Addressing Sweat and Electrolyte Loss with Dr. Robert Kenefick

We discuss sweat rates, sodium losses, general guidelines, and how everyone's sweat rates vary with Dr. Robert Kenefick.

When our bodies need to cool down, evaporative cooling is often the best method to beat the heat. While we lose fluids and electrolytes through urine production and breathing, it’s the sweat we create to fuel our evaporative cooling that is our biggest source of fluid and electrolyte loss.  

Some of us are efficient, sweating just enough to evaporate the heat away. Others will leave intricate patterns on our shirts and puddles on the ground; neither of which are overly helpful in cooling our bodies and more importantly, it pushes us closer to dehydration.  

To make it more complicated, not only do sweat rates differ from person to person, but your individual sweat rates can vary every day. Hydration levels, environmental conditions, acclimatization, clothing; all of these can alter both your volume of sweat and its composition, making it difficult to determine your fluid and electrolyte needs. 

On today’s episode, we are joined by Dr Robert Kenefick of Research and Development at Entrinsic Bioscience. Previously, as a researcher at the US Army Institute of Environmental Medicine, hydration and sweat rates were important topics for Dr. Kenefick, who is now considered one of the top experts in the country.  

He will take us through an understanding of the physiology behind sweat and why our bodies are willing to give up both fluid and electrolytes. We will also discuss our replacement needs and how to determine your own rehydration strategies. 

Joining Dr. Kenefick, we will talk with two exercise physiologists, Jared Berg, and Dr. Stephen Cheung. Jared Berg is a coach and exercise physiologist who used to work side by side with Rob Pickels at the Boulder Center for Sports Medicine. He will discuss his recommendations on what we need to replace as we sweat. We will also touch base with Dr. Stephen Cheung, one of the top experts in the world on exercising in environmental extremes, like hot weather.  

So, grab your towel and favorite sports drink, and let’s make you fast! 


Baker, L. B. (2017). Sweating Rate and Sweat Sodium Concentration in Athletes: A Review of Methodology and Intra/Interindividual Variability. Sports Medicine, 47(Suppl 1), 111–128. Retrieved from 

Barnes, K. A., Anderson, M. L., Stofan, J. R., Dalrymple, K. J., Reimel, A. J., Roberts, T. J., … Baker, L. B. (2019). Normative data for sweating rate, sweat sodium concentration, and sweat sodium loss in athletes: An update and analysis by sport. Journal of Sports Sciences, 37(20), 1–11. Retrieved from 

Bio-sensing textile based patch with integrated optical detection system for sweat monitoring. (n.d.). 

Buono, M. J., Ball, K. D., & Kolkhorst, F. W. (2007). Sodium ion concentration vs. sweat rate relationship in humans. Journal of Applied Physiology, 103(3), 990–994. Retrieved from 

Cheung, S. S., & McLellan, T. M. (1998). Heat acclimation, aerobic fitness, and hydration effects on tolerance during uncompensable heat stress. Journal of Applied Physiology, 84(5), 1731–1739. Retrieved from 

Duvillard, S. P. von, Braun, W. A., Markofski, M., Beneke, R., & Leithäuser, R. (2004). Fluids and hydration in prolonged endurance performance. Nutrition, 20(7–8), 651–656. Retrieved from 

Kenefick, R. W., & Cheuvront, S. N. (2012). Hydration for recreational sport and physical activity. Nutrition Reviews, 70(s2), S137–S142. Retrieved from 

SAWKA, M. N. (1992). Physiological consequences of hypohydration: exercise performance and thermoregulation. Medicine & Science in Sports & Exercise, 24(6), 657. Retrieved from 

Smith, J. W., Bello, M. L., & Price, F. G. (2021). A Case-Series Observation of Sweat Rate Variability in Endurance-Trained Athletes. Nutrients, 13(6), 1807. Retrieved from 

Episode Transcript

Rob Pickels  00:04

Hello and welcome to Fast Talk, your source for the science of endurance performance! I’m your host Rob Pickels here with Trevor Connor and Dr. Robert Kenefick.


Rob Pickels  00:13

When our bodies need to cool down, evaporative cooling is often our best method to beat the heat. While we lose fluids and electrolytes through urine production and breathing, it’s the sweat we create to fuel our evaporative cooling that is our biggest source of fluid and electrolyte loss.   Some of us are efficient, sweating just enough to evaporate the heat away. Others, like myself, leave intricate patterns on our shirts and puddles on the ground; neither of which are overly helpful in cooling our bodies and more importantly, it pushes us closer to dehydration.   To make it more complicated, not only do sweat rates differ from person to person, but your individual sweat rates can vary every day. Hydration levels, environmental conditions, acclimatization, clothing; all of these can alter both your volume of sweat and its composition, making it difficult to determine your fluid and electrolyte needs.   On today’s episode, we are joined by Dr Robert Kenefick of Research and Development at Entrinsic Bioscience. Previously, as a researcher at the US Army Institute of Environmental Medicine, hydration and sweat rates were important topics for Dr. Kenefick, who is now considered one of the top experts in the country. He will take us through an understanding of the physiology behind sweat and why our bodies are willing to give up both fluid and electrolytes. We will also discuss our replacement needs and how to determine your own rehydration strategies.  Joining Dr. Kenefick, we will talk with Dr. Stacy Simms and Dr. Stephen Cheung; leaders in the fields of Hydration and Environmental exercise physiology. We’ll also here from Coach Jared Berg and Pro-Tour Rider for EF Education- EasyPost Alex Howes .  So, grab your towel and favorite sports drink, and let’s make you fast!


Rob Pickels  02:03

From training peaks to whoop sifting through your data can feel like it requires a master’s degree. Good news, we have those plus over 30 years of coaching and data analysis experience. It’s crucial to understand how to test and how to change your training based on your data. You go res leave the analysis to us book your data analysis session now at fast Doc


Trevor Connor  02:27

Well, Dr. Kenefick, it’s a pleasure having you on the show. I know this is your first time being on fast talk. And we’re really excited to talk to you. As a matter of fact, Rob and I when we were planning for this episode, we’re trying to think of good guests. And he actually has a bit of a history with the lab that used to work at. Oh, tell me about that curious.


Rob Pickels  02:50

It’s interesting. We’ve almost crossed paths twice in my life. I grew up in New Hampshire. And if you grew up in New Hampshire running track, you spend a lot of time racing at the University of New Hampshire in their indoor track there because it’s one of the only available in the area. Had I probably continued you would have actually been a professor within the program at UNH during the time that I would have been going. But I developed such a hatred of that indoor Field House after running 1000 circles around it that UNH was the last place I was going to go to school. So that was number one. I eventually went to Ithaca College. And while I was there, one of the professors a lady named Betsy Keller had put me in touch with some people that US Areum that ultimately led to what was supposed to be a job or a research opportunity, my senior year of college. So I actually left college early Ithaca graduated me early so that I could take this sort of research assistantship or whatever it was to be at the time. And unfortunately, I went home, I moved all of my stuff, and I got a phone call. That was, Oh, we’re getting your security clearance. And then we found out we didn’t actually get funding for your position. So unfortunately, we don’t have a job for you. And so I was fortunate that Ethica would take me back and they actually hired me on as a research assistant because I had no school or anything to do at that point. So I drove back to Ithaca. And I spent the last semester of what should have been my senior year as a research assistant instead of an actual student. Wow. Okay.


Dr. Robert Kenefick  04:23

Well, let me first apologize for not getting positioned in Syria. That is unfortunate. But you know, Third time’s a charm. So here we are.


Rob Pickels  04:31

Here we are. We’re making it work today. Yeah. Well, I


Trevor Connor  04:34

will just say that Rob spoke very highly of you. So obviously, there were no issues on his end.


Dr. Robert Kenefick  04:40

But it’s very kind.


Trevor Connor  04:41

So today we’re talking with you about sweat and I know you’ve done some research on this. We have talked about heat adaptation, best ways to train and race in the heat. But we haven’t really dived into this whole question of how do we sweat How much do we sweat? How does it impact Back to your performance, how do you replace those sweat losses. So that’s really the focus of today’s episode. And just like every topic that we cover, this can get complicated. So I think today’s episode, we’re probably going to be a little broader and not quite as deep, but really get an understanding of sweat. Right. And I hope by the end of this episode, we can have some good suggestions on what you can do to make sure that you’re not losing too much fluid, or at least replacing that fluid as you train and race.


Dr. Robert Kenefick  05:30

Yeah, I know that sounds great. I mean, you mentioned that one thing that that is interesting, who would have thought something as simple as sweating, could get so complicated so quickly? And there are so many nuances and and I have to say, you know, I’ve been doing this for 30 years now. And I’m still learning new things about this. There’s so many aspects to it, and complications. So yeah, I mean, I’ll do my best today. But just know that this is always an evolving topic.


Rob Pickels  05:56

And what’s interesting about the topic for us today is it is about sweat rate, we’re going to cover hydration, but hydration itself is a totally separate topic. And I love the fact that we’re really going to discuss sweat in depth, because there is so many different nuances there.


Dr. Robert Kenefick  06:11

Yeah. So I mean, what’s interesting, too, and at least, you know, you mentioned in my time working for the US Army, that you Syrian US Army Research Institute of Environmental Medicine, and one of the things that we did there was build sweat equation, sweat prediction equations. And, you know, to just hear that and say, well, that’s interesting. Why would you want to know that? Why would the army want to know that? By being able to predict how much you can sweat, I can predict, or I can understand how much fluid you would need for a particular activity? And why is that important for the military? It’s because water is the second most difficult thing to to move around in training or in theater. And so you want to be able to understand, how much do we need to bring for particular activity for so many individuals, you know, what’s the environment, what’s the work intensity, what are they wearing, what might they be carrying, and to be able to have enough food but not have more fluid than you need, because now it’s more costly. So and the first thing is more difficult to move around, it’s actually fuel. So when I think of sweating, in particular, it’s hard for me to to divide the hydration piece from the sweat piece, because they are actually very intricately intertwined. But we can obviously, we’re just given the topic today just focus on the sweat piece. But I think it’s just important. And it seems intuitive, that this sweat piece is going to have that intimate relationship to how much fluid that you’re going to need for an activity.


Rob Pickels  07:34

It’s interesting that you have experience at the army. And when I say experience, it’s an understatement. You have a lot of experience, the research that has come out of that facility, on things like sweat, among other topics is really interesting, because, you know, we think a lot as athletes as endurance athletes, like our listeners are identifying as it’s a whole different situation. It’s a whole different level of seriousness, when we are talking about hydration and sweat needs of somebody who is in combat of somebody who is carrying hundreds of pounds of gear, and they’re not wearing a mesh singlet, you’re up against a much more difficult task in question.


Dr. Robert Kenefick  08:11

Yeah, I mean, it’s interesting to recognize that they are athletes in their own way. I mean, a lot of times, they even for some of these sweat prediction equations for the military, the exercise intensities they go to are as high as what you would see for athletes or cyclists or for runners or for other for other kind of athletes. But there are other issues that you said, I mean, you mentioned carrying loads that plays a big role, uniforms, body armor, wearing helmets, and then you know what kind of environments if you’re at altitude, there’s a whole other aspect there. And then other aspects of fall in play as well, because, you know, while individuals will have MRIs to eat, and these MRIs have an appreciable amount of sodium and other electrolytes replace what’s lost in sweat. A lot of these times in training or in theater, these men and women can’t eat, because they’re up tempo, they don’t have time to eat, they’re moving too fast, you know, or they’re not hungry, or they’re doing an activity, whether they’re in training, or they’re in combat. And so there’s these extra added pieces, when you’re looking at a military scenario that are a little bit different than for the athletes. And then you can, you know, actually the physiological stress, psychological stress of actually what they’re doing the simple fact that they may not have slept very well, that but they still have to carry on doing a task, they still have to maintain the high cognitive function for decision making. So a lot of ways it’s similar in a lot of ways, there are some differences.


Trevor Connor  09:35

Interesting. Since we have a lot to cover. Let’s start with basically a 101 of sweat and start with the simplest question, why do we sweat? That is


Dr. Robert Kenefick  09:47

a great question. There are actually two types of sweating. And the sweating that we’re talking about today is the sweat that is involved in thermo regulation. And so I’ll explain that in a little more depth, but there’s also the nervous sweat that You have and everybody has this experience, like, you know, if you’re on a job interview, that kind of sweat, you may not be thermo regulating in that situation, but you may be sweating. And that’s a different type of sweat. So you have Ecrins, sweat and apochromat sweat, the African sweat is more that nervous, sweating, tends to have kind of an odor to it. And so we really want to focus on accurate sweating that’s involved with thermal regulation. And so the basic idea is you want to figure this through through evolution. And, you know, aren’t too many other animals that actually do this, we’re leveraging the concept that in the biophysics of the the idea that when you take a fluid and put it on a surface, and it evaporates, in certain circumstances, it can carry heat away. So for the thermal regulation, portion of sweating, what we’re really looking at is heat that’s gained, or generated by somebody doing an activity, so somebody exercising, so obviously, people sweat when it’s cold out, so that heat is being generated from exercise, or you can get it from being in a hot bar, or both, right. So you can be generating heat as you exercise, and you can be in a hot environment. But either way, we have to maintain a temperature homeostasis. Obviously, you know, Fahrenheit, most people know 9897 598, Mark what people typically understand and Fahrenheit as the temperature setpoint, if we want to call it that, that we need to maintain homeostasis, and we can go up a little bit, we’ll go down a little bit. And then there’s actual physiological adjustments or adaptations that help us reset or establish homeostasis to bring that temperature back down to the setpoint. So if we look at the scenario, where we’re gaining Keats, and when we’re exercising, and so the byproducts of exercise involve co2, so you’re breathing out co2, and we all know that water, so metabolic water is released when you exercise, and that water goes right back into your body tissues or into circulation. And then he does the other aspects. And so while you’re exercising and generating this heat, that heat is causing heat gain, so you’re gaining heat, your core temperature is actually beginning to increase, and you can also be gaining heat from the environment. So your skin temperature will be rising. And these two inputs go into your brain to your hypothalamus and give signals that you need to thermoregulate to get our body core temperature back to that homeostatic setpoint 37 C 9798. So there are particular adjustments, physiological adjustments that happen, first thing that’s going to happen is you’re going to need to take the heat and bring it to a place where it can thermo regulate, or you can dump that heat. So when you’re looking at a scenario, let’s say you’re cycling, or you’re running, and you’re generating that heat from those muscles that are doing that activity, that heat has to go somewhere and one of the main places it’s going to go is into your your blood volume. So a portion of your blood, as we know is made up of water. Water is an incredible heat sink, that means that you can put a lot of heat into water and everybody knows is using your boiling water, you’re putting that heat into that water on the pan, causing that temperature to rise. And that’s what’s happening in your body, you’re taking that heat from your exercising muscles putting into the liquid portion of your blood. And now what we need to do is move that heat and bring it out to the periphery of your body. So you’ll notice most people become flushed when they exercise. And that’s because the blood vessels not at the periphery, beginning to open up. So we have what we call peripheral vasodilation. And lots of times you’ll see people especially in the upper extremity on their face, so the chest or the shoulders will become very red. That’s a reflection of this peripheral vasodilation. The next thing that’s going to happen is you’re going to sweat. So the sweat glands are going to be initiated and fluid and that fluids coming out of your bloodline again, is going to go on the surface of your skin, and the heat from your skin is going to cause the evaporation of that sweat. And when the sweat evaporates, it carries heat away from it. So that is how human beings summer regulate. And it seems like the simple answer is thermal regulation. But then when you explain it all out, you can see there’s a lot of complexities. And there’s a lot of adaptations, adjustments that can happen. And like with most things in physiology, you know, you’re doing one thing as an adaptation because you need to thermo regulate, but there are going to be some downstream consequences of regulation.


Rob Pickels  14:25

Yeah, these systems are incredibly complex, right, as you’re pointing out in here in the thermal regulation side of it is very important. You know what, we’re going to lose heat through a few different methods, right, convective radiation, depending on the environment, maybe some conductive to a degree depending on the situation. But the sweating that we’re talking about that evaporative heat loss, I mean, we’re talking orders of magnitude above like 10 times the amount of heat loss compared to some of these other methods that we can exchange that energy Correct.


Dr. Robert Kenefick  14:55

Yeah, and I’m glad you brought that up the the other mechanisms by which we can gain In order loose heaps, and you’ve mentioned some of them, you know, so radiation most of the time are thinking you’re gaining heat from the radio flow from the sun or the radiation you would get from standing next to like a space heater. But you can also radiate are sure you’ve stood next to an individual who’s radiating heat, you feel that radiation to a degree conduction is another mechanism that you’ve mentioned. So always a good example of that is like sitting on a bleacher seat and the fall of a football game, and you’re conducting heat from from your body into that cold seats, or you can gain heat on the same way. And and convection is the other way that most time heat is lost, we’re talking about a lot of times talking about airflow carrying heat away. But the other way we think of it is, you know, movement of some type of media. And so water is another great example of that. And that’s a different circumstance. But you can talk about swimmers as well, they’re going to be losing heat to a larger time up to 25 times more than being in the air just because of water in the flow of water carrying heat away. But you’re correct. Sweating is the predominant way that we are going to thermo regulate


Trevor Connor  16:03

something that you brought up that’s really important to point out is it’s not the sweat, it’s the evaporation of sweat, that causes you to lose the heat. And a lot of people don’t understand this, if you’re sitting in a gym somewhere and you’re sweating, and all that sweats ended up on the floor, it’s not doing anything for you.


Dr. Robert Kenefick  16:21

That’s 100% Correct. So, you know, we call that wasteful sweating. And you know, I can’t run any more. But I’ve taken up boxing and was in the gym this morning in a class and there’s just a puddle around me. And I know a lot of people have this experience from maybe doing hot yoga or being on a bike to a cycling class or running on a treadmill. And so while that sweat is some people look at it as a sign of like I am working hard, your body’s trying to thermo regulate, but it is wasteful, you are actually not summary getting so dripping sweat is not going to be advantageous to helping you lower your body temperature


Trevor Connor  16:56

that’s actually particularly relevant to swimmers because if you’re in a warm pool, you are sweating. A lot of swimmers don’t realize this. But if you’re working hard, you often are sweating, that sweating is immediately getting taken away from your body. So there can be issues with thermoregulation when you’re in a pool.


Dr. Robert Kenefick  17:12

Yeah, I mean, that’s another great example, I swam for a bit when I was in college. And there were certain times when the pool temperature was too high. So another great example, now you’re conducting heat from the water, right? You are getting hot, you really can’t sweat as well, because you can’t evaporate, but you are still sweating, right? And so you are losing fluid. And I can tell you after some of those workouts, you could actually smell the sweat, you know, when you have 20 or 30 guys in the pools doing a hard workout. And that’s another another good example. So you’re generating heat while you’re exercising, you could be gaining heat because the pool temperature is very high. And still having fluid loss though a lot of times it’s almost insensible. I can’t recall a lot of times when I was like, whoa, I’m really thirsty when I’m swimming, but you are losing fluid.


Rob Pickels  17:55

And you guys are touching on a really interesting topic that not many people think about. But it’s something that I’ve had to think about in my previous life, right working in advanced development for Pearl Azumi, the apparel company, in that if we’re talking about evaporating sweat from your skin is removing a lot of heat. But if that moisture is then absorbed into your clothing, and it’s being evaporated from the outer layer of clothing, instead of your skin, you’re losing a lot of the efficiency, you’re losing a lot of the heat loss that you otherwise would be getting, it’s kind of like that puddle on the ground isn’t cooling you off, you know, something that’s evaporating from layers away from your skin isn’t cooling you off nearly as well either. So material choice can really become a big deal. Depending on the environment that you’re in. I’ve definitely noticed that in hot humid environments, mesh where you’re able to have more airflow directly on your skin is significantly more efficient than something that’s maybe more of like a solid knit.


Dr. Robert Kenefick  18:52

That’s That’s an excellent point. And if you and you mentioned apparel companies and this is another huge area where you see a lot of intersection from human physiologists like myself, engineers, individuals who know a lot about biophysics, people who do modeling mathematical modeling. And so I just happened to be out at the American College of Sports Medicine conference last week doing a presentation and talking to some individuals who work for some larger payroll companies. And this is what they’re doing there. They are delving into the science, the technology of the different types of fibers fibers that might be able to spread this sweat more evenly understanding where the body actually sweats from doing sweat mapping and having an idea of where would individuals sweat more from how would we design clothing, just as you said to maximize evaporation from those particular areas, technology that should have been involved but hasn’t been given a lot of thought and that’s women’s apparel. So how do we design sports bras to be more efficient to allow for sweating so there’s this is a huge area obviously the apparel companies all around the world are working on this and they are increasingly getting more More sophisticated. And then the other piece that they’re bringing into is marrying that up to algorithms, predictive algorithms and wearable technology. So ideas because you mentioned you know what to wear. And when I was running before, one of the big problems that you would think about is, you know, I’m going to run, I think it’s going to be a moderate type of run, maybe I’m gonna bring water, maybe I won’t, but it’s like late fall, and maybe it’s in the winter, am I going to wear too much, because then I’m going to sweat a lot, I can get too hot, or I don’t want to be cold. And what kind of predictive algorithm could tell me what would be the best thing for me to wear when I’m out running, so I don’t generate too much heat or I’m not too encumbered and interfering with my ability to thermo regulate. So very complicated, obviously, a lot of commercial aspects to this, they’re very important. And for any of your listeners who are very interested in this, especially younger people who are looking at careers, a lot of careers, a lot of interesting intersections, like I said, engineers, mathematics, modeling, computer programming, it’s endless. And it’s it’s really fascinating.


Rob Pickels  21:01

Yeah, doctor, kind of Vic, you’re a man after my own heart by advice, saying all of that, that’s a world that I absolutely love and have lived in for many years. I do want to cap this off, though, and get back on to our, our main outline. But before I do that, if there is any listener who’s really interested in these topics, I am going to plug a Canadian researcher named George heaviness, who does some really, really great work in this area. Sweat mapping, has worked with some big companies out there. So there’s so much reading, there’s so much learning to do in that area. It’s definitely available.


Dr. Robert Kenefick  21:32

Actually, George is in the UK. He’s at Loughborough University. Oh,


Rob Pickels  21:35

he’s in the UK. He’s in the UK. Yeah,


Dr. Robert Kenefick  21:38

so I’ve been to George’s lab, he is an awesome guy. And his lab is probably the premier lab, one of the premier labs in the world doing work, just as you’ve mentioned, in this area, and also looking at a few other areas for thermal regulation that you wouldn’t think of things like vehicles, and there’s a lot of other types of application to and for public health as well.


Rob Pickels  21:58

I’m sorry, Canadians, I tried to get a win for


Trevor Connor  22:01

appreciate you trying to give Canada some credit, there was very, very good


Dr. Robert Kenefick  22:06

Canadian researchers, Glen Kenny, I can mention many, many of them. So they’re certainly well represented and feel. Appreciate it.


Trevor Connor  22:14

So let’s switch to the other side of sweat. So we’ve talked about the fluid part of sweat, but a lot of people focus on the fact that there is electrolyte loss in your sweat as well. So again, let’s start with the kind of the 101 explanation. What are you in? Aren’t you losing in sweat? So I’m going to use some big terms here? Is it isotonic? Is it hypotonic? Meaning, does it have the same concentration as your blood? Is it less than your blood? Is it more than the blood?


Dr. Robert Kenefick  22:42

Yeah, those are that’s an excellent question. And it gets into some ideas of there are actually different types of dehydration. And we can talk a little bit about that. But most of the time, people think that your sweat contains a lot of particularly people who are thinking sodium, sodium chloride, but when we’re thinking about all of the electrolytes that include magnesium, calcium, sodium chloride, those are being lost, because remember, we said, Where’s your sweat coming from, it’s coming out every blood vibes coming out of that liquid portion coming out of your plasma. So it represents what is in your plasma, but your sweat is hypotonic relative to your plasma. So that means the constituents that are in there, those electrolytes are in a less concentration than what is in in your actual plasma. So when we talk about sweating, we talk about the state that individuals get in through thermo regulatory sweating, and that there’s a reason I’m saying femoral negatory, sweating, because that alludes to a type of dehydration, right, you are becoming hypertonic, and hypovolemic. So your sweat is hypotonic. So when you’re losing fluid, and some electrolyte that actually makes your plasma more concentrated, and so your classes become more hypertonic. And you’re losing volume, that’s hypovolemia. So a lot of times people think and when I’ve had students, you know, they’ve they’ve said, when I taught at UNH, they were like, well, you know, I’ve tasted sweat on and whomever, and it tastes very salty to me. So do I need to replace that electrolyte? And that’s a it’s an a great question. As to what do I need to replace? When do I need to replace it? Under what circumstances do I need to replace it? And how do I how do I know what I need to replace? And those are a lot of areas that are, I think, becoming very big right now with a lot of different types of sensors and wearables. Other prediction algorithms that get into this for particularly for athletes, who really want to try to individualize what they’re doing their training, and have a better understanding of what their particular needs are to kind of tailor that for whatever they’re going for whatever their personal goals are. So we can delve into that if that scenario, what you’d like to go into.


Trevor Connor  24:51

Well, certainly one thing I have found interesting. So when I was studying this many years ago in my graduate program, it was Something that was a little revelatory to me that I’ve really applied to my athletes, which is, as you pointed out, as you’re sweating, the blood becomes more concentrated. So everybody is very concerned about replacing those electrolytes. And there’s certainly a lot of manufacturers that take advantage of this and create these drink mixes for when you write that are really high in sodium. But the fact of the matter is in your gut, that whatever you’re drinking is trying to it’s first going to get transferred into your blood volume, and the blood volume is already concentrated. So if you’re drinking a concentrated mix, you’re not going to actually get that concentration gradient that’s gonna allow everything to transfer from the gut to the blood, you in some ways can actually accentuate the problems. So when you’re exercising, and you’re sweating a lot, it’s more important to replace that fluid. And what I was taught in school, and I’m very interested in your response to this, is you should actually, at a certain point be drinking mostly straight water, or a not very concentrated mix, just to replace a little bit. It’s really after exercise that you should be focusing on replacing those electrolytes. But what are your thoughts?


