Fat or carbs, what is the superior fuel for endurance athletes?
Like the battle between the Sith and the Jedi, high carb low fat and high fat low carb athletes have engaged in epic battles over the subject for years now with no clear answer as to who is the Jedi (good guy) and who is the Sith (bad guy). In this blog, I tackle this age old debate in an effort to give you some actionable items that you can apply to your own training and nutrition, and explain why teaching your body to use both fats and carbs, i.e. be metabolically flexible, is important for endurance athletes.
There is a very interesting case study which I think demonstrates why an endurance athlete that is metabolically flexible will perform better than someone who is reliant on just one fuel source. In the case study the researchers attached a continuous glucose monitor (CGM) to two ultra-endurance athletes on the day of a 100 KM race. Runner A was an elite ultra runner having participated in the 100 KM World Championship, while Runner B was a recreational ulta runner with only a couple 100 KM finishes. Runner A also had a higher VO2 Max than Runner B, indicating they had a better aerobic fitness than Runner B.
Runner A finished the 100 KM race in 6 hours 51minutes while Runner B finished in 8 hours 56 minutes. Surely part of the reason why Runner A completed the race faster than Runner B had to do with their experience and training. However, we also know for sure that Runner A did NOT finish faster because they consumed more carbohydrates.
During the course of the race Runner A took in 1125 kcals and 249 g of carbs, while Runner B took in 1599 kcal and 366 g of carbs. Unfortunately, we don’t know the amount of protein and fat the runners consumed. We do know, via some simple math, that Runner A consumed 129 calories worth of fat and protein and Runner B consumed 135 calories worth of fat and protein. At most runner A consumed 14-15g of fat and Runner B consumed 15g of fat. Could the fat and protein calories make a difference? Yes. Do I personally think it had a huge impact on the performance of either athlete during the course of the race. Probably not.
If we take a look at the CGM, velocity, and caloric intake data from the day of the race we can see a very important factor that played into Runner B’s performance.
The top graph in the figure above is Runner A’s data and the bottom graph is Runner B’s data.
The solid line in the graphs above are charting the athlete’s glucose as measured by the CGM. The dotted line is measuring the athletes velocity. The bar graph is measuring their energy intake (during the race only).
The first takeaway after looking at the data is the difference in fasting blood glucose. Runner A’s fasting blood glucose appears to be in the 90’s, not bad, ideally we would like it to be in the 80’s, but the 90’s isn’t terrible. Runner B’s fasting glucose appears to be somewhere between 100 and 110, that is at a pre-diabetic level, not so good.
The second stark difference between the two runners CGM data is their response to what I presume is their breakfast. It might look like Runner A has a pretty terrible glucose response to their breakfast judging by the intense spike in glucose. In fact this is normal, and exactly the response we want to see to what is presumably a fairly high carbohydrate meal. That spike that we see at around 2:41 and then immediate drop afterwards back to fasting levels shows Runner A is very insulin sensitive (this is a guess on my part as we don’t have the data to back it up, however the ability for you to return to baseline glucose levels after consuming a large bolus of carbohydrates can be a good proxy for insulin sensitivity). The spike we see at the 2:31 mark in Runner B’s data is smaller than Runner A but only because Runner B’s fasting glucose is starting out high to begin with. In addition Runner B’s glucose never returns back to their fasting level and continues to rise up until the start of the race. All of this points to Runner B is more insulin resistant than Runner A.
Finally let’s look at Runner B’s blood glucose and speed towards the end of the race. You can see their speed drops after the 9:36 mark. Runner A’s speed also declines around the same time. This is likely due to fatigue.
However something dramatic happens to Runner B at the 12 hour mark, their blood glucose plumbits…they “bonked”. Most athletes think that they bonk because they did not consume enough carbs during the race. However we know that Runner B took in more carbs than Runner A and Runner A did not bonk, despite running at a higher intensity throughout the race.
How did Runner A perform at such a high level without consuming nearly as many carbs? There could be a number of reasons, from training history, running economy, lifestyle factors leading up to the race, etc. If we assume for the most part everything was equal and just use the data presented in the case study, there seems to be sufficient evidence that Runner A is much more metabolically flexible than Runner B. Runner A is able to get away with consuming less carbs and run at a greater intensity than Runner B because they can use both fat and carbs throughout the race.
The first step in becoming more metabolically flexible is determining how insulin sensitive you are and how well you can utilize fat.
This test just requires you to not eat, AKA fast.
Pick a day and once you finish your last bite of food at night, see how long you can go before you need to eat again (it is fine to consume water). Can you make it 12 hours, 14 hours, 18 hours, 24 hours? If you can make it 12 hours you are doing pretty good, but you could do better. Hitting the 18-24 hour mark would be ideal. If at 10 hours you are ready to eat your own hand, can’t focus, and are dreaming of a pile of pancakes, then you have some work to do.
This test is a bit more fun…it is the opposite of the fat adapted test, it involves eating your favorite carbohydrate 😉. Basically you eat a decent amount of carbs and then observe how you feel afterwards. For example, you could eat something like a potato, a bowl of rice, a spotted (ripe) banana, or any other carb rich whole food item. After eating your carbs (YUM 😋) observe how you feel. Do you want to take a nap? Does your energy crash? Are you wanting to dive face first into a chocolate cake? Are you having trouble focusing on your work? These would all be signs that you might not handle carbohydrates very well.
A more “nerdy” way of performing the same test is to do something like Robb Wolf’s 7 day carb test. You will need a glucose meter to do this, but those are fairly cheap and easy to get.
Another option would be to perform my good friend Dr. Mike T Nelson’s Pop Tart test 😉.
How did you fair?
The average person will most likely not do well at either test, so if you do well at even one you already have a head start!
If the fat adapted test gave you some trouble then we need to teach your body how to utilize fat better. A simple way to do this is to start spacing out the time you eat between meals and work your way up to doing some intermittent fasting. If you are someone who eats three meals a day but needs to snack in between each meal or else they get “hangry” you will want to start by eliminating some if not all of those snacks over a few weeks. After you can make it through a day without snacking between meals you can start working on expanding your intermittent fasting window. Pick one day and do the fat adapted test above and gradually extend the time you can go between dinner and breakfast. Take it slow, no rush. It might take weeks or months before you get to the point where you can easily go 18-24 hours, but once you can, you will have a pretty good ability to utilize fat for fuel!
If the carb test was an issue for you, you are likely not very insulin sensitive (like Runner B from the above case study). One proven way to to improve insulin sensitivity is to do some fasted aerobic exercise. This could be something like an easy walk, run, hike, bike, or row (bonus points if you can do this outside!). To ensure that you keep this exercise session easy it is best to use a heart rate monitor and keep your heart rate between 110-140 beats per minute. If you don’t want to use a heart rate monitor or don’t have one, do the workout only breathing through your nose.
Once you can pass both tests you are well on your way to becoming metabolically flexible! By being able to utilize both fat and carbs during endurance exercise you are giving yourself a huge advantage out on the course, you have the ability to put the pedal to the metal when you need to by accessing the glucose from the carbs you take in, but also have the ability to access your stored body fat to help you avoid the dreaded bonk!
Working on improving your metabolic flexibility is not a quick process and it is important that you give it time, but the payoff will be huge!
When you need help improving your metabolic flexibility and improving your endurance performance sign up to work with me directly. Also be sure to sign up or my newsletter to get articles just like this delivered to your inbox on a weekly basis!
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