, to increase lipid utilization during exercise, and to maintain good comfort actually, and then finally, was convenience. The reason maybe some of the more professional athletes weren’t doing it was because they were finding wasn’t benefiting their training in some cases, or they were feeling really poor during high-intensity training sessions, and they felt they couldn’t train at the same intensity. There are things that we might get into in the overall discussion as we go on. So, I hope that kind of sets up at least a little bit of the why and what we’ll start to get into and discuss this afternoon.


Trevor Connor  26:55

Yeah, I think you’re touching on a really important point, which, again, might explain why top-level athletes are a little more hesitant, but people who are doing this more for health and fitness reasons are more interested in this. There are two questions here about the fasting. One, is there health benefits? And I would say the research is mostly pointing in that direction. The second question is, does this help performance? And I know we’re going to get into that, but I think that’s where you get a little grayer, and there’s certainly evidence that no, this can actually hurt performance. So, if somebody is exercising to improve their health, I think there is a big benefit to this. Actually, this is a tangent, but bring up the fact that I just did an interview a couple of days ago with Dr. Bredesen, who is about to publish a second book about Alzheimer’s, and he has seen a whole lot of benefits in Alzheimer’s patients using an intermittent fasting type protocol. He’s particularly big on the time-restricted feeding.


Trevor Connor  28:01

Let’s hear what Dr. Bredesen has to say about the health benefits of fasting.


Dr. Bredesen: Health Benefits of Fasting

Dr. Bredesen  28:06

Fasting turns out to be so incredibly helpful for many things, you know, for your blood pressure, for your glycemic control, for your lipids, it’s amazing how important and helpful routine fasting is.


Trevor Connor  28:23

And you mentioned this when I was reading your research, he talked about metabolic flexibility, which is the brain can use glucose and fat in the form of ketones for fuel, and you want it to be able to use both, and when you use this time-restricted feeding, you can produce ketones and give the brain both sources of fuel, and it seems to help with people who are having some sort of neurodegeneration or cognitive decline. But that’s a tangent.


Dr. Brian Carson  28:55

iI’s an interesting tangent and one of the things that we always struggle with when we’re asked to kind of define, you know, what’s your research area? And what kind of populations do you work with? In some ways, I could be criticized for not being more focused, but I tend to work across what we would maybe term the health performance spectrum. So, we work with people who are metabolically impaired, right? Those who are competing at the elite end, and part of my response to that is the fact that the mechanisms within the muscle are very similar in terms of adaptation to exercise, and nutrition, regardless, whether it’s for a health benefit, or for an actual performance benefit. Now, the magnitude of those changes it’s very, very different between those two populations. But generally, the mechanisms of adaptation within the muscle, at least are very, very similar. So, it allows us to work on both those populations without kind of compromising our overall research area. So, I would agree with your assertion around the use of fasted exercise for certain health benefits. I think there’s a lot of evidence there for that. I think there is a lot of evidence there also in terms of driving the endurance phenotype within the muscle, and we’ll get into that, even in elite athletes. The performance pieces is a little bit more difficult, it’s a little more unclear. We see adaptations in elite athletes, but then it doesn’t always translate to a performance benefit, in terms of our tests, and there are certain reasons for that, which, again, we might get into later in the conversation.


Chris Case  30:32

So, Dr. Carson, let’s get into some of the molecular science here, in the molecular level, what is happening metabolically speaking during the fast?


What Happens Metabolically During a Fast?

Dr. Brian Carson  30:45

Okay, so it’s a good question. As the fasted period extends, and obviously our energy sources from exogenous sources, so from our meals, they begin to deplete and they begin to be removed from the blood and taken up into the relevant tissues. As the fast extends, what we’ll see are an increase in the availability of what we would call non-esterified fatty acids, so free fatty acids circulating in the blood. We will also see prevalence of some small ketone bodies would become available at that time and other kind of liquid source. We’d also see glucose, which predominantly is common, so blood glucose is really tightly regulated, really tightly maintained, so we don’t become hypoglycemic, and that glucose will be supplied from liver glycogen from gluconeogenesis in the liver, and they are the predominant energy sources which are prevailing later on in the fast. The other thing that we’ll probably see present in the blood during the fasting period is the presence of catecholamines, principally norepinephrine and epinephrine, and their function at this point in time my guess is too much mobilization and energy sources. So, they’ll be driving the low paralysis of the adipose tissue, of the fat tissue, to release those non-esterified fatty acids and into the bloodstream. They’ll also increase lipolysis in skeletal muscle to mobilize those energy substrates from things like the intramuscular triglycerides, so that they’re ready for use during that period, and to maintain our normal metabolism throughout that fasted period. What we’ll see then is an increased fatty acid utilization during that period. So, if we were to measure something like your respiratory exchange ratio, at that time, we’d see that we’d have a lower which indicates that our fuel sources being used are predominantly from liquid, rather than carbohydrate. If you were to provide a meal at the end of that fast, and you mentioned metabolic flexibility, what you’d see very quickly is the body would quickly move to utilize the available carbohydrate coming from the source, so you’d see a quick shift in that exchange ratio. However, when we’re talking about fasted exercise, obviously, we’re not going to have that meal. So, as we go to begin the exercise, what we have is high circulating fatty acids, some ketone bodies, and we have pretty normal blood glucose at that point in time, with the presence of some of the catecholamines in higher concentrations than normal. So, at that point, then we go and we’re getting to actually what’s happening in terms of metabolism during the exercise bed. So, because of the increased availability of those fatty acids, we’ll see increased utilization of fat sources. So, we look at and we’ll see increased fat oxidation coming from those free fatty acids, but also coming from the intramuscular triglycerides, which I mentioned earlier. We’d see decreased glucose oxidation and during that period, so to increase utilization of fat as a fuel during, let’s say aerobic exercise, I’m going to refer to here for the cycling community, we’d see that predominate at lower intensity, so probably less than 70% of your VO2 max. So, that would be what you guys probably considered pretty easy ride, and that will be your kind of long, easy ride maybe 60 to 70% of your VO2, we see that increased fat oxidation. The evidence out there would suggest that this isn’t overly influenced by time, that it’s more influenced by the intensity exercise and by the training level of the participants. So, the higher trained you are the more fat oxidation that we would see during that period. We would see across the exercise period, we’d see greater variation in glucose and insulin from pre to post, in the fed state compared to the fasted state. So, if we compare the same athlete having fed prior to exercise or exercise while fasted, we would see greater variation in that pre-exercise glucose and insulin concentration to post-exercise and glucose and insulin concentration, suggesting greater glucose oxidation, which you would expect, and insulin is low in the fastest state, and that allows the mobilization of those lipid resources. We wouldn’t see any difference on the pre to post, non-esterified fatty acids in either condition, what you would see normally is in the first 15-minutes, and in the fastest state, you’d see those circulating fatty acids come down, because the demand is probably outweighing the supply there, and then over time due to the increase that’s occurring because there’s no exogenous energy availability, we actually see those fatty acids come back up because we’re mobilizing those fats from the adipose tissue itself. We’d see in the fed state there because there’s increased insulin, that will not happen because insulin will negate glycolysis, and you will have the availability of glucose as a substrate in that fed condition. So, there’s a kind of contrast between the two. In the fed state as well, you’d also have decrease oxidation of those intramuscular triglycerides, and an overall increase in carbohydrate oxidation. That would be fair to say.



Trevor Connor  36:27

Let me just take a quick step back here to just explain this to some of our listeners who might be a little less familiar with this. The way to think about this is your body has a limited supply of glucose and glycogen. So, glycogen is stored form of glucose. It wants to keep spare that glucose, because it is still the primary fuel of your brain, and your brain gets priority. Fats are essential when you’re talking about exercise, your fat storage is unlimited. So, if you are in a fasted state, your body’s basically saying, okay, now my limited store glucose is getting even more limited at this point, you’ve probably used up your liver glycogen. So, your body goes in this mode saying, I’ve really got to spare that glycogen, I’ve got to spare that glucose. So, it’s going to ramp up your use of fats for fuel. In a fed state, you probably have just hit yourself with a whole bunch of carbohydrates. If you’re eating a normal meal, your body goes okay, we got an availability and we actually we don’t like to have too much glucose in the system, we like to keep it right at a certain point. So, we’re actually going to prioritize using this for fuel, and this is really directed as you’re saying by insulin. When insulin is high, it says to store fats use glucose. When your insulin levels are low, it says spare the glucose use fat. Is that a fairly accurate, high-level summary of this?


During Exercise

Dr. Brian Carson  38:01

That’s a really nice summary. Typically, the body will take, you know, if you’d like to think of it this way, the body takes the easy way out. So, it costs us a little more in terms of oxygen to oxidize fats. So, we tend to do that when we’re at rest, and when things are a little bit easier when lots of oxygen is available and lots of energy is available. Then we also do that in periods where we haven’t got exogenous sources, like glucose readily circulating in the blood. However, soon as you feed that glucose, if you feed a bunch of carbohydrates, you’re going to have high blood glucose, blood glucose, and you’re going to have high insulin. Basically, what the body says, okay, well, I’ve got these available, this is very easy, and it’s much easier to oxidize these, we’re going to shift to this. And that’s part of that metabolic flexibility piece, the ability to shift between these two. So, in very trained people, they’ve got that ability to a greater extent. So, where carbohydrates are available will always switch to this glucose oxidation state. I mentioned the respiratory exchange ratio, which would increase at that time. But when we’re fasting, we tend to have lots of these fatty acids available, and because we’ve mobilized them, and we’ll predominantly use those and try and spare that glucose for when we might need it later, and for kind of a fight or flight type incident or, or a situation later on, so we’ll try and preserve that glucose and muscle glycogen as best we can, and use those fats which are readily available, which we’ve stored for periods like this. So, that’s a really good summary.


Trevor Connor  39:40



Trevor Connor  39:40

Another really important high-level thing to remember here is we don’t actually directly use glucose or fat for fuel. Both are used to produce ATP, and ATP is what we ultimately use to produce our energy.


Dr. Brian Carson  39:55



Trevor Connor  39:56

Or energy activity. It’s important to remember that a fat molecule can produce a lot of ATP, upwards of 100 ATP, but it can’t produce them quickly, it’s a slow process. Glucose doesn’t produce a lot of ATP, but it can produce it really quickly. So, if you are doing that high intensity, type activity, explosive activity, you want to prioritize glucose, if you aren’t needing energy quickly, but you need a lot of energy like long, slow endurance rides fat is great.


Dr. Brian Carson  40:32

Absolutely. So, fat is more energy-dense in terms of, as you pointed out, ATP. If you think of glucose as almost like the convenience food in this scenario, when you need a fast and you need it easy, then glucose is going to be the preferred source. If you’ve got time and you’ve got plenty of resources available, then you can switch to fat. I think that’s a good high-level summary.


Chris Case  40:56

One question I would have here for clarification sake if for no other reason, knowing the popularity of the Ketogenic diet, these buzzwords that you’ve heard a lot of, we’ve mentioned ketone bodies, when you’re talking about what we’re talking about in this fasted state, to clarify, we’re not talking about being in a state of ketosis. Is that correct?


Dr. Brian Carson  41:24

That’s correct. A state of ketosis is going to take him some time to develop, and that wouldn’t be the focus of my own research, or what we’re talking about today. What we are talking about are short-term periods of fasting, such as an overnight fast, somewhere between maybe 12 and 15 hours, probably at a maximum, then moving into exercise. Whereas something like ketosis is going to take carbohydrates manipulated for a much, much longer period than that for significant amounts of ketone bodies to develop, to be used as a fuel source, and even people who would follow a low carb, high-fat diet, would say that probably three weeks is the minimum in terms of getting an adaptation there to be able to use those ketone bodies successfully during exercise and so on. So, no, that wouldn’t be what we’re referring to here.


Chris Case  42:16

So, maybe we didn’t finish our overview of the metabolic processes here. I think we left off maybe post-exercise? Is that where you’d like to pick up, Brian?


Dr. Brian Carson  42:27

I’d like to make one more point around during exercise, and it’s relevant for your listeners because many of them will undertake long rides, which you know, might go four to six hours. One of the things that we’ll see in a longer duration exercise, is that after about two hours, the substrate utilization between both a fed and faster condition, it starts to move closer together. So, they become more similar, even in a low-intensity ride, you’re still using some glycogen, and if it’s slightly higher intensity, you’re going to be using enough glycogen for it to start to deplete, and not deplete completely, but where you’re going to need to use and some different fuel sources. So, what we see is that the two conditions, the fed and fast, start to migrate towards each other in terms of metabolism, start to look very similar. You’ll see increased fat oxidation begin to occur in the fed state so that it now looks a little bit more like the fasted state. That makes sense because if you think of what you’ve had in your, in your meal prior to exercise in the fed state, you started to use up those resources, and now you look similar and to watch the athlete who began without breakfast, for example, it looks like after two hours, so they start to look more similar over time.


Chris Case  43:50

Okay, well so now we know what took place before your exercise, during your exercise, what’s happening post-exercise?



Dr. Brian Carson  43:59

We know what’s going on during the exercise period, now in the post-exercise period in terms of metabolism, what you’ll see again in the fasted state, is you’ll see a decrease in overall glucose oxidation, you’ll see a decrease in the amount of insulin that’s circulating. So, those two things match up and there’s less insulin circulating, therefore, we’re less reliant on glucose. In the fasted state, again, compared to the fed, you’ll see an increase in those circulating and free fatty acids is still prevalent. And again, that matches up, because we see an increase in the number of catecholamines and the concentrations of catecholamines that are present. So again, they’re mobilizing that fat resource, we see those increase free fatty acids, and then what we’re going to see is an increase in fat oxidation in that fasted state, and a decrease in glucose oxidation overall. An interesting point, sometimes we might flip flop between the healthy or the elite athlete right down to the metabolically impaired, but an interesting point in the post-exercise period is that when we look at participants in research who are medically compromised, most of these studies show no difference in glucose, insulin, or free fatty acids between the fasted and fed conditions after the exercise, which isn’t true in the athlete population. So, this is we think probably relates to that metabolic flexibility, which we’ve mentioned a couple of times, which is also influenced by training status, and then also influenced by their body composition. So, an increase in fat mass and a decrease in the amount of muscle mass in these metabolically impaired individuals, so they have poor metabolic flexibility. So, what they do is immediately they will still try and rely on that easy access, that easy resource of glucose oxidation. So, it looks even after fasted exercise, it looks similar to fed exercise in the post-exercise period, and that we will see similar levels of glucose oxidation, similar insulin concentrations, similar fatty acid concentrations, and similar reduced catecholamine concentrations, in those particular individuals. I think that’s a really interesting point, and that feeds into some of the confusion that you’ll see in the literature. I mean, we had a systematic review, a meta-analysis in 2018, and I guess some of the findings are confounded by the different populations that are being used in the literature. So, it’s really important when you’re interpreting these studies to consider the population involved.


Trevor Connor  46:39

I think that’s a really important point, because that was, I remember going through some of the literature this week, and that really was very contradictory. So, for example, I have this 2015 study that Dr. Noakes was involved in, they showed in recovery, they saw a reduction in inflammation, they saw a reduction in oxidative stress, but I think it was in your review, where you saw post-exercise in a different population actually increase in some of the inflammatory markers like aisle six. So, you can see very, very different responses depending on the population.