Dr. Robert Kenefick  26:07

So? Yeah, I mean that a lot of times people ask questions and scientists and they and give you the answer at the pens.


Trevor Connor  26:14

Before we hear Dr. Kedah fix answer, let’s check in with another hydration expert, Dr. Stacey Sims in here her answer to this question of having too much sodium in the gut and what impact it has.


Dr Stacy Simms  26:27

Because sodium is in different compounds. So most people associate sodium with sodium chloride, and they think about, you know, table salt and that kind of stuff. If you take too much sodium chloride, the chloride ion disassociate in the intestine, and changes, the membrane potential allows intestinal cells open up, releasing endotoxins. And this can contribute to an increase in core temperature rise because it’s a toxin, and also contribute to GI distress and reduces the integrity of the contractile mechanism of the intestine. The other aspect of taking in too much sodium is with water their sodium. So if you’re ingesting too much sodium, then water is going to come to the sodium that’s in the digestive tract. So to find balance, and this is where a lot of triathletes, and to some extent, age group, cyclists go awry, when they start taking salt tablets, when they’re taking sodium chloride into taking high doses, sodium chloride, people are trying to take one to three grams of sodium to match quit losses. And they start doing that it’s way too much, the upper in that you should take per hour as a heavy sweater would probably be 1000 milligrams.


Dr. Robert Kenefick  27:38

So this is another depends. So I’ll try to run through a different a number of different types of scenarios here. So you’re correct. I mean, for most individuals who are not exercising, I would say, you know, maybe more than an hour a day or even a half an hour, an electrolyte replacement, beverage could also be some type of fuel during activity probably isn’t really necessary given to things for that short duration of exercise, even if it’s like a hit workout that’s very intense, where you may actually be working very hard and lose an appreciable amount of sweat, it’s not going to be that much in that short period of time. And you are going to have opportunities with food and fluid. And that’s where a lot of electrolytes kind of come from food intake to replace those electrolytes. So that’s really scenario driven, right. So you know, it’s not absolutely necessary. Some people would say, Well, you know, I’m maybe doing an activity, that’s, that’s short, but I want to increase the amount of carbohydrate I have available. Because I want to work more intensely, it’s going to be very intense workout, maybe running a 5k, maybe I’m an athlete who could run up to 10k, and, you know, less than 45 minutes to an hour. So I want to take him carbohydrate to do that, because I want to have the fuel. And in those circumstances, it’s very difficult to separate out most sports drinks because they contain electrolyte, you know, a fuel type of replacement and a beverage because you want hydration and then the electrolyte portion. So there is that other circumstance, one of the things that you mentioned that I think it’s also important to understand when you’re taking in various types of fluids is that the concentrations of what they are a very important, as you said, you can put in concentrations very, very high glucose can also contain high concentrations of electrolyte that actually cause fluid to be drawn from the blog blood volume into the intestinal lumen. And so that’s because somatic diarrhea, and that does happen sometimes. And so that’s something to be concerned about. The other issue too, is when you put taking beverages that are very high concentrations, typically of glucose actually delays gastric emptying, so the fluid is slower to leave your stomach and then get into your gut to be absorbed. Some would say that’s advantageous because it allows a more efficient uptake of glucose, but when you actually try to speed the fuel getting into circulation, that may not be something that you want. So when we look at other circuits stances were now things start to become more important. Now we’re getting into scenarios where exercises going for much longer durations, longer than our situations where people may not be able to take in food, I remember when I used to ride a bike it was it was nice to be able to have the opportunity to eat, which is more difficult than when you are running, obviously, when you’re sweating. But, you know, especially the other aspect is intensity for individuals who can exercise for relatively higher intensities for longer periods of time. So now you’re looking at greater sweat losses greater appreciable losses, and over time, and it’s going to take time, you are going to begin to start to lose some electrolyte. Now you’re really looking at scenarios where people who are are exercising for many, many hours. So you know, triathlon would be a great example, some adventure racing. So I’ve done some of these events with my partner who is an ultra runner and gone out, you know, run for 24 hours. And there are lots of events like this, where individuals may not be exercising for at high intensities, but they are exercising for very long durations. Now you’re starting to see scenarios where they are starting to accumulate electrolyte loss, and that’s a situation where you probably need to start thinking about replacing it. Again, circumstances that allow you to eat you can do that long distance, you know, trail runs, ultra runs, you’ll see most of those, those stops at stations will have salty foods, snacks like that, pretzels, peanuts, potato chips, and that helps. The other thing that the higher amounts of sodium in beverages do is they actually hope you hang on to it more. That’s another reason to have something that’s higher, typically, it’s higher than what’s in sport drink, and that sodium causes food retention. So you know, we hang on to it longer, so you don’t urinate it out, you don’t make urine from it. So that’s another advantage to it. Another another scenario that we’ve seen in military too is individuals who will be exercising for you know, days on end, and situations where people may not be acclimatized or acclimated. So when we talk about heat climatization, which you know, as the summer is coming in, are going to become naturally acclimatized to the heats. But before that happens, you’re losing more sodium and more electrolyte sweat. And as you become more acclimated, you lose less. So in situations where individuals for acclimatized are mentioned acclimation that second, that’s a situation where particularly in there’s some military scenarios where we think well, you know, it’s early spring, where the south starting to get hot, you know, these, these trainees are coming from, you know, northeast or from from northern climes, they’re definitely not acclimatized, their sweat is not going to have had that adaptation. So they’re gonna be sweating out a little more electrolytes. So that might be a scenario where we think, okay, maybe we want to supplement or make sure that these individuals are eating so that they’re getting the electrolyte in, if they’re not able to eat, and maybe we want to look for a beverage that has a little bit more sodium in it. And then as you mentioned, there’s a bunch of out there, I just want to come back a long way around things, just question but artificial adaptation to heat can happen at any time of the year. And so that’s acclamation. And this can be induced by doing some type of exercise in the hotter environments. And people have used microclimates, you know, dress, dresses, in heavier clothing, and exercise for a period of time, typically not at higher intensities, just enough to raise the core temperature for a period of time to induce these types of adaptations, like I’ve just mentioned, for sweating, or other adaptations as well, cardiovascular adaptations and adaptations to your blood volume. And so that’s a whole other topic that I think you’ve mentioned that, you know, it’s important for athletes, when they’re going into an event and who are thinking about, Well, boy, I may be coming from a situation where I’m training in the northeast, but I’m going to be competing in a hot environment. I know I’m not climatized to heat yet. And the circumstances of sweating, when I’m losing my sweating might play a role in my performance. And so maybe I want to start thinking about doing some type of acclimation type program. There’s lots of them out there. There’s a fair amount of research in this area to in order to get ready for that.


Trevor Connor  34:20

So I think you’ve got to lead to the the next place we want to talk about which is first going into some general recommendations, and you’ve just given a whole bunch and then I think we’re going to finish out talking about individual variants and individual needs. But are there general recommendations that you have for athletes in terms of fluid replacement based on sweat loss, and I’m thinking in particular, you recently just published a study where you were saying, to prevent dehydration, you generally need to replace about 37 to 54% of your sweat rate. But I found really interest is in this was in runners. You pointed out that under an hour of activity You might not need to replace anything at all.


Dr. Robert Kenefick  35:02

Yeah. And then that gets back into that situation I was describing earlier of, you know, it depends. So one of the ideas when we start talking about sweating, and that sweating relationship to fluid loss and dehydration, is its impact on performance. And if we have to draw a line in the sand, and there’s people who don’t agree with us, and that’s, and that’s fine, but we just have to draw a line in the sand. And within that sort of literature, when we talk about a 2%, loss, 2%, dehydration, and that’s relative to your body weight. So if you were to lose 2% of your body weight through sweat, sweating, so measure, weigh yourself and then exercise for an hour, where yourself again, if you haven’t taken anything, any fluid or food during that time, you can just do the math, and you can calculate what your sweat rate was in that hour, how much did you lose? You know, did I lose, you know, half a leader, or leader, and whatever that is, and so that that can give them an an idea to athletes to say, I understand what my sweat rate is, that sweat rate can change, I mentioned a climatization does it become acclimatized or acclimated, I actually will probably, I will sweat more of my sweat will become more hypotonic, I’ll serve more sodium, but I’ll lose more sweat, I’m more able off sweat earlier and activity, I’ll sweat more profusely. So my water, my fluid needs are going to be greater. So one of the things that I would say my colleague, Sam Shabbat, who writes quite often with me, one of the things that we would say, for individuals who are serious about performance, that they need to understand this idea of when they’re going to be approaching this 2% loss, because that’s where performance is going to start to become altered. And if you can maintain your fluid balance such that you’re not approaching 2%, then you should be okay, at least as far as performance goes. And you know, there, I’ve mentioned this before, there are downstream effects to becoming dehydrated to sweating. Now, as your blood volume becomes less, and you become hypovolemic, there are cardiovascular adaptations that happen as well. And some of those, like an increased heart rate can also play a role on performance, it also plays a role on perception ratings of perceived exertion, how hard do I feel I’m work. So all of these things should be taken into account activities, like you’ve mentioned, for an hour, you probably won’t approach 2% loss, because the duration just isn’t long enough. And for most individuals, it’s just not intense enough for you to generate that much heat when you lose 2%. In that period of time. It could be possible for very, very large individuals who are no, no, I don’t want to say this and say, Well, geez, you know, people who are blind men are playing football in the south. And you know, in August, wearing all that equipment, you know, those individuals might be able to do that, for the most part, or any any activities, it’s possible, you wouldn’t need to drink at all during an event of an hour or less, because you won’t approach 2% loss, or for the most part, when you are going to do activities, or you’re going to compete, and you are going to approach that 2% loss or you’ve calculated and figured out how much do I sweat, I believe it’s important for individuals to have a plan, how am I going to think about fluid into to attenuate my losses such that I don’t cross that 2% mark. So an example for myself in 2012, I was running quite a bit and I wanted one of my goals was to qualify for the Boston Marathon. And I sweat a lot. And I knew my sweat rate at the time. And I knew that for my circumstance, I had failed on a number of longer runs half marathons approaching marathon distances. Because I was becoming too dehydrated, I couldn’t drink enough to offset how much I was sweating, I was approaching that 2% going over that 2% Mine, not having enough fluid. And that affected my performance such that I couldn’t qualify. So what I needed to do is to determine circumstances under which I wouldn’t sweat so much. And that would have have to be a cooler environment. So I wanted to find a race in December. And in going to that race because it was so cool. I wouldn’t have to thermal radiate as much I wouldn’t lose as much fluid in still having a plan to drink in order to attenuate the fluid losses. So my performance would be less affected. So I would say that those circumstances when you’re looking at longer activities, now we’re talking endurance activities, could be cycling, could be running could be adventure, racing, any of these types of activities that are going on for those longer periods of time, you do need to start thinking about, Okay, how much food could I particularly lose in this event? If I know my sweat rate? How much do I really need to be thinking that I’m taking in those circumstances to it’s important to have fluid that is not hypotonic? So fluid that has an electrolyte and the other thing we haven’t They mentioned is drinking in too much fluid in the dangers of hyponatremia. So situations where people lose an appreciable amount of sodium, we see this a lot and longer, longer duration events, their training scenarios in the military. And then people drink that just water, there’s, so I would call it hypotonic fluid intake, and that actually dilutes your sodium in your plasma. And that can have serious detrimental effects to the nervous system. It can cause seizures, it can cause death. And so that’s very, very serious. So there’s another idea, you know, you need to understand if you’re in these events, and you are losing sodium, because it’s, you know, it’s going on for so long. Again, you need to be planning okay, what should I be drinking? Or what should I be eating so that I am not putting myself at risk for hyponatremia by drinking, just playing water and putting myself at risk?


Trevor Connor  40:52

Dr. canopic just explained the impact of losing fluid through sweat. But before we continue that conversation, let’s hear from another exercise physiologist Jared Berg in his thoughts and how much fluid and sodium we can afford to lose. So in your training or racing, should you be fueling or drinking to replace your sweat and electrolyte losses is that the goal that you should be seeking when you’re when you’re training on rice?


Jared Berg  41:17

Yeah, I totally think that’s, that’s that the safest, the healthiest and the best performance, maintaining potential is to make sure that you’re you’re you’re keeping your, your body well well hydrated to replace the fluid, you loss replace the electrolytes that you are losing. And there’s lots of lots of reasons that we can dive in for that. But, you know, simply simply put is, when we start to lose, you know, start to lose some of that fluid, we’re losing that our plasma volume. And so our basically, our blood volume is starting to decrease and our blood is where we really are going to, that’s where we transport that oxygen to to our body and that oxygen is so important in the aerobic metabolism and are basically our aerobic ability, our ability to to produce ATP or produce energy. And so if we’re actually reducing our aerobic capacity, if we are not replacing our our fluids, we’ll have less less blood moving around. And then the electrolytes are certainly important they are functioning in very fiber, a muscle contraction, you know, it’s it’s those electrolytes that are allowing the actual muscle fiber, the nerves to communicate with the actual muscle fibers that allow certain those actin myosin crossbenches to interact with each other, and form the actual mechanical action of a muscle contraction that week. So that’s really important to keep those electrolytes up there. And if we don’t, they can start to have some sort of muscle contracting misfires, which can really make it tried to produce power, uncomfortable or even debilitating.


Trevor Connor  43:01

So how plastic Do you think the body is meaning? How much fluid can you lose how much electrolytes particularly sodium can you lose, and still perform at your best?


Jared Berg  43:12

Oh gosh, let’s go go with the fluids first, that’s mostly what I understand the most is you know, and when you go into all the the peer reviewed journal articles, you’re gonna see that when you start to approach 3%, of fluid loss, you’re gonna start to see a significant decline and, and performance. Okay, and so you’re basically that’s representing a, a decline in in your fluid that’s impacting your boat, like your ability to transport oxygen throughout your body and you’re also not going to be as effective at cooling yourself as you start to get beyond that also, right? As far as electrolytes, I feel like we are there is a little bit more variability among among athletes, where some people can lose more more sodium than others and still not experienced the negative effects like like cramping you know, basically muscle misfiring where I think that’s a little bit more variable. But you know, as soon as we start to sort of upset that balance, our our body’s gonna go to compensate and unfortunately, it can compensate by you know, causing those cramps, resulting in cramps.


Trevor Connor  44:27

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Rob Pickels  45:02

Dr. Kenefick, I mean, just listening to you, you raise so many incredible points that it’s it’s hard to focus on one and ask a question. But I’m going to, I’m going to try to do this, you mentioned 2%, as being a threshold where performance begins to degrade. I know for myself, I love to do really big, long bike rides. And in all honesty, I probably can’t take enough water with me to hold to a 2% loss. So I know I’m going to lose some performance. But what I’m wondering is based on your research with the army or someone else’s, is there a line where it starts to become dangerous that you really don’t want to go past? Say, 5% 10%? Where do we need to be to stay safe?


Dr. Robert Kenefick  45:42

Yeah, that’s a great question, I just want to come back to the 2% thing real quick. 2% is sort of a rule of thumb, but you know, individuals would say, Well, you know, I can do 2%, my performance isn’t degraded, when I’m in a cooler environment, we’ve done some of that work. And that and that’s true. So a lot of it can be environment driven as well, you know, circumstances where you’re cooler, you can stand a little bit greater degree of dehydration, percent loss and kennen, hotter environments. So just want to put that caveat in, when we talk about, you know, there’s obviously this relationship is, you know, the, the greater the fluid loss, the greater the percent loss, the greater the decrement on performance, and the greater the degree of the cardiovascular adjustments that have to be brought into play. And, you know, you can also see that the changes in ratings of perceived exertion just makes the work feel harder. And you’re gauging that off your heart rate. And in other aspects, the idea of when does this become dangerous, as is an interesting one, you know, so research in and of itself, when I worked for the army, most institutions will have a limit, as far as what you can go to typically, we would go to about 4%, sometimes 5%, body weight loss after that, it wasn’t really considered ethical. But we do know from research and just some practice, individuals who, who practice weight loss or weight sports, combat sports, or regularly lose large amounts of fluid, and can sustain very large percentage changes over time. And, you know, sometimes, and you know, when I’ve done some work with some of those individuals, you know, who were working fighting with the UFC, you know, a lot of times when you talk to them, when they did these very large losses of you know, 568 10%, over now, that’s a circumstances over typically a number of days, but could be over a day, that they really felt that their performance was degraded, they’re allowed to try to get that fluid back, you know, before the event, but it’s very, very difficult to do so. So you mentioned, you know, your your aspect of it, I can’t take that much water with me, and how can you adjust for that, and you raise a great point. And that’s this idea of, you know, especially when you’re if you’re running, you can only take so much water for Sacred takes so much water circumstances where you may not be able to get water. So even if you’re on a bike ride and could stop depends on where you are. And so in those circumstances, you know, that you can only do the best you can do can take water as much as you can carry, you can start the plan out, how much do I need to drink, when can I drink it to try to stave off this level, but you’re correct, a lot of people are going to finish an event, but I just want to bring that point up, but you may try to attenuate getting to this 2% line, you’re not going to be able to do it. But you know, you probably get over that maybe two or 3%, maybe greater towards the latter half, or maybe towards the very end of the event. So it doesn’t really impact your performance for the most part, during all the other parts of the events, maybe just that last portion. But you know, you raise a difficult question for athletes is, you know, how can I replace all of that fluid? And is that a realistic expectation for longer rides? Who runs when water isn’t available? And you’re correct as those sorts of circumstances aren’t going to allow it. And you can only do the best you can do in trying to replace float during those events. But to the other question, you know, there are some classic studies that were done, particularly right around World War Two. Man in the desert is a book by Edward aid off was considered one of the fathers of thermo regulatory science and environmental physiology. And he did a lot of this work well, before they were IRBs did a lot of it out in the Mojave Desert. And then other studies that were done, done in Florida, where individuals were put into life rafts and looked at dehydration, but not given them through it. And people could last a number of days and achieve very, very large losses before it became dangerous. And so while it’s difficult to predict the actual change, or the percent loss, we typically have those circumstances mentioned plasma osmolality how concentrated does your blood get when we’re now we’re starting to look into circumstances where your plasma osmolality becomes so high that it begins to actually cause death. So, you know, typically in athletics, you may see circumstances where people will lose four or five 6%. There, okay, they’ll replace it difficult to say how much the performance has been changed about probably will be another aspect that I want to bring up too, is that there is of another loss that’s happening in your exercise in the slot. And that’s the loss of muscle glycogen. So you are actually breaking down glycogen to use, you know, breaking it down into glucose. And that actually changes your body weight. So my colleague, and I mentioned that for sure, have written a paper to compensate for that, to take it into a calculation when you can actually use predictive calculations to say, well, if I’m exercising for two hours, and I’m measuring my body weight, how much of that was actually fluid. And how much of that is actually the muscle glycogen that I that I combust the better use for the activity. There’s another important point another important point that’s been left off. Typically, in their early literature, that calculation wasn’t taken into account, where individuals may have exercised for very long periods of time, and they report body weight changes. But the reality is, if you had corrected, the body weight changes are still pretty big, but they aren’t quite as big. Given the fact that this correction wasn’t taking into account.


Rob Pickels  51:14

I love that you bring that up, because I’m nerdier than the average bear. And I may or may not have created my own spreadsheet that calculates out, you know, all of these concentrations needs and everything else for me as an individual. And I definitely have utilized your equations into my calculations. So I love that you include your honey needs, I did, I’m actually nerdier than the average poohbear For those who listen to the show regularly. Thanks, Trevor. But that sheet, I won’t go on that sheet, I want to bring up real quick because it really highlighted something for me. And that’s the impact that the length of an event can have. I know for myself, I’m a fairly heavy sweater, a little above average, and I have about average sort of sodium concentration. But that still means I sweat out a lot of sodium, right? Because I’m sweating out a lot of average sodium amount. What’s interesting for me is that if I go like one bottle and an hour of plain water, my sodium concentration will actually stay pretty good for hours and hours. But I end up in a situation where I surpass that two to 3% loss, really within about two hours, right. And so if I’m going to be writing for longer than that, and I want to stay within a good percent body loss, if I’m going to go for six hours, then I need to be at two bottles an hour, 40 ounces of water an hour. And that’s where I end up in a situation or where riders would end up in a situation that they would become hyponatremia after time. And you know, in those situations, perhaps that we need that extra electrolyte supplementation, and it’s very much the length of the event that is going to cause the shift in need, in my opinion, in my experience.


Dr. Robert Kenefick  52:59

Yeah, I mean, the basic idea of hyponatremia as your over drinking, you know, so you could you hear about there’s a radio contest a few a number of years ago now, I think they call the the radio contest is hold your we for a we have one of those little we devices, and then people were drinking copious amounts of water, and they’re actually diluting their their plasma sodium, just by sitting there drinking fluids. So the main idea is just drinking too much hypotonic fluids such as water, but now you’re introducing so the concept of electrolyte loss, in addition to drinking hypotonic fluid, and the simple fact that you’re losing more electrolytes in the more the longer you go, the more you lose. And then I would say in those circumstances, you know, sports drinks, the sodium content electrolyte content is, is going to be for most I don’t want to say all sports drinks, but for most commercial sports drinks, for those circumstances, the fuel is fine, the electrolytes probably going to be too low. There are other types of products that have higher electrolytes, some people would say, look to an oral rehydration solution. And that might be too high as are typically for another type of dehydration, where the dehydration is isotonic, you have large fluid losses and electrolyte losses. So they have very large electrolyte concentrations about 66 to two mil equivalent to sodium, and because that’s what being lost vomiting and diarrhea, like so you really do need to look for something that has a higher electrolyte content. And that can only say and, you know, my question for you is are you taking food with you because food, food is another great way to get electrolyte you know, various things you can take. And when I remembered I wrote he passed me to have a sandwich turkey sandwich in my jersey in the back, pull that out.


Trevor Connor  54:41

So far, we’ve talked about general fluid and sodium needs, but over the rest of this episode, we’re going to show just how individual it really is. However, before we dive into that conversation, let’s hear from Alex house and how he’s personalized his sodium intake.


Kiel Reijnen  54:57

So I’ve had my sweat analyzed a few times, and it varies so much. And I think a lot of people kind of take that for granted, they think, Oh, I’m heavy salt, or I sweat too much, or I don’t sweat enough or whatever, it changes the lot with the seasons with, you know, relatively minor heat adaptation. You know, we started to hold on to or so quite a bit better than you know that first couple of times we get shocked in the heat. And so being aware of how much that changes definitely changes how I look at, you know, what I need to drink, and how much I need to drink throughout the season.


Trevor Connor  55:33

So what in particular do you look for to change? And what do you change about what you drink?


Kiel Reijnen  55:38

So in the winter, I know that just because I have limited access to heat, that makes any sense. You know, I spent a lot of time writing outside in the cold, when I do do a hard session on the train or something like that, I know, I need to consume quite a bit more sodium with whatever fluid I’m taking in versus in the summer, I can dilute it a little bit, because I know that the amount of sodium coming out of my sweat is quite a bit less.


Rob Pickels  56:07

You know, I do think that listeners do need to be taking in all of those different sources. I think that what we’re struggling with right now is the generalized recommendations, right? Because we know that people are highly individual, how much they sweat as individual, how much sodium they need as individual, maybe even how much their tolerance is to being dehydrated as individual, I would love for us to dig into how do we actually take this to more of an individualized recommendation for people so that we can take it out of some general terms?


Trevor Connor  56:41

Yeah, and I before you go there, this was probably in the reading I did particularly one of your studies, was really surprised by was the extent to which there is individual variability. So for example, in one of yours studies, you mentioned that the individual variance in terms of their daily fluid needs was huge, anywhere from two liters per day up to eight liters per day. But also, we read this review by Dr. Lindsay Baker, that said that even within a particular individual, their day to day variability in terms of their sweat rate can vary 22%. And their sodium loss can vary 17%. So it seems like that makes it very hard to make any sort of recommendations.


Dr. Robert Kenefick  57:27

I mean, you’re 100% Right. And, you know, I can just give a shout out to my colleague, Lindsey Baker, she’s she’s an excellent scientist working on Gatorade, sports science to encourage people to read her work. Among others. You’re right. I mean, the the circumstances that we talk about what contributes to variability. And one thing that contributes to variability is actually the variability in what you’re doing a measurement with. And so that’s something that needs to be taken into account. So a lot of times you’re seeing now there’s a lot of different tools coming out to measure things like sweat, electrolytes, and, you know, prediction algorithms, they all have variability in them. And I would, I would just, I’m not going to endorse any products. But I would just tell listeners to do their research, do their own research, and to think about, you know, how well does something actually predict or how well does something actually measure something because there’s going to be variability in that measure. And then some things are maybe better than others, and some variability is just going to be inherent and can’t get around that. So that’s, that’s one part. The other part too is, you know, other contributors to variation for an individual that could be the time of day. Other aspects that can play a role is how hydrated you are, start, maybe I’m a little dehydrated effect can affect my sweat rate, you know, how acclimatized I am, or acclimated, just trying to think of other aspects, you know, the environment that I’m measuring something in my exercise intensity, you know, so if I’m, if I was going to predict for myself how much I might need to take in fluid for a 10k, I really need to understand the exercise intensity, the environment that I’m doing that because depending on the environment, and depending on how fast or slow I try to run this event, or cycle or whatever, I sweat rate is going to be different. And then how I’m going to use that information is going to be different. You mentioned another aspect is that is how much electrolyte do I use? climatization acclimation plays a role there are some individuals we talk about as being either heavy sweaters people who sweat a lot, or people who are salty sweaters, people who lose a lot of salt, or sodium and electrolytes in their sweat. And do they need to have more electrolyte because they lose more? And so those are other considerations that people should understand when they’re trying to especially serious people. You know, I like to think of this idea, and I think I’ve written a paper about it, you know, performance. When you enter a race, not everybody’s going to win, but the important aspect is, you know, if you are concerned about your performance, and you are serious about it, no matter if it’s like I’m going to run a three hour marathon or a six hour matter I find if that’s what my goal is, and I’m serious about it, then I really want to think about all the aspects that I can take into account and create an individual plan for me so that I can be successful and to achieve whatever goal that is. And that would be an understanding of how much am I sweating, and then maybe an understanding of what is in my sweat when I lose that. So I can have an idea of what I should try to be thinking about replacing during the activity.


Trevor Connor  1:00:25

So what I found really interesting, the Dr. Baker review, I mean, she just went through all the issues with the individual variability with the measurement, as you pointed out. And she really concluded her study by saying, you can’t really state here is one particular individual’s sweat, right, all you can do is classify them as low, moderate, or heavy sweater. But then in one of your papers, this made a ton of sense to me, you pointed out that if you just have athletes drink, add libido, them. So basically drink when they think they should drink, over 50% are gonna end up, dehydrate, and they’re gonna under hydrate. So it’s really important for them to understand their sweat rates, and make sure that they’re replacing at least 50% of the sweat losses. So when it’s so individual, that you can really only generally classify somebody as saying, well, you’re you’re a low sweater or a heavy sweater. But you need that information to make sure you’re properly replacing how, where’s the balance in this? How do you take something that’s so variable, and come up with good numbers for you as an athlete?


Dr. Robert Kenefick  1:01:34

Yeah, a good question that in a challenge, you know, I’ll touch base on, which I don’t think is personally think needs to be a controversial topic, but has become a controversial topic. And the idea of drinking to thirst is that, is that sufficient hydration to keep you at this 2%. And so there’s a number of papers out there pro and con, and I tried to take the middle ground there, where it goes back to saying, Well, you know, if we tried to draw that line in the sand, the same 2%, there may be events where you could drink thirst, you’d be fine. Because you’re not going to have an appreciable amount of sweat loss, you’re not going to get there. So if that’s of 45 minutes, or less, or an hour or less, but where I think it’s important, as you mentioned, you know, when it gets to be longer, and you start to get to these larger losses, you should start to be thinking about, how do I do that? And that begs the question of, well, there’s so much variability, how am I ever going to figure these things out? And so, so my recommendation, and I’ve given this recommendation to others. And this isn’t a one off, you know, one of my mentors, Dr. Jonathan would say, you know, Robert, you’re, your body’s your laboratory. And I still like to think that and I like to encourage athletes to think of themselves in the same way your body’s laboratory, you are going to have to be thinking this through and doing a few experiments, if you don’t think about a scientist that isn’t, you know, just one experiment is a series of experiments in trying to take these things into account, like I just mentioned, so let’s say you wanted to determine what your sweat rate was, you can do that by simple math. And there’s a number of papers out there, but it’s as simple as weighing yourself doing an activity for a period of time. Typically an hour is good, because we express sweat rates in meters per hour. And you know, you weigh yourself, you stay in metric, it’s no kilograms, converts to liters, which just makes life easier. If you run for an hour. If you take in any fluid, you subtract that out if your weights are if you eat anything, but if you don’t eat, if you want to run for half an hour, you can just do the math and calculate that just double it for an hour. And then actually calculate, okay, I just went for a run and I lost a liter. So my sweat rate is a liter for an hour, or it’s two liters an hour, or whatever it is. It’s 500 mils. So you do that. And maybe you did that once a week in a certain type of environment, and certainly intensity. But then you say, Well, look, I’m going to do a training run today, or a training cycle, it’s going to be at a particular exercise intensity, however, you’re quantifying that heart rate. Now you’ve got some device built into your bike, your pacing, watch, whatever it is, and you’re going to try to stay around race pace, and you do your sweat rate then. So that might give you a better idea of what you might expect that circumstance. Maybe you want to do that in a hot that, because now you want to have an idea of like, what’s it like when I run that race pace on a hot day? And how does that affect my sweat rate. So now I have a better idea of what I’m going to need to do on the day. And the other piece too, and it gets more complicated. And Lindsay’s work would point that out. You know, when we’re trying to get an idea of how much electrolyte is in your sweat, there are those devices that can do that. That will give you a kind of a ballpark. And you can use that for yourself to figure out if I know the concentration of let’s say, sodium that I’ve lost in my sweat and I can do the math of how much sweat that I’ve lost, I can figure out how much sodium I’ve lost and then maybe I want to try to replace that. And that’s going to be probably more of a ballpark but that’s probably okay. You’ll probably be okay for that. Because the circumstances under which you’re doing these experiments, we would call them a field experiments. There’s a lot of variability. You’re really trying to get ballparks versus What we can do in a lab, which can be much more precise, we have indwelling catheters, and people are taking blood and we’re actually met taking their sweat and putting into electrolyte analyzers. And you know, we have a very good idea of what the exercise intensities are. And that’s a different circumstance, people can certainly go to those lengths to try to figure that out, if they can have the availability or there’s a lab, though, to that work, or if they want to be a subject in a research experiment, and learn all this. But the other way to do it is to actually look at all the circumstances that I’ve just mentioned. And, again, your body’s your laboratory, to do those experiments and start to try to figure these things out for yourself. That’s, that’s the best way to to address this, this concept of variability. And you know, the other idea too, is that’s going to change when your fitness changes, that’s going to change with the seasons. If that can change, as I said, with environment. So, you know, if you’re if somebody now it sounds like Rob’s pretty much into this, you probably have already done this, start keeping spreadsheets, start recording some this information for yourself, I don’t think I’d be too crazy about it, you can start to get some ideas about what you need, when you need it, and start to use that information to start to make plans for yourself. Does that make sense?


Rob Pickels  1:06:11

It does. And if I can kind of summarize this, for some listeners, I think that the take home message here is don’t worry about suffering from analysis paralysis, right? Just just start recording some data, because that’s going to be the most important thing. You can have one laboratory session, I would question the validity of that I would almost rather have 100 field sessions that may be a little bit less accurate, right? Because it’s the volume of the data across all of those different conditions, different environments, different intensities. And if you are going to keep a journal for yourself, you ought to be recording that pre and post Wait, record what you’re consuming during the event. also record the conditions. Was it Sunny? Was it 70 degrees? Was it humid? How hard were you going maybe even what you’re wearing, but you can begin to create this notebook. And ultimately, what you’re looking for out of that is trends, right? Because we’re never going to consume exactly 1.1 liters per hour. It just doesn’t work that way. And so I think that these ballparks that you’re talking about, that’s a really important concept for people to remember. Even though we’re talking about the variability, that doesn’t mean it’s discounting or discrediting the data being collected.


Dr. Robert Kenefick  1:07:24

No, that’s that’s an excellent point. And I would agree with everything you’ve just


Trevor Connor  1:07:28

tested to determine or sweat and sodium losses can be tricky. Let’s hear from Dr. Stephen Chung, a top expert on hot and cold weather extremes vote whether there’s a value to testing,


Dr. Stephen Cheung  1:07:40

I think there is room for it. And if you use it properly, I think it’s like any test if you just use it to get a G wiz number, then nobody is has benefited. But it can be useful for individuals to get an idea of the electrolyte content that they should be using. And because while most of us get the sufficient amount of electrolytes just through a daily diet, if you are exercising, for long period of time, in very hot conditions, you are going to be sweating a lot. And if you are a very salty sweater, so if your sweat has a lot of electrolytes in it, there does come a point where you can be critically limited and you can get into trouble from losing too much electrolyte at once. So it is worth it, especially if you think you are one of those people and to get your electrolyte tested in your sweat. And then see if you do need to take in more sodium than than normal. So yeah, I think it is useful for a limited range of people. I think for many of us, we’re probably not in that extreme salty sweater category. But I think there is a significant percentage who are and those be the people that would really benefit to confirm that yes, I do need more salt and to supplement with more salt in my drinks as I’m exercising.


Trevor Connor  1:09:13

The thing I found really surprising getting ready for this episode, which you’re hinting at here was in Dr. Baker’s review, which was really a review on the different methods of measuring sweat loss and electrolyte loss. And she went through all the different methods that are used in the lab. So these are pretty sophisticated methods. And it was just page after page of here all the issues like when you collect the sample the sample can dehydrate. So how do you store it? How long can you store it? And it would just page after page of these are the issues that actually make it really difficult even in the lab that measure this and then ultimately at one point in the review, she says really the most effective method is surprisingly taking your weight your naked weight before and after. Just be careful As you said, to account for food or water that you drink, or if you go to the bathroom, and you should really do it naked, but otherwise, that that action really is your most effective method.


Dr. Robert Kenefick  1:10:10

Yeah, I mean, given the circumstances most people are in and the availability of what they have at hand. That is, that is the best way of doing. And when we do, all of our research studies have been used, when we’re looking at calculating our levels of dehydration, salt, it’s all uses body weight. And so most people, they may not have scales quite to the level of accuracy. But you don’t need to have that, you know, simple, simple scale. Now, if you have a, most athletes are probably gonna have a dumbbell around. So you know, just throw that on there before, you know it’s five pound dumbbell, throw it on there, it always measures five punch, and a reasonable idea that your scales doing, okay. And then you can make those measurements. And yeah, and you know, the other piece too. As we said before, you know, you can only do the best that you can do. And I agree, you’re trying to collect some information on yourself, you learn, you learn quite a bit about yourself. And the reason I like physiology is I like I like to think about adaptations. The simple fact that we adapt to training with that environment, and we’ve adapted theologies as well. So you know, we’re always trying to maintain some kind of homeostasis. And adaptation is a big part of that. And what is exciting, it’s always been excited for me, when you do that for yourself, when you start tracking things over days, and then a few weeks or even months, you can start to see these adaptations happening for yourself. And it’s pretty exciting to see when you’re looking at data, particularly when it’s your own data, and seeing, you know, these physiological concepts that you’ve read about, maybe you’ve heard about in a podcast, or talked about with friends or in a magazine, and then actually see it happen for you. And you can explain it. And I’ve always found, for myself personally, that it’s so cool to be able to see that. And when I’ve been able to have work with people and point that out something they’ve found exciting too, because the when you’re talking about these ideas, climatization, or these other types of adaptations, like heart rate, or even for sweating, they’re very, very abstract. But when you are actually experience it for yourself, seeing it happen, truly exciting, and then you kind of you own it, right, you own it. And now you have information, you’re armed with information that you can use to help train yourself and to help prepare yourself again, you know, and you don’t have to be preparing to run a three hour marathon. You know, it’s, it’s whatever performance goals that you set, however you want to put yourself in the best situation to achieve those goals, you should try to do that. And take advantage of some very simple things that you can do like like weighing yourself,


Rob Pickels  1:12:34

I just want to point out that podcasts are the only place that you can get information. You should discount everything read in magazines or websites, only listen to podcasts only get your information right here on fast talk labs.


Trevor Connor  1:12:46

With that be nice. So I just really have one last question for you, which is there are companies that are popping up that are claiming they can take a sample from you and determine your optimal drink mix. So whether you should have more electrolyte, less electrolyte? Considering all these discussions we’ve had about measurement issues and variability, are we there yet? Can we legitimately do that?


Dr. Robert Kenefick  1:13:13

It’s a good question. You know, I don’t want to, again, I don’t want to disparage any these devices, there’s many, many of them out there lots of different services, I can’t keep track of them all. And I don’t try to and I’m sure some are better than others, some, some have better science, some are doing a better job at with their analyzers, or how they’re analyzing things and what information you’re getting. I guess the only thing I can say is, you know, we kind of alluded to before, when you’re doing this, it’s a snapshot of that circumstance of what you’re doing at that time, and everything that’s going on in you at that moment. It’s a snapshot in time. And so you’re getting information about that snapshot, in how well that snapshot represents the totality of what you’re going to be doing. Am could be a question. And so, you know, as we said, before, you probably if you really want to have a better idea, you probably need to get more snapshots to try to totally represent the totality of what’s going on. But you know, if it’s if you want to have that snapshot, and that’s valuable to you, and that’s kind of almost you see, you’re saying like, Hey, I would like to serve aside, I’ve done some research, I understand what they’re doing. I’ve asked good questions about it before I plunked my, my dollar bills down on the table, or my credit card into the website. I want to take a shot at this and I want to get a benchmark by this and give some ideas and maybe I just want a benchmark for which I can then say, alright, this is objective benchmark. And now I’m gonna need to do it myself or I’m going to do some training and then maybe at some other part point, I’ll get another benchmark and see how that works. That’s that could be another way of looking at these. I would say yes, I mean, the technology, all technologies have limitations. Are they going to get better? Undoubtedly, you know, that the interest in this the wearable market, that technologies to the things that I’ve seen In the simple fact that many, many governments the Army’s does this thing is it’s only going to get better. So I wouldn’t shy away from it. I just would urge caution to understand exactly what’s being done, how it’s being done, and understand again, just a snapshot. And you know, when I would just look at the circumstances under which you’re doing it and be thinking about how applicable it’s going to be to every circumstance you’re going to be.


Rob Pickels  1:15:27

For many of us in North America, the road racing season is winding down, you can test your end of Season fitness with fast talk labs, just scheduling inside advanced tests with us, your inside test results will reveal your VO two Max up to date training zones, anaerobic threshold, carb Max fat Max VLA Max, then it will suggest a path forward for better training and fitness. Learn more at fast talk Trevor, Dr. Kenefick? You know, again, so many great topics in here. For me, I’m always looking for the simple and the actionable. If you guys we’re gonna break down and Trevor, we can start with you, Dr. Kenefick, will give you the last word, if you can break this down. What are the take homes? What are the takeaways from today’s episode?


Trevor Connor  1:16:17

That’s a tough one for me, because I think one of the messages here is we have only skimmed the surface, I’m looking at all my notes and every topic area that we got to, we probably only covered about a quarter of what I had written down. So there is a lot to this, there’s a lot of really interesting research, a lot of really good useful information on hydration and electrolyte loss, and sweat loss that we haven’t even touched on yet. I know that’s not much of a take home. So I guess my take home is probably still in the one that everybody else is going to want to use. So I’ll just start it, which is we’re all individuals, we vary between us. So I’m going to be different from Rob, who’s going to be different from Dr. Kenefick. But we also vary day to day, and whether your heat acclimated or not, and a whole bunch of other factors. So I think unfortunately, there’s no miracle one answer to this. It’s exactly what you guys have been saying, which is get to know yourself. Keep those records and start finding what works for you. And with that, Rob, take it from there.


Rob Pickels  1:17:24

Yeah, Trevor, my take home is that I hope that listeners heard a few things today that made them say, Hmm, that’s really interesting. I want to know more about this. And the thing I love about this sweat rate topic is that you can know more about it. Unlike testing your VO to max or your economy or your even blood lactate. All of those require kind of some pretty special equipment. This is data that you can collect on yourself, at least in terms of the volume side of things, maybe not the sodium or the concentration side. But this is a large volume of data that you can collect yourself. And I encourage everyone to start that journal, you can just start a little spreadsheet on your phone if you want. And then you always have that with you. But gather that data, it’s going to give you some really incredible insights that will help you understand both yourself and the concepts that we’re talking about. Dr. Kenefick,


Dr. Robert Kenefick  1:18:16

I can only echo that and Harper center gradient can just go back to what my mentor said here, your body’s your laboratory, it’s no, it’s it’s important to know some of these things about yourself, especially when you have goals. And that’s one of the things I just really want to echo. You know, a lot of people have very different goals. And, you know, it’s it’s important to try to achieve goals because they mean so much to people, whatever they are, you know, if you’re working with athletes, or working with other people to try to understand what those are, and then try to figure out okay, what’s, what’s the what, how can we leverage everything, all of our knowledge to be able to help people achieve what they want to achieve, and whatever that’s going to be. And so this is one way to do that, you know, understanding some of these aspects. And again, you don’t have to get crazy, but you can, you can maybe do this once a week or once a month to start to build that database start to build get some benchmarks and baselines to start to understand some things do a little reading. Honestly, I know, as we said, the podcast is one of the easier ways to get information. But there are other ways lots of excellent resources. And then for the diehards, you know that you can delve right into the scientific literature, you know, hundreds and hundreds of papers. You know, PubMed is a great way to find a lot of these resources. And so I would encourage people to do that. And the last thing I can say You’re 100%, right. I taught at the University of New Hampshire for 12 years. You know, everything we talked about just one of these topics would be a number of lectures throughout the semester. And you know, Exercise Science programs are four years long so there’s a lot to talk about a lot of depth a lot of information and so you’re right we’ve we’ve only just really scratched the surface on this topic,


Trevor Connor  1:19:56

the Dr. Cohen effect, hopefully that means we can get you back and take one particular area and dive a little deeper, but it was an absolute pleasure talking with you today.


Dr. Robert Kenefick  1:20:05

Yes was when I enjoyed it very much. I’m happy to come back whenever you like. So if you have please feel free.


Rob Pickels  1:20:11

Terrific. Thank you to both.


Rob Pickels  1:20:13

That was another episode of Fast Talk. Subscribe to Fast Talk wherever you prefer to find your favorite podcasts. Be sure to leave us a rating and a review. The thoughts and opinions expressed on Fast Talk are those of the individual.   As always, we love your feedback. Join the conversation at to discuss each and every episode. Become a member of Fast Talk Laboratories at and become a part of our education and coaching community.   For Dr Robert Kenefick, Dr. Stacy Simms, Dr. Stephen Cheung, Jared Berg, Alex Howes, and Trevor Connor, I’m Rob Pickels. Thanks for listening.