To understand the importance of higher carbohydrate intake, read up on part 1 of our discussion here: High Carbs for Big Adventures
How much carbohydrate you need is an important question to address for exercise performance. No one needs almost 500kcal of sugar per hour in their daily diet, and there may be evidence that intentional fasted training may have some (limited) benefits to metabolism for male athletes (note there is research showing this is not the case for female athletes, a topic we cover in greater depth with Dr. Stacy Sims in our Coaching Female Athletes Craft of Coaching module).
Many athletes are most comfortable consuming at a safe and known upper limit of 200kcal/hour and determining with more accuracy than that seems unwittingly daunting. However, with a bit of math (and honesty) we can make dramatic improvements to your fueling strategy by asking just a few questions:
- How important is your performance?
- How many carbohydrates do you have stored?
- How quickly do you use those carbs?
- How long is your event?
Simply put, we know that if you are likely to burn more carbohydrate calories than you have stored over the duration of your event, then you need to eat some carbohydrate—and exactly how much you need to eat depends on the deficit you’re likely to incur.
The below table outlines a range of workloads and durations for example male and female cyclists. You can see that depending on the exact combinations there are many scenarios where an athlete does not need to supplement—they likely have enough stored in their system to get them through the event or workout.
For numbers unique to you, let’s go through how to calculate how much carbohydrate you need.
How important is your performance?
This is always the best question to start with because each subsequent question relies on this answer. If the goal for this session is to accumulate a duration of time at a base intensity zone, then fueling enough to avoid bonking is likely the strategy that you ought to employ. Modify that with the inclusion of a hard workout the next morning and you’ll likely be best eating more during today’s ride to avoid performance-robbing glycogen depletion when the workload matters.
Because most training is accumulating duration at a relatively low intensity, I’m going to proceed through this article based on the minimum calories needed to avoid running out of stored carbohydrate and the subsequent significant hit to performance. This is likely more than you are eating now. You may find that consuming more than the recommendation in this article helps you achieve peak performance, so be sure to experiment before your next event to individualize the program.
How to calculate your own carb needs
To determine how much carbohydrate to supplement per hour, we need to calculate 3 key factors:
- Carbohydrate burned
- Carbohydrate stored
- Duration of the event
Here’s the formula for the first factor:
Carbohydrate Burned = Total Caloric Expenditure × Carbohydrate-to-Fat Ratio
Total Caloric Expenditure
Our total caloric expenditure is directly related to the amount of work that we are doing. The math behind this calculation can be quite complicated, so I created an “efficiency factor” to convert workload to caloric expenditure.
Approximate Total Caloric Expenditure per Hour = Workload × Efficiency Conversion Factor
- An inefficient rider at 200 watts burns approximately 880kcal/hour (200w × 4.4kcal/watt/hr)
- An average rider at 200 watts burns approximately 800kcal/hour (200w × 4.0kcal/watt/hr)
- An efficient rider at 200 watts burns approximately 720kcal/hour (200w × 3.6kcal/watt/hr)
Carbohydrate to Fat Ratio
We can also make assumptions that riders of long events are going to be averaging a high-base effort level. Based on this we can assume that efficient riders will burn approximately 50% of their calories from carbohydrates, inefficient riders will burn 85% of their calories from carbohydrates, and an average rider ~70%.
- An inefficient rider at 200 watts burns approximately 750kcal of CHO per hour (880kcal/hr × 85% carbs)
- An efficient rider at 200 watts burns approximately 360kcal of CHO per hour (720kcal/hr × 50%)
- An efficient rider at 415 watts burns approximately 750kcal of CHO per hour (1500kcal/hr × 50%)
- An average rider at 200 watts burns approximately 560kcal of CHO per hour (800kcal/hr × 70%)
This is the second key factor we need, and we can calculate it with this formula:
Carbohydrate in Liver + Accessible Carbohydrate in Muscles = Stored Carbohydrate
Carbohydrate in Liver
Our body stores carbohydrates (in the form of glycogen) in two main areas: the liver and the muscle. What’s important about liver glycogen is that it can be transferred via the bloodstream to any working tissue. A general estimate is that males store ~450kcal of carbohydrate in their liver, while females store approximately 375kcal.
Accessible Carbohydrate in Muscles
Carbohydrate stored in muscle is locked in the cell and unable to travel throughout the body, which means that for most endurance sports we’re only concerned with the carbohydrate stored in our lower body. We are also operating under the assumption that athletes are beginning their ride with full stores, however depending on daily diet or recent exercise this may not be the case.
To determine how much carbohydrate we have accessible we need to determine an athlete’s lean mass (typically ~80% bodyweight for males and ~70% for females), and then determine how much is being used for exercise. For a cyclist this is approximately 55% of lean mass for males and ~60% of lean mass for females. Once we know much muscle we’re using, we can assume that the muscle has approximately 25kcal of carbohydrate per kg of muscle.
Lean Mass × Percent of Working Muscle × kcal per Gram of Muscle = Accessible Carbohydrate
Based on these figures you can see that the amount of stored carbohydrate varies with the size, lean mass, and working muscle mass of each individual athlete. Often this range is approximately 900kcals for an average female to 1,300kcals of stored carbohydrate for an average male.
Putting it all together
Now that you know your carbohydrate burned, carbohydrate stored, and the duration of the event, you can use this formula to calculate how much carbohydrate you need per hour:
(Carbohydrate Burned – Carbohydrate Stored) ÷ Event Duration = Carbohydrate Needed per Hour
Knowing these values means that we know an inefficient male rider can completely run out of stored carbohydrate in less than 2 hours when riding at a base intensity, because 2 hours at 750kcal of CHO per hour is more than the 1,300kcals they likely have stored. However, an efficient rider using only 360kcal of CHO per hour can go multiple hours before they use up their 1,300kcals of stored carbohydrate.
Most riders, however, will run out of stored carbohydrate after riding for 2 hours and 20 minutes.
Over the course of seven hours, our rider is going to amass a deficit of 2,700 CHO kcal. Replenishing evenly over the course of seven hours means a rate of 385kcal/hr to prevent running out of gas. Fortunately, this is within the 480kcal per hour (120g/hr) ceiling that is recently established. By upping carbohydrate intake, this athlete can ensure they’re adequately fueled for their event.
Applying it in the field
Everyone has unique needs for their daily diet, but most athletes benefit from looking deeper at their energy intake during training and racing. In some instances, increasing carbohydrate intake can have a drastic effect—allowing athletes to maintain effort and intensity deeper into an event, helping them feel fresher and more alert, and leading to better recovery.
Ultimately this exercise can help create a framework that gets athletes closer to their ideal carbohydrate intake. Some may have their best performances at rates that are less than or greater than those calculated. What’s important however, is that we begin the process of individualizing recommendations so that we’re able to find our own optimal intake.