Dr. Brian Carson  47:18

Absolutely. And these metabolic impaired, you’re more likely to see that oxidative stress because the mitochondria are effectively not trained, and to complete this exercise without incurring damage. One of the other things, something to watch out for, is around immunosuppression when, particularly when carbohydrate is low, and again, these populations might be a little bit more susceptible, and to that immunosuppression, and their training status and their body composition has a lot to do with that. So, it’s just an important consideration when we’re looking at the literature in this area. Just one more point I’d like to make about the metabolism, post-exercise and some really nice research, and I’ll flag up this particular group for anybody who’s interested in the area, James Batson, Javier Gonzalez, and Rob Edinboro, who is one of their lead investigators at this time. They have some nice studies. But one particular study stood out to me in that, after exercise, and this is again from the health perspective for those who might be looking to maybe improved or their metabolism or actually improve their body composition in some ways, and one of the benefits that they observed when they skipped breakfast and before exercise, was that they found that their 24-hour energy balance was negative in the fasted exercise condition by about 180 calories compared to breakfast before exercise. So, that was quite significant over that period. I guess the energy expenditure there was combined between the exercise energy expenditure and the restricted energy intake by the admission of the particular breakfast. So, the overall ended up with a negative energy balance of around 180 calories. Well, it’s actually about 400 calories, but it’s about 180 calories more than the group who performed exercise but had their breakfast. So, I think that’s really interesting finding and for those who are listening and who are interested in their own metabolic health, I think it is a strategy that can be incorporated into an overall regime to potentially improve those markers.


Trevor Connor  49:29

Agreed. it’s worth pointing out, I mean, you might say 180 calories, that doesn’t sound like very much. I remember back in school when we were studying obesity, going through studies showing that people who became obese, who hit very unhealthy weights, were only over-consuming by about 150-200 calories per day. That sounds small, but over time, both for weight loss and weight gain, 150-180 calories is actually quite a lot.


Dr. Brian Carson  50:02

Yes, it’s a huge amount. So, even the overall negative energy balance they achieved was about 400 calories, but 200 of that came from the exercise itself, but the emission of the breakfast, they didn’t compensate for that in the rest of the day. I think that’s a really important point. Often what we might assume is that, okay, they didn’t have their breakfast, they did the exercise, but the meals throughout the day or the snacking, which might occur later on, might actually compensate and keep the body and energy balance. I will say that that was an acute study. So again, we have to temper our interpretation and literature. I think that group in particular, are worth keeping an eye on the researcher, they are doing a lot of stuff around fasted type exercise, and around breakfast in general. So, they’re really good group, producing really high-quality research.


Chris Case  50:54

So, maybe it’s time we turned our attention to what a lot of listeners out there, probably dying to hear, which are these performance gains, the potential performance gains from taking up a regime, fasting regime, or the adaptation benefits. So, let’s dive into that.


Performance Gains

Dr. Brian Carson  51:11

Great. I’m sure all your listeners are really interested in this particular aspect of the talk. The performance outcomes and I’ll break this down, I guess, into more endurance, kind of aerobic performance, and then, and I’ll talk a little bit about anaerobic performance, much less evidence in the anaerobic piece, but I will come to that. Most of what I’m going to discuss here can be found in the review, which I mentioned by my former Ph.D. student, Dr. Tom Heard, which we published in 2018. So, what I’m discussing here is based on what we found in that particular review, and we conducted a meta-analysis of all the research in this area. A couple of caveats before I get stuck in, and I might refer back to these if I need to, one of the caveats is that how performance is measured is very important, so when you’re talking about performance, you’re really referring to, you know, race day performance and getting over the line with your, with your wheel in front, whereas a lot of studies tend to look at things like time to exhaustion, which are not as relevant in terms of performance unless you’re going to take part in an endurance race to see who can last the longest, well, that’s probably not appropriate. So, some of the measures are confounded by that, I would say to a certain extent, whereas ones that focus on things like time trials, I would say, are better evidence. The other issue that I think we often have is and, I kind of refer back to this when we talk about the adaptations, but it’s the sensitivity of the measure in the lab. So, how sensitive is the measure that’s being performed, and what have the researchers don’t to maximize the sensitivity of that, and let’s base somewhat under reliability of the measure. So, if we measure your performance today, and we measure the exact same conditions in seven days time, you know, how tight is the number that we’re going to get from that? So, what’s the percentage error? What’s the critical difference? What would be a meaningful change in performance? So, there are two of the caveats that I would have at the beginning, a lot of labs will control that really well as best they can, but there is always some level of error in there, which makes it difficult to tease outperformance and adaptations, because performance adaptation in Tour de France in terms of the stage win, might be a win by a second or less than a second in a sprint finish, which is a really fine margin, whereas we’re looking for percentage increases over 20-kilometer or 40-kilometer time trials. So, any loss of sensitivity there is difficult to overcome. So, those caveats aside, what we found is for acute exercise, so if we exercise somebody in the fasted state, and then we exercise them another day in the fed state, an acute about of exercise, not a chronic training intervention, most show benefit for performance in the fed condition. Importantly, what I should say here is, there is no study showing benefit of the fasted condition. There are studies which show no impact of the fasted condition, but never a benefit greater than the fed condition, if that makes sense?


Chris Case  54:32

It does.


Dr. Brian Carson  54:33

That’s entirely intuitive, entirely what we would expect and what we know about carbohydrate performance. For example, you know, there are lots of studies. I mean, there’s a whole bank of evidence around the importance of carbohydrate for performance, and, you know, if you’ve go into a race with low glycogen, then your performance is likely to be down somewhere between 10-20 percent, over 90-minutes. So, that’s not an unexpected finding, there are a number of studies, though, in terms of positive if you like for fasted based exercise where the performance isn’t impacted. So, if we look a little bit closer at that, and when the challenge is greater than one hour in duration, what we see is more of an effect for feeding, and again, this makes sense. So, where the challenge is extended? Where it’s longer? Where it’s more of a challenge, where it might go beyond what your glycogen stores that you naturally have and providing some exhaustion as fuel has been shown to be more beneficial. So, in the analysis that we did within that particular paper, what we saw with the effect size was greater when we looked at studies which looked at performance outcomes, greater than one hour in duration, versus those which were less than one hour in duration. So, again, that intuitively makes a lot of sense. So, the effect becomes stronger, the longer the duration of the exercise. So, if you’re going into, I mean, the take-home there will be if you’re going into a particular event that’s going to last longer than an hour, then you absolutely should be fueling on the day of the event to perform to your maximum. That’s probably very unsurprising to your listeners, I imagine it is anyway.


Chris Case  56:26



Dr. Brian Carson  56:27

However, when the performance outcome is less than an hour, in almost 60% of the studies we looked at, performance was not impaired by fasting. So, if you’re in an event that’s going to be less than one hour, and you see benefits for doing it in a fasted condition, maybe it’s something like the GI discomfort we mentioned earlier, then potentially you’re not negatively impacted, and in 60% of those cases. The shorter it comes I would say the less need that you have for exogenous carbohydrate in advance of the actual challenge. So, that’s a positive in terms of those who might be interested in entering some competition in a fasted state. For your listeners, though, you know, feeding is going to be important on days when you need to perform a competition, that’s also true for your training. So, in terms of performance and training, when do we need to perform in training? We don’t always need to perform, but we have different intentions or different goals for each training session that we’re going to undertake, and on certain days, you might, for example, want to be doing a hard day, and that might include some thresholds or some hard type intervals, type of work that you want to achieve, that particular session is going to be looking at really high-intensity training. Well, then, in my view, and if that’s extended, particularly beyond the hour in my view, then you need to be fueling appropriately for that training session, and for that intention. So, you need to be able to ride at the speed that you want to ride in the race, and you don’t want that to be impaired in that particular session. However, if the goal of your session isn’t so much about performance, it’s about maybe adaptation, and maybe it’s a recovery ride, maybe it’s kind of a longer ride, then potentially the option to do that particular ride in the fasted state is one in which you might explore. But if you need to perform in a training session, I would say that you should fuel for that training session. One of my colleagues at Limerick, Dr. Catherine Norton, and her Ph.D. student Margaret Cozier are working on a term and around that intention, and they call it Perry Training Nutrition. So, try and fuel or use nutrition around the exercise session, around your intentions, and that’s in the pre, during, and post period. So, Perry Training Nutrition around that particular session and its intentions. It is really important.


Chris Case  59:05

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Trevor Connor  59:08

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Trevor Connor  59:31

Yeah, I mean, you’re seeing that sort of direction in the research, I know what’s becoming popular is exploring this idea of doing your interval, your high-intensity session in the late afternoon or evening in a fed state, then don’t fully refuel, sleep, get up the next morning, and do a low-intensity session in a fasted state.


Dr. Brian Carson  59:54

Yeah. That’s been shown to be a pretty good model. I know James Morton and John Holley have done research in that area, they will term that Perry training nutrition, I mean, that’s including all macronutrients, but they would talk about periodization of carbohydrates around training and aligned to the intention of the particular session. The idea that you would train maybe twice a day makes sense, and that you might feel differently for each of those sessions makes sense. So, I think, John Holley calls a model, it’s a train high sleep low. So, that will be where maybe you have a session in the evening, which is a high-quality session, whereby it will be high intensity and you should feel appropriate for that particular session, but that you might not feel appropriate in the recovery from that session. So, you might manipulate the carbohydrate in the diet and recovery, keep that carbohydrate low, sleep overnight in a low condition, and then move into a fasted exercise session the next morning, presenting a greater challenge for that particular exercise session in the fasted state with low carbohydrate availability, low muscle glycogen stores. Again, to drive that phenotype response and endurance phenotype in response to that session the next day, likely that session will be at a much lower intensity in the session than before. So, that would be more of a recovery-type session or extended low-intensity session, perhaps.


Chris Case  1:01:28

One thing I would wonder there, and this maybe gets ahead of ourselves a little bit, but it seems like that’s slightly dangerous. If you don’t do that the right way you might negatively impact the benefits of the training bout, you might prevent yourself from recovering well enough so that the subsequent days you aren’t ready to ride again, and it might be counterproductive. Is that is that true?


Dr. Brian Carson  1:01:58

Potentially. If you were doing that continuously, without maybe cycling it in and out of your week, or having planned into your overall program, and that could be the case, but in the example, I gave there, so it’s one hard training session the night before, poor recovery, if you like in terms of nutrition, or sub-optimal recovery in terms of nutrition, and overnight and into morning ride. What I was talking about, there was more like a recovery ride or an extended kind of low-intensity ride, so you won’t be fueled appropriately for that particular session. But after that, you would feel appropriate, and then perhaps that might be your only ride that day, and you’ll be ready to train again properly the next day. If you did that day on day, then perhaps, yes, you’re going to be compromised over time. We mentioned a little bit about immunosuppression, and so on and likely the quality of your sessions will go down but doing that maybe once or twice a week might be, you might be able to incorporate that. One thing here is very important, using periodization, in another form here is the periodization of your training around your competition phase. So, you probably do this type of work, I would say out of season or in the preseason, it’s not something you’re going to be doing and during your race season, during your race season, you’re going to want to be feeling appropriate. If that makes sense?


Trevor Connor  1:03:29



Chris Case  1:03:29



Dr. Brian Carson  1:03:30

And just coming back on the performance in the fasted state, I mentioned that there are a few anaerobic type studies available in the literature at this time, when we did the meta in 2018, there were four such studies available. What we found was that fasting, in most cases, didn’t impair performance during anaerobic type exercise. There was study which showed that when you’re doing perhaps, a wingate type exercise and I’m not sure if your listeners are familiar, maybe they are, wingate is where you’re looking to maintain really high power output, it’s an all-out and bout of exercise for just 30 seconds, what you’ll find is the peak power output, which occurs in the first four-to-five- seconds of the 30, and will be reduced in the fastest state, but overall, maybe the mean power output might be impacted across the 30 seconds. Most of those studies showed no impact on anaerobic performance. Now prior to that, and one caveat here is three of those studies were hit-type studies. So, they contained an interval. So, where there was a period of recovery between each particular high anaerobic bout, and that might play into it there so that the athletes were able to recover sufficiently in those intervals, and to perform when the high-intensity interval came back around again, and to not be impaired. We found that interesting, and that kind of formed the basis for some of the current work we’re doing, and some of the future work also, which I’m sure we’ll come on to later on. But there’s really very limited work in this area. So, further research is required in this pace. As I said, there, we might come back to that later on in the conversation.


Trevor Connor  1:05:27

Something I found really interesting in your review, talking about the windgates, was that when athletes did a wingate workout, so as you said, a series of windgates with recoveries between, in a fasted state you saw a ramping up of AMPK, which are key pathways in aerobic adaptations.


Dr. Brian Carson  1:05:58

Yeah, and actually, that brings us on to the next part of the conversation, I think really, really well, because we can talk about the adaptations to training in the fasted state. So, what we’ve talked about now is kind of what happens just before the exercise, during the exercise, immediately post, and what happened in terms of our performance during that particular exercise bout. But one of the main reasons is that people undertake fasted training, as we discussed at the very beginning, is to drive an adaptation towards an endurance phenotype. As you said, even in something like a windgate, what you see is, and this will be typical any way you would see phosphorylation of the signaling kinases, such as a AMPK, and because you’re basically turning over ATP, as soon as you start to turn over ATP which results in ADP and EMP being available, you will phosphorylate MPK. So, that’s not unexpected and you’ll do that in the fasted or the fed state, but it seems to be augmented in the fasted state, which is really, really interesting. There are a number of other signaling kinases that we activate in pathways that we activate, such as p38a kinase and p53, has become a more recent discovery seems to be important, and calcium calmodulin kinase or camp k two, and something like ACC, which is really a surrogate of AMPK, but it’s involved in the switch between carbohydrate and fat metabolism. These signaling cascades are all activated by exercise, in some ways, they’re somewhat augmented by doing that in a fasted state. A lot of those pathways converge around transcriptional coactivator called, PGC-1 alpha, you know, it used to be coined or still there’s a lot of time, this kind of master regulator of metabolism in the muscle because it’s a transcriptional coactivator, its transcription is linked to increased transcription of other metabolic genes. That’s why it was considered this kind of master regulator, some recent evidence, which is kind of refuting that role in that there’s some, I guess, other pathways, so that if we knock out PGC-1 alpha, for example, and animal models, we can still get upregulation of these other genes. So, there are pathways we could step in if PGC-1 alpha isn’t there in opponents, but that’s an aside. So, a lot of these pathways converge on PGC-1 alpha, and as I said, it kind of co-activates and regulates the transcription of a lot of other genes associated with metabolism in the muscle. Some of those are things like NRF-1 and NRF-2, PDK-4, which is really easy to manipulate with exercise because it acts as a little bit of a switch in fuel metabolism in between carbohydrate and fat. So, in the post-exercise period, you’ll see PDK-4 upregulated to a huge extent. What you’re seeing in that post-exercise period, if you haven’t fed, is obviously a switch to fatty acid oxidation, and that’s aligned to that. Some of the other genes related to fatty acid oxidation will be, which are regulated by PGC-1 alpha, which is very important and CPT-1, CD-36, which are, I guess, fatty acid transporters to an extent, and UCP-3, and then what you see then is, so, we see these millions of different genes are upregulated and it their upregulation is transient usually peaks somewhere around three hours after the exercise bed, and then can be manipulated by the availability of nutrients. So, some of these are augmented. For example, we’ve seen in our own research that isn’t published yet, impacts on delta on NRF-2T and on CD-36, which are augmented in the fasted state, compared to a pre-carbohydrate feed, and with high-intensity interval training, or a better term being here, sprint interval training. So, some kind of menu is very transient to talk about three hours, be back down, probably around 12- hours after the exercise, any effect would disappear. The idea would be that, over time, that you’re transitive, increasing these genes, you’re increasing the copying of these, the production of these proteins, and over time, you’ll see a creation of these proteins in the muscle, and that they will exert their function then in the muscle. So, these genes are all associated with both lipid and carbohydrate oxidation, and ultimately mitochondrial biogenesis. So, increasing both the number and content of the mitochondria. Which, as you mentioned earlier, are going to be where we produce our ATP, where we convert our fuels, such as our fats and carbohydrates into ATP, which the muscle can actually use.


