Sports nutrition is a complex entity. Given that each athlete has unique physiology, different training and competition demands, and preferences for various food, it’s no wonder that we haven’t landed on one cookie-cutter approach for how to best fuel athletes — as it likely doesn’t exist.
A generally accepted notion among sports science practitioners (and athletes) is that carbohydrates are a preferred fuel source for high-intensity exercise. Researchers in this area like Louise Burke and John Hawley publish extensively on the data showing that when it comes to high-level competition, “carbohydrates are king.” While we won’t get into the biochemistry, this has to do with the way carbohydrates and fats are oxidized by the body during activity.
Carbohydrates are a “quick” fuel source for glycolytic (higher intensity) exercise, while the breakdown pathway for fat is a bit slower and thus, efficient for lower intensity and more extended exercise durations.
Basically, as exercise intensity increases, we rely more on glycolytic metabolism (of carbohydrates) and less on fatty acid oxidation.
For this reason, high-carbohydrate diets have long been prescribed for athletes. To support heavy training, replenish muscle glycogen, and fuel for competition, carbs are just what the doctor ordered. But the prescription is starting to change.
The increasing popularity of low-carb and ketogenic diets for health has also helped blossom a “niche” area of applying ketogenic diets to athletic performance. While this includes body composition and strength-related sports, a big focus of this area has been for endurance sports, which will also be the focus of this post.
A few proposed advantages of low-carb diets for athletes.
- Low-carb high-fat diets increase the ability to use body fat as an energy source at rest and during exercise — this flexibility might mean an athlete can have access to more fuel stores and access fat stores at a higher exercise intensity. This would also help to spare muscle glycogen, a big advantage for ultra-long endurance sports.
- Low-carb diets are proposed to lead to less inflammation and oxidative stress (due to less glycolysis), and this might help with performance and recovery. Exercise does damage to muscles, and an “anti-inflammatory diet” could help athletes rebound quicker from hard efforts.
- Ketogenic diets might favorably impact body composition — namely by lowering body fat % and total fat mass.
The theory is sound, and anecdotes of ultra-endurance athletes adopting ketogenic diets support the theory.
But the theory in opposition is also sound. Eating a diet low in carbohydrates, while boosting fat-burning capacity, may actually impair the ability to perform at higher intensities. When your body wants to burn the carbohydrate “kindling” at 80–100% of your max, the slow-burning logs (fat) might not cut it.
N=1 is nice when we want to build a support structure around an idea. But when we really want to test the efficacy of an intervention, we look for the research. There have been quite a few studies on ketogenic diets and athletic performance. Let’s take a look at what they’ve shown.
An interesting study by two pioneers in the ketogenic diet sphere — Jeff Volek and Stephen Phinney — wasn’t necessarily an intervention, but provided some interesting observations on differences between “keto-adapted” endurance runners and their high-carb peers. They took 20 elite ultra-endurance athletes (10 low- and 10 high-carb) and put them through a VO2 max test and a 3-hour treadmill run, measuring their fat burning capacity during each.
Unsurprisingly, the Keto-adapted runners were able to burn 2x the amount of fat during the 3-hour treadmill run compared to the high-carb group (see above). Peak fat oxidation also occurred at a higher exercise intensity in the low-carb runners (70% of max) than the low-carb (50% of max).
No performance outcomes for this one, just evidence that low carb athletes can burn more fat. Whether this would contribute to a better race outcome has yet to be studied, but could be promising over something like a 50 to 100 mile run.
Another study (Lambert 1994) compared a high fat diet (70% fat/7% carbs) to a high carb diet (74% carb/12% fat) after allowing trained cyclists to adapt to each diet for 2 weeks. The high fