Carbohydrates and Performance: Necessary Or Not?

The age-old anecdote is that when it comes to performance, carbohydrates are king. When I got into endurance sports 10 years ago, I read countless articles about how I needed to be consuming carbohydrates in the form of energy gels during my runs to perform my best. And when I wasn’t exercising, I needed to be eating carbohydrates at every meal. With carbs appearing to be the universally agreed-upon fuel for performance, I did as I was told and ate my carbs.

There is no doubt that the best athletes in the world are not skimping on their carbohydrate intake. Case in point, Olav Aleksander Bu, a sports nutritionist and coach to some of the world’s most elite cyclists, has his athletes consume 240g of carbohydrates per hour to keep up with how much they are burning while exercising and racing.

If the top athletes in the world are consuming 240 grams of carbohydrates per hour while exercising then surely carbohydrates must be important…at least that was my thought 10 years ago.

The Low-Carb Revelation

Then one day I heard Mark Sisson on the Ben Greenfield podcast talking about his book Primal Endurance and he proceeded to blow my mind from a nutrition perspective. Mark advocated for a whole-foods lower-carb diet for endurance exercise. His argument made sense: Endurance exercise is by definition done at a lower intensity, and since it was lower intensity the amount of carbohydrates that contributed to your energy needs was lower. This is because metabolically healthy individuals will primarily use fat for their fuel during exercise.

The other argument he made was that even the leanest people have enough fat stored on their bodies to fuel multiple endurance events. In contrast, the average human only has several hundred calories of carbohydrates stored on their body, which can quickly be depleted. This meant that if you could burn mostly fat during your endurance racing there was little need to eat massive amounts of carbohydrates. In addition, Mark argued that by working your fat-burning metabolism you can increase the amount of fat you burn at a higher and higher intensity.

This blew my mind and as I dove deeper and deeper down the “fat burning endurance athlete” rabbit hole I quickly became convinced that we did not need carbs to achieve peak endurance performance. So I set out to prove it myself. I followed the training advice in Primal Endurance: I slowed down my runs to rely more on fat, rehauled my diet to focus on low-carb whole foods, and I felt amazing and my training was getting better.

Case closed then…we don’t need carbs for performance and we can perform perfectly fine with little to no carbs at all, right?

The Low-Carb vs. High-Carb Study

A recent study [1] seems to suggest that it is true, that we can perform just fine without carbs. This study used experienced long-distance triathletes as test subjects with a crossover design to compare the low-carb and high-carb conditions. A crossover design means is that the same participants did both the low-carb diet intervention and the high-carb diet intervention. The low-carb diet intervention consisted of 50g of carbs per day, with 70-80% of calories coming from fat and 20-25% coming from protein. The high-carb diet intervention consisted of 380 grams of carbohydrates, with <30% of calories coming from fat and 20-25% coming from protein.

After eating these diets for 6 weeks the athletes performed 2 times on exhaustion tests. During both time to exhaustion tests participants cycled at 70% of their VO2Max until they could no longer maintain that pace. In the first time to exhaustion test they were given a placebo drink. During the second time to exhaustion test they were given the same drink but this time the drink contained 50g of carbohydrates.

After the initial 2 time to exhaustion tests there was a 2 week “wash out” period where the participants did not follow any specific diet and then they switched to the high-carb diet for 6 weeks and repeated the 2 time to exhaustion tests. After performing all 4 time to exhaustion tests here is what researchers found:

• When consuming a low-carb diet and consuming the placebo beverage during the time to exhaustion test participants were able to cycle for approximately 84 minutes.
  
• When consuming the high-carb diet and consuming the placebo beverage during the time to exhaustion test participants cycled for approximately 88 minutes.
  
• When consuming a low-carb diet and consuming the beverage with 50g of carbohydrates in it participants were able to cycle for 109 minutes. That is 25 minutes longer than with the placebo beverage.
  
• When consuming the high-carb diet and consuming the beverage with 50g of carbohydrates in it participants were able to cycle for 100 minutes. That is 12 minutes longer than with the placebo beverage.
  

Why are these results important?

First, the differences in performance between low-carb and high-carb diets are not that great, at least in the context of scientific research (more on this in a bit). Second, it shows that you can consume far less carbohydrate during exercise than what is commonly recommended and still experience a significant performance boost. The common recommendation is to consume 60-100 grams of carbs per hour during endurance exercise. However, in this study participants were consuming just 10 grams of carbs per hour and still saw a performance improvement.

The researchers theorize that the reason we experience performance decrements in endurance exercise is not due to our carbohydrate stores getting too low, but instead it is due to our blood sugar getting too low. So the only thing we really need to do is to consume enough carbohydrate so that we avoid the dip in blood sugar. The other theory as to why the low-carb condition performed better on the second time to exhaustion test is that the ketones the athletes produced provided a “metabolic buffer” allowing them to better fight off the drop in blood sugar.

So how does this align with Olav Aleksander Bu’s comments that he is having some athletes consume 240 grams of carbohydrates per hour?

Here is the difference:

The athletes that Olav is working with are not your average athlete toeing the line at your local 5k. They are world-class athletes.

For example, the athletes who participated in Nike’s project called Breaking 2, which had the goal of having the first human run a sub 2-hour marathon, had a VO2Max of 71 ml/kg (which is bonkers) and were running their marathons at 88% of their VO2Max. To contrast that with the study I referenced above those participants exercised at 70% of their VO2Max.

Let’s look at fuel usage between these two intensities:

https://examine.com/research-feed/study/dpGjQ9

As you can see by the middle column in the chart above most of the energy used at around 70% of your VO2Max comes from “Plasma FFA” and “Muscle triglycerides”. These are just fancy terms for fat stored in your blood and muscle. However, when you go above 80% of VO2Max as represented in the 3rd column most of the energy comes from “Muscle glycogen” and “Plasma glucose”. These are just fancy terms for carbs stored in your muscle and blood.

Athletes participating in the Breaking 2 project or those that Olav are working with are working above 80% of their VO2Max and therefore it makes sense that they benefit from consuming large amounts of carbohydrates because most of their energy is coming from carbohydrates.

The Low-Carb vs. High-Carb Conundrum

Finally, I want to point out something with regards to eating a low-carb vs. high-carb diet for athletic performance. If you recall from this study when consuming the placebo drink during the time to exhaustion test that the low-carb diet was only 4 minutes slower than the high-carb diet. In research terms that was considered “not statistically significant” meaning the 4-minute difference wasn’t large enough to scientifically say that higher carb diets resulted in greater performance. However, a 4-minute difference at the world-class level is a MASSIVE difference where a difference of seconds may mean in them getting paid or not or winning the gold medal or missing the podium.

So where does this leave us in the debate as to whether carbs are really king when it comes to performance?

Here is something that seems to be almost universally true:

The athletes at the top of their sport seem to always leverage some amount of carbohydrates to enhance their performance. We can debate over what amount is enough and we will likely never come to a conclusion on what is the optimal amount due to individual differences. What I do know is that people who are reading this post are likely not athletes performing at a world-class level (and if you are I am truly humbled lol). Your livelihood likely does not depend on how you place in your sport. This means you likely don’t need to be eating 400g of carbs a day and consuming hundreds of grams of carbs per hour while exercising.

At the same time it may not be the best idea for you to consume 50g of carbs a day and eat 0 carbohydrates while exercising. The reason why most people are more likely to do better staying away from the extreme ends of the carbohydrate spectrum is that it can result in negative health impacts.

On the high end of the carbohydrate spectrum, consuming 400g of carbs per day and then consuming even more carbs during exercise may result in dysregulated blood sugar levels and when this happens chronically it can result in poor metabolic health. In addition, most people are not nearly as active as the world-class athletes. Even the average recreational endurance athlete doesn’t even come close to the amount of training that a world-class athlete does. This means they don’t need the same amount of calories.

On the low end of the carbohydrate spectrum, consuming 50g of carbs a day and then exercising for multiple hours can put a stress on the body that can result in low energy and dysregulated hormones. I know this all too well as this is what happened to me when I tried to eat a keto diet while doing several hours a week of endurance exercise. Admittedly, this may not specifically have to do with the amount of carbohydrates but rather a side effect of low-carb diets being very satiating resulting in too low of a caloric intake for the amount of energy being expended. It may be the case that if you can somehow find a way to eat enough food on a low-carb diet then maybe the negative effects will be mitigated. However, I find with most people that is very hard to do.

Finding Your Carb Sweet Spot

For most people here is what I suggest:

• Use a benchmark workout to establish a baseline using your current carbohydrate workout. This workout should be hard, it should be at or very close to your same exertion you would be expending during your competition.
  
• Once you have a baseline spend 3 months eating at your new carbohydrate level (this could be more or less carbs depending on what you want to experiment with).
  
• Repeat your benchmark workout. If it resulted in better performance then you have some data to prove you might perform better at that carb level.
  
• When it comes to figuring out what carb intake to use during exercise a similar approach should work there as well.
  
• Use the same benchmark workout above at a given carb intake during that workout.
  
• The next time you perform that workout modify your carb intake during the workout and note your performance.
  
• Note not only your performance but also pay attention to things like your recovery and your digestion.
  

When it comes to carbohydrates and performance there is not going to be a universal guideline as to what is optimal. What we will likely see is that both the scientific and anecdotal evidence will show carbs have a dramatic impact on performance and at same time other sources will say that they are not as necessary as we once thought. So what do we do if we have so much conflicting information? The best guideline I can offer is to consider the intensity at which you are exercising.

As we see when the intensity is low you can get away without consuming a large amount of carbohydrates. If you are going out for an easy zone 2 run or doing some bicep curls you likely don’t need to have a giant bowl of pasta the night before and consume candy during your workout. If however you are going to attempt a long duration cardio workout at a fairly high intensity or going to be doing a 90 minute intense full body strength training session some carbs in your meals prior to the workout and some during your workout might be warranted.

That said, you need to consider the context of the person. Your digestion might do well if you eat during your workout or your blood sugar might do well consuming carbs in general. Assuming your livelihood does not depend on your performance then these types of trade offs don’t make sense, you should not be sacrificing your health in the name of performance. It can be quite hard to nail down the right approach, believe me I spent years experimenting with various carb intakes. If you would like help navigating this complex topic, I have some one on one coaching spots open and would be happy to work with you to help you figure this out. If you are interested you can fill out this form and I will reach out to you to set up a time to talk some more.

References

[1] Prins PJ, Noakes TD, Buga A, Gerhart HD, Cobb BM, D’Agostino DP, Volek JS, Buxton JD, Heckman K, Plank E, DiStefano S, Flaming I, Kirsch L, Lagerquist B, Larson E, Koutnik AP. Carbohydrate ingestion eliminates hypoglycemia and improves endurance exercise performance in triathletes adapted to very low- and high-carbohydrate isocaloric diets. Am J Physiol Cell Physiol. 2025 Feb 1;328(2):C710-C727. doi: 10.1152/ajpcell.00583.2024. Epub 2025 Jan 9. PMID: 39786965.