Dr. Brian Carson  1:11:13

What we also see then when we look in these mitochondria is we see markers of these mitochondria, such as citrate synthase. Citrate synthase is an enzyme, part of the TCA cycle, and so very important in the production of ATP. Then we also see an increase in markers associated with fatty acid oxidation, such as beta HAD. So, both of their content is upregulated. What we’ve found, and again, I’m hand waving a little bit because this research hasn’t been reviewed by my peers as yet, we’re still preparing the papers and we’re doing some kind of follow-up analyses to better describe the mechanisms. But when we compare the fasted condition to the fed condition, we see better upregulation of those two markers, of mitochondrial biogenesis and fatty acid oxidation, when we exercise without carbohydrates, first in a sprint interval training model. So, over a period of three weeks, so in a very, very short period, just nine sessions. So, they’re quite interesting findings and we hope to get those into a good journal maybe later this year. They’re the type of adaptations we’re interested in, and overall, I suppose is that greater endurance phenotype. And even though I’m referring to sprint into a train there, those adaptations will be important for endurance performance also.


Trevor Connor  1:12:44

So, to take the big step back, basically, the one two-sentence summary of this is, even taking something like a wingate, wingate is used to test your anaerobic power, it is a sprint workout, you are doing that anaerobically. Even doing that sort of workout in a fasted state, what you’re seeing is this big ramp-up of all of your aerobic machinery, and seeing greater adaptations in the aerobic machinery, that would kind of be the one or two-sentence summary of all this.


Dr. Brian Carson  1:13:20

Exactly that. One way I describe, because this happens very early in my first-year module, on our sports sciences degree, we talk about transcription and translation. What I tried to say to the students to put it more high-level is that effectively the body is very good at adapting. So, when you challenge the muscle today with a particular exercise challenge, it’s not very good at it if you’ve never done it before, but it’s very smart, and it says, well hang on a second, why was he asked me to do? Why couldn’t I do it to the level he wanted to do it? So, what I didn’t have was this thing off the shelf, I didn’t know this, I didn’t have this, and if I had those, I’m better able to do what he was asking. So, I need to make more of these. That starts immediately after the first exercise bout. So, we start transcribing the DNA to develop these genes, to produce more of these genes, to them eventually produce more of the proteins associated with them, so they can exert their function. That machinery is then there, so when you come to exercise in three, four weeks time, after you’ve done this repeatedly, you have more of these things that you need available, and therefore, you can actually conduct the exercise or you can perform the exercise better to a higher level, that’ll be kind of the overall summary that I will give there.


Trevor Connor  1:14:45

We’ve had Dr. Inigo San Millan, the head physiologist for UAE on the show many times now. We asked him about fasted training, and he had a great point about all these adaptations we may be seeing. Yes, we are seeing increases in RNA Transcription, but does that mean we’re actually seeing adaptations? So, we are going to do a show on intermittent fasting. So, I’ll just hit you with the big question. What’s your feeling about using intermittent fasting in training? Helpful? Not helpful? Really makes no difference?


Dr. Inigo San Millan: Opinion on Fasting During Training

Dr. Inigo San Millan  1:15:21

Well, in my opinion, it might be detrimental. This is from what I have seen in athletes. It goes back to the restriction, right? Of fuel, right? That’s kind of intermittent fasting, and there are a few studies showing that, I feel it’s a maybe like, if you will, a more elaborate than the carb train law would be higher, restricting carbohydrates, right? And energy but that’s not a good idea in my opinion. One of the things is that there are few studies showing that restricting carbohydrates might increase the transcriptional activity of those genes related to mitochondrial biogenesis, right? But didn’t show those studies. They are very sexy and elaborated studies, make no mistakes, but one thing is to show an increase in the RNA activity of that transcriptional capacity apart for different mitochondrial precursors. The other thing is actually to demonstrate through proteomics to see that the protein is important. The studies are not there yet. So, we’re still at that RNA transcriptomic level, which is the signal, right? That’s why it’s very easy then to interpret that data, these data shows that people who go out there and they don’t eat carbohydrates, they’re you know, mitochondrial biogenesis precursors of the transcriptional activity increases, therefore, they’re going to increase mitochondrial function, but not necessarily, only about 40% of the RNA is transcribing to protein. So, we might not have the right threshold, right? I will say the same that analogy with f1. F1, as you guys know, is the precursor of areata presses, right? So, it’s key and that responds to oxygen, and you find it’s cool, so cool that last year’s Nobel prize, is the three ones where because of the work they have done in f1. So, if you are for 20-minutes in a very hypoxic environment, your f1 is going go up the chart off the charts, really high bike, high increasing f1. Now, are you going to increase red blood cells, because you were 20-minutes in hypoxia? No, you’re not. So, this is kind of what’s happening at this level of mitochondrial function, right? So, I think we need to understand this a little bit better.


Trevor Connor  1:18:22

We talked about this before on the show that in humans, there’s no such thing as purely anaerobic muscle fiber, all fibers are able to at least do some work aerobically. So, even when you’re doing this big, hard, short effort, if you’re doing it in glucose sparing state, as you said, the muscle is going to go, well, hey, you’re asking me to do this big work, I can’t really do it right now using glucose, using pure anaerobic machinery, so I better ramp up that aerobic machinery.


Dr. Brian Carson  1:18:56

Absolutely. Most of the type twos are mixed fibers anyway. So, they’ve got the mixed functions. They’re glycolytic, but they can also do a certain amount of aerobic, or they’ve got a certain aerobic capacity. So, you’re absolutely right there. It’s somewhat counterintuitive in that, why would doing very short sprint interval type training require nutrients? How would that offer a bigger challenge? You’re not going to deplete the glycogen stores given the volume of work that’s there, it’s perhaps in the recovery period, or perhaps it’s in the interval between, and we’re not a hundred percent sure at this point. I can just say that in our hands that we’ve had differential effects between fast and fed in this particular model, and we’ve actually looked at 25 mitochondrial genes in a multiplex essay which we developed. So, we’re interested and very keen for people to see those findings in time. But there are a few new players in there as well. So, what I went through were probably the classic stuff that you’ll see in an awful lot of studies and anybody who’s up with the literature will have known all of those genes that I mentioned, but maybe an understudied cohort and they are related to the one that you mentioned a few moments ago, cert one, but they’re involved in NAD metabolism, so NAD biosynthesis and NAD scavenging in the electron transport chain. So, we see a couple of particularly interesting genes which haven’t been shown to be regulated by sprint interval training and have never been shown to be regulated by fasting versus feeding. So again, I’m hand waving a little bit, this will all have to go under scrutiny, but there are some of the results that we’ve seen in our hands at least,


Trevor Connor  1:21:03

I’ll be very excited to read your study when it comes out.


Dr. Brian Carson  1:21:06

Yeah, hopefully, gets in somewhere nice, and it comes out probably later in the year because there is some more mechanistic work that we’re doing just to try and explain what exactly what’s going on in terms of metabolism.


Chris Case  1:21:19

So, Dr. Carson, you brought up the impact on fat metabolism. What about the impact on glucose metabolism, whether positive or negative?


Impact on Glucose Metabolism

Dr. Brian Carson  1:21:30

That’s an interesting question because we talked about the adaptations, we’re talking about changes in markers of both lipid and carbohydrate oxidation being increased over time. But there’s a slight paradox in the literature, at least in relation to performance and that one thing that you might not expect, sorry that you might expect, over time, and with fasted training is that you actually increase the amount glycogen store, and so there’s more glycogen there. Glycogen is associated with our muscle glycogen stores, are so associated with improved endurance and aerobic performance, you think that has to be a benefit, but actually some of the research out there and shown and some, I think a paper around 2008, showing that, even despite this higher glycogen store, actually, there is some reduction in the ability to oxidize glucose sometimes and after a period of fasted training. What athletes find difficult than are in the performance test is we don’t see differences in performance, maybe because they can’t access that glycogen store to the same extent, were no seeing that same level of carbohydrate oxidation. So, that’s one of the potential reasons why we don’t see maybe the difference in performance between fasted and fed training after a specific period in the research.


Chris Case  1:23:00

And for all those listeners out there that say we don’t get deep into the science, I hope those acronyms, those many genes Brian just listed off are satisfactory.


Dr. Brian Carson  1:23:13

Should be. I guess the three. So, for those who might not want to listen to all those gene names and that just went over their head, it probably could divide them into three biological functions associated with the ability to oxidize lipids, and the ability to oxidize carbohydrates, and then the ability to either scavenge or synthesize NAD, which is important in the electron transport chain, for production of ATP.


Trevor Connor  1:23:42

Basically, the short summary of what I am hearing from everything you’re saying is, you are seeing genes involved in all sides of aerobic metabolism from your ability to use fats, which is the primary aerobic fuel, all the way through the Krebs cycle, to you know, the end stage, the electron transport chain, you’re seeing genes upregulated at all stages. So, I mean, it’s really hitting all sides of your aerobic metabolism.


Dr. Brian Carson  1:24:13

Absolutely. Again, it’s probably not surprising in some ways. What might be a bit more surprising but is that the fasted is driving that adaptation to a greater extent. That’s the bit that’s probably surprising to more people. And again, exercise is going to do that all the time, so it’s important to remember in all this, we can sometimes get taken away with the nutrition, and the exercise is key here, and the exercise is the main driver of all this response. And, then I guess the nutrition is just trying to optimize it as best we can. Some of this is driven as well by the glycogen availability, and that’s where, you know, the model you talked about earlier in terms of that kind of training high and then sleep blow. For example, if you’ve low glycogen availability, that enhances the phosphorylation of AMPK, for example, which is the first signal transducer, and that’s where the signal starts, that then transduces at the level of PGC-1 alpha, its coactivator number of genes, and then we see these other outcomes. Whereas things like free fatty acid availability, which we also associate with fasting, would phosphorylate, things like ACC, and also p-38. And MAP kinase. So again, at the top of the chain, that initial signal is being augmented, and then what we’re seeing is that translated down through the levels in terms of transduction of that signal, and then the translation.


Chris Case  1:25:47

Yeah, so you mentioned Dr. Carson, a little bit about you know, some of your current research, what are the next questions that you want to look at here? What’s the future hold?


Future of Fasting Research

Dr. Brian Carson  1:25:59

So, I’m still interested in the manipulation of these nutrients around the exercise belt, and particularly in this pre-exercise period. So, one thing might be interested in is rather than necessarily fasted type exercise and exercise potentially without carbohydrate, but with other macronutrients evolved. So, one thing that we see in the literature is that, particularly endurance athletes, and this isn’t always associated with endurance athletes, but they actually have a higher requirement for protein, and then your normal person and particularly elite athletes, they have a higher requirement for protein even though their sport is insurance-based. So, one thing I’m considering is looking at a protein feed prior to exercise. So, a similar model where we come to the exercise session after an overnight fast, we don’t provide any carbohydrate perhaps, and we provide a small amount of protein because what we have seen is some of these mitochondrial genes that I mentioned earlier on, we’ve seen that these can actually be impacted by protein supplementation in in-vitro models initially, but also in vivo, because effectively the protein is going to provide the amino acids which are the building blocks for these particular proteins. So, once we increase transcription, and then translation into these proteins, they require these amino acids, so almost giving them the substrate to do that in advance of the exercise bed, and without having the blunting effect that carbohydrate seems to have on the expression of these genes. So, I think that’s a row that I would like to investigate a little bit more. There’s not a lot done in this particular area. There are some suggestions in a couple of reviews, that pre-protein feeding prior to insurance exercise might be beneficial. There is some evidence to say that, you know, some of the amino acids will be utilized during the actual exercise period. I would like to see how it impacts the adaptive response over time. That’s somewhere where I think I’ll probably go next with this piece.


Trevor Connor  1:28:16

You said that carbohydrates have a blunting effect on this gene expression. Is it the carbohydrates themselves that are directly having that effect? Or is it insulin? Because consuming protein will also raise insulin?


Dr. Brian Carson  1:28:32

Yeah, that’s a really good question. To be honest, so far, we don’t know. So, what we know is once carbohydrates been provided that we see this blunting, I guess, and the response or an augmented response when carbohydrate isn’t present, and whether that stands the insulin or not, we haven’t actually investigated in humans, it potentially has an impact, and you’re absolutely correct with the provision of protein, we’ll see postprandial insulin increase. So, that could have an effect on the utilization of fats as a fuel, as intramuscular triglycerides as a fuel, or as muscle glycogen as a fuel. So, we know less than this area, because we know less, I guess that’s an area that we can explore a little bit.


Trevor Connor  1:29:17

I do have one question for you that actually isn’t really related to fasting. This was something that I saw in your review that I found really interesting, which is you did talk a bit about, okay if you’re going to train in a fed state, some of the things that you saw that were beneficial and not beneficial, and one thing that really caught my attention was eating a low glycemic index meal versus a high glycemic index meal prior to exercise or an event. What was it that you saw there?


Eating a Low Glycemic Index Meal vs. High Glycemic Index Meal

Dr. Brian Carson  1:29:53

There were a couple of studies that they had different groups, or they have different conditions, where they had both low and high GI. It seems that the high GI condition would have resulted in a slightly different outcome, a less favorable outcome. So, the low glycemic index seems to have a more favorable outcome in those particular studies, for both performance and adaptation, and that would be what we’ve seen, but there’s very little research on that. I think that’s based on maybe two studies. So, there’s a little bit more scope for some research in that area.


Chris Case  1:30:36

Well, yes, I do have one final question for you, Dr. Carson, I realize this is putting you on the spot a bit, calls for some speculation. I know you’re a scientist first, not a cycling coach. I know there’s a lot of research still to be conducted when it comes to fasting, and exercise in a fasted state, but I’m sure there are people out there saying, okay, this sounds great, I mean, they’re potential performance gains, there are potential adaptations. How do I get the most out of this? So, if you can, could you design a protocol or a regime for our listeners, I realize there’s a lot of factors there, but how many times should people be doing this per week? What part of the season perhaps should they be trying? When should they avoid it? And you know, what does it look like on a particular day? How fasted should they be? How long should they remain fasted after the bout of exercise, and so forth? I know that’s a long question with a lot of facets, but hopefully, you can take it away and give us some recommendations there.


Fasting Protocol

Dr. Brian Carson  1:31:58

Thank you, there’s quite a bit in that all right. I’m glad you outlined that it’s allowed to be a speculative piece, as you said, I’m not a cycling coach, I’m a scientist. So, we always do well to link with the applied practitioners, and I think the more we can do that the better. I’ll do my best to maybe answer the question.


Periodization of the Season

Dr. Brian Carson  1:32:23

I’ll start with the periodization of the season in terms of training. I think if you were to ask me when in the season that it would be best for athletes to engage in this type of fasted training, I think probably in their offseason or in their preseason phase. So, for me, that would be the best time. When they’re right in the middle of a competition phase, I suggest that’s probably not the time for them to engage in this practice too much at that point, they want to be feeling for what’s required for the competition at all. times. So, I would say early in the season is the time to explore this type of training.


How Often Should You Fast?

Dr. Brian Carson  1:33:07

How often? I think there’s scope to do this and probably at least two times per week, but optimally three sessions per week. Those sessions could be varied in what they look like. So, if you planned a recovery ride, I think that’s the pretty optimal time to engage in this. So, when you don’t have a huge intention for the ride, if you’re not trying too hard and fast work, it’s maybe a longer in duration and lower intensity. I think that is somewhere which you could target in your overall program to introduce a fasted session. Another area which you could explore, I think is in some of the anaerobic type work. So, I mentioned a little bit about our own work and that is minimal enough research in this area, but and obviously a cyclist, you know, you’re mostly aerobically focused, but there are periods where, you know, your anaerobic metabolism is really important within the race. That’s a very obvious period at the finish, but there are periods where maybe tactical moves need to be covered or specific hills need to be attacked, and where this might be important. So, if you had planned maybe a short training session, which was focused on training the anaerobic system because the overall load of that session is not high, the overall energy demand and probably the time involved is not high, I think, based on our own research, and sprint interval training, I think that is a session that you could target within the week. So, I think two to three, maybe one recovery ride, one type of anaerobic session, and maybe one other type of session during the week. I think that will be the amount, and again, the timing, being more early in the season, just to try and drive that adaptive response and that endurance phenotype early in the phase, and then when you get closer to your competition, focus on fuel and for what’s required. I guess that will be my general recommendations. Something for you to take away from this.


Chris Case  1:35:12

Yeah. A question for maybe both of you guys, given your experience here Trevor, and your research Dr. Carson, so say somebody wants to try this, they decide, okay, the easiest way for me to do this is I’m gonna, you know, have a meal, have dinner, not eat anything, the rest of the night, sleep, wake up, no breakfast, go for a ride, sort of easy to incorporate that into the routine. So, if somebody has a two-hour ride planned the next morning I would assume that would be, you’d want to ride the entire ride, you could have your water but try to limit anything else for intake. What if you had a four-hour ride or a six-hour ride? Is it practical? Is it possible to get to that point? Is there a transition period where you need to work up to that where you’re able to, again just have water and nothing else for up to that length of ride? Or should people try to do such a thing right off the bat?


Dr. Brian Carson  1:36:33

Okay, so there’s a couple of parts to the question. One was maybe having dinner the night before, and what’s the period of fasting before the exercise session? What I recommend is that the period of fasting be kind of a minimum of 10 hours, closer to 12 If possible, so it looks like you know, maybe having your dinner pre-eight o’clock, and then exercise pre-8am the following morning, I think something like that will be suitable. If you’re talking about some of the earlier strategies around intermittent fasting, or alternate-day fasting, you know, the idea that you would do it maybe on the alternate day when you were fasting that would be highly appropriate anytime before you act within that modified period between 12 and two, so somewhere before midday effectively to conduct the exercise, but there’s a bit of work to be done there in terms of the duration of the fasts, and what time of the day exercise might come in. There’s a lot coming out around chronobiology, and the clock genes, and when we might optimally time exercise and nutrition, so there’s a little bit of work to be done there.


Dr. Brian Carson  1:37:42

The next part of your question was around, you know, during the exercise, so we don’t have breakfast, and we take off into the exercise bench, should we have anything during the exercise bed? If it’s two hours, then I think, I don’t think that that’s a requirement. One thing, again, this isn’t maybe empirically proven, but one thing that I will consider, and we did this and some research that I’ve conducted is we left a period of time after the exercise bout into recovery where we didn’t feed because there are studies which weren’t designed to look at fast versus feeding, but they fed in that post-exercise period, and they saw a little bit of blunting of the gene expression, some of the genes we mentioned earlier, and three hours as a result. So, just taking that evidence, I think if you can leave it an hour after the exercise bed, I think that is beneficial. But again, how convenient is that for you, you’re gonna have to factor into, and how do you feel? I also think what’s very important here, how does the athlete feel themselves? If they don’t feel comfortable then they should eat. You then asked about, you know, a longer ride, five to six hours. I think that would be difficult after an overnight fast. Maybe some of your listeners are already doing that, and have no issues with that at all, and if they are, they should be no problem for them. It may be somewhat individualized, I think their training status will have an impact on that, and the intensity of that ride over those six hours is definitely going to have an impact on that. I don’t think you’d be able to go into a post-exercise period, and not eat for another hour. So, I would say maybe in the later stages of that, and that you’re looking to have some type of fuel, whether that’s carbohydrate meal or not, and we’ll be offering debate as well. But again, the research today is not in place to support, and anything that I’m saying here is very much speculation.


Chris Case  1:39:46

Understood, understood. Thank you for taking that step. I know it’s hard for scientists to do that.


Trevor Connor’s Experience With Fasting

Trevor Connor  1:39:54

You asked both of us, I’m just going to give a bit of an experiential side because I did experiment with this a bit myself. When I saw that, that this idea of fasting was becoming pretty popular, I did my usual coach thing and said, well before I recommend this to any of my athletes, I’m going to try it. I actually did it for a while. So, I’ll tell you my experience, but my experience is pretty consistent with what I’ve heard from other coaches who have used it. As you said, there isn’t much in the research yet, but it’s the little bit that’s in the research that tends to match up with what I experienced. So, starting with the timing, I tended to do the time-restricted approach, or that alternate-day approach, where I just ate very little in a day. Definitely, you want to use that overnight fast, for me, I can’t sleep if I don’t eat soon before I go to bed. So, I tried the have dinner and then don’t eat for four hours before sleeping, and I just had insomnia. So, what worked for me was fast while you sleep, and then get up in the morning, don’t eat anything, and I wouldn’t eat until about two o’clock in the afternoon, and I was fine with that. My brother tried it, he was the exact opposite. He just liked to not eat at night but have a big meal when he woke up. I did just because why not, try doing some of my fasting days on Tuesdays because I had a big training race I love to go to on Tuesdays. So, it was about an hour-and-a-half, two-hour training race. I found very quickly, the first time I did that, where I went to it in a completely fasted state, I lasted about 20- minutes, and I was seeing stars but kept at it and there was some sort of adaptation. After a couple of weeks of that I was able to do most of the race, but what I was never able to do is when we got to that final 5-10 minutes, where the pace got lifted up, guys were attacking, they were getting ready for the sprint, I was out the back instantly, and was never able to find that really high-intensity. So, that’s consistent with the research that Dr. John Holley was doing, saying, yeah, you’ll lose that top end. The last thing I will point out as I did once, try the six-hour ride in a fasted state without eating, and it was absolutely miserable.


Dr. Brian Carson  1:42:28

I would imagine.


Trevor Connor  1:42:29



Dr. Brian Carson  1:42:29

It didn’t sound good to me, anyway.


Trevor Connor  1:42:31

No, what I really noticed, I was able to struggle through it, maybe there were some gains from it, but its impact on my recovery was so dramatic, I would never do it again.


Dr. Brian Carson  1:42:47

There’s also the fact that I mentioned earlier that after two hours of exercise, even in the fed state, we start to see both the fast and fed conditions migrate towards each other. Whereby, those exogenous fuels have been used up to a certain extent, and the fed state now starts to look like the fasted state. So, perhaps, you know, doing fasted exercise for six hours, it’s probably excessive, you know, you probably see four hours of your six looks like the fasted state even when you fed. So, there might be no additional benefit of doing that. It doesn’t seem sensible to me to probably recommend someone going to a six-hour ride fast.


Chris Case  1:43:31

Right, right.


Trevor Connor  1:43:32

I had the okay, I’m gonna fast, I’m gonna do my six-hour ride fast, and then I had a certain point where I said I can eat. When I got to that point, I wasn’t a scientist, I wasn’t a coach, I was a starving human being. I called up the pizza place that had the biggest pizzas I knew of and ordered a large pizza, wolfed that down, and then just kept eating and the rest of the day.


Trevor Connor  1:43:58

Payson McElveen, mountain biker and host to The Adventure Stash, shared his thoughts with us on fasted training and whether he feels there are benefits.


Chris Case  1:44:07

Have you ever dabbled in? Or are you a believer in intermittent fasting at all?


Payson McElveen: Thoughts on Intermittent Fasting

Payson McElveen  1:44:12

I had an interesting conversation on my podcast about this recently. For one, I mean, I think we all do because we sleep at night. Most people don’t eat while they’re sleeping. So, I think we all fast to an extent in that regard. Sometimes it’s a little funny to me that we glorify this idea of fasting when our first meal of the day is literally called breakfast. But the point being, I haven’t gone too aggressive with that. I haven’t done any full days, multiple days, anything like that. Not because I don’t necessarily think it doesn’t work, but just because I haven’t been intrigued by it enough to do that. That said, I do think it’s healthy psychologically as much as anything else to get in kind of a challenging place calorically now and then, or a fueling place now and then. A friend of mine, also Redbull athlete, Sasha Julian, who’s one of the top climbers in the world. We were chatting recently about how it’s kind of funny how so frequently we just micromanage our caloric intake, you know, we have breakfast, lunch, and dinner, on the bike or exercising, you know, start early, eat often every 30-to-45-minutes if you can, and we get so tied up in the minutiae of all this stuff that we forget what our bodies can do when things go sideways. So, in her sport, when powered await is even more critical than cycling, probably because she’s hauling it straight up a vertical face. She’ll take very little fluids, and very little food and be fine. She’ll go for a day and eat like 500 calories while exercising the whole time. Once she comes down, yeah, you’re not very comfortable when you’re super thirsty and super hungry, but it’s fine, and you got the climb done. So, obviously, from a performance standpoint, it doesn’t make sense to do that when you’re trying to race a bike fast. I think it’s probably good to be a little bit flexible and resilient, psychologically. So, in the offseason, sometimes, you know, you’re doing a really big high-country ride, and I don’t necessarily do it on purpose, but you run out of food and you run out of water with three hours to go, and you just get through and you get to eat a bigger meal when you get home. It’s not too big a deal. So, obviously, from a performance standpoint, I stay away from that, I haven’t been done much in the way of fat-adapted training or gotten into that can of worms.


Payson McElveen  1:47:07

Yeah, I know that what I said was a little tangent, but that’s kind of my only answer.


Chris Case  1:47:10

Yeah, I’m curious, do you ever get up, not eat breakfast, and go for rides or do workouts just for that experience? And if so, do you see benefits to that?


Payson McElveen  1:47:23

Well, like we were talking about earlier, yesterday, I did because I rolled out of bed 20- minutes before my gym workout, and there wasn’t time to eat, but not really intentionally. I was talking to, I know you’ve had Sepp Kuss on before, and he’s one of my good buddies, we kind of grew up racing, we’re on the same team for a couple years as juniors, and he’s sort of laughs about the whole fasted training thing to an extent. He told a story on my podcast one time where at one point his team was having him do a two egg ride, he called it, where basically, when he was quite fit from a, you know, energy economy standpoint, he ate two eggs for breakfast, nothing in the way of carbs, literally just like fried eggs, and then got on the bike and did a five-hour ride. Nothing gnarly in terms of intensity, just five hours aerobic. Obviously, he bonked like crazy, and you just push through, he said he’s not sure that sort of training works, because then you go home, just like, wow, food.


Chris Case  1:48:34

Fight, right.


Chris Case  1:48:38

Should people actually feel different, or are these adaptations taking place such that they’re happening sort of behind the scenes, and you’re gaining from them, but it’s not extremely apparent at the moment?


Dr. Brian Carson  1:48:52

No, it shouldn’t be terribly apparent at the moment. It may be the DRP, you’re kind of perceived exertion might be a little bit harder, but if it’s below a kind of a 70% VO2 maximum threshold, it shouldn’t be a huge impact, you shouldn’t feel terrible. I said that depends on how the athlete feels. If they don’t feel good, they probably shouldn’t do it. So, a lot of these changes, you know, what I’m talking about at the molecular level, we won’t know they’re going on. It’s a slightly different challenge is what I would say for the muscle, and so it shouldn’t be something overly dramatic. So, no, it shouldn’t feel terribly different. I wouldn’t think.


Chris Case  1:49:39

So, Dr. Carson, you’re new to the program. I know that you’re going to be able to do this well, you’ve been a great guest, very eloquent, given us a lot of great things to digest and think about here. Our take-home messages, one minute on the clock. What are the most important messages you think people should take from this episode on exercising while fasted?


Dr. Brian Carson Takeaway Message

Dr. Brian Carson  1:50:03

Okay, thank you. Thanks again for having me on. If I were to pull together a few take-home messages, I would say that fasted exercise is a potential tool for you to have in the toolbox for your overall training regimen. Some of the benefits associated with it are the ability to manipulate substrate utilization during exercise, whereby you move towards a greater contribution, and from liquid sources, and sparing of your glucose sources and you’re in your muscle glycogen for later in the exercise period. The other aspect of faster training, which I think is really important, and which the research definitely supports is the driving of an endurance phenotype in the adaptive response, we see increased muscle glycogen store, increased markers and mitochondrial content and function, and increase markers and enzymes that are associated with carbohydrate oxidation, and which are really important for exercise and overall performance. I think they’re the key take-home for me, in terms of practical application, and as I said, this is something which you should have in your toolbox, and which you might use a couple of times a week in your training, or predominantly use that in the offseason, early preseason, and during competition phases, focus on fueling for the exercise required, and I think they’re the key things that I would take home from our conversation today.


Chris Case  1:51:37

Excellent. Trevor, what do you got?


Trevor Connor Takeaway Message

Trevor Connor  1:51:41

I think the important thing to point out here is research on fasting, both for health and for performance is very new, and Dr. Carson, thank you for doing a lot of that research. But it is important to point out that while there are no indications that it has an impact on adaptations, that there seems to be an indicator that it helps with health. We’re still very early, there’s still a lot of questions, there’s still a lot that we don’t know. So, my one minute is, be careful. There was a reason I start at the beginning of this whole conversation with, you know, some of the ways these things can go bad, where I’ve talked to people on the nutrition side who have read in the research that, hey, there are health benefits to fasting for a day, and then there were a few studies that said, hey if you do two days in a row fasting, it seems to amplify the benefits. So, suddenly, people are doing a seven-day fast, because that’s gonna make it even better, and no now you’re starving yourself, and there’s gonna be a lot of negative effects. So, try it, experiment with it, there seem to be some benefits, but I’m really glad that you even said, do this in the offseason, be careful, we’re still learning, and if you overdo this, you can quickly take it the wrong way. Chris?


Chris Case Takeaway Message

Chris Case  1:53:12

Yeah, I mean that’s essentially just what I was going to say, this is one of those areas where people want to take what little we know right now and apply it, and that can sometimes be dangerous. I think keeping abreast of all the latest research in this area is going to be a really helpful thing for those that you know, are the experimenters out there, the people that like to experiment with these things, also the people that have actually seen benefits from it and have had personal experience from it. But yeah, the more we know in the future, the better we’ll be, all of us, at applying this to our training and hopefully honing those performance and adaptation gains.


Chris Case  1:54:06

Well, thank you again, Dr. Carson. It’s been a pleasure. We hope we can get you back on the program again in the future. This has been a great conversation.


Dr. Brian Carson  1:54:14

My pleasure, guys, I really enjoyed the conversation. Thank you very much for the invitation.


Chris Case  1:54:21

That was another episode of Fast Talk. As always, we love your feedback. Email us at, or send us a voicemail. Subscribe to Fast Talk wherever you prefer to find your favorite podcast. Be sure to leave us a rating and review. Your thoughts and opinions expressed on Fast Talk are those of the individual. For Dr. Brian Carson, Jim Miller, Petr Vakoc, Payson McElveen, Dr. Inigo San Millan, Dale Bredesen, Trevor Connor. I’m Chris Case. Thanks for listening.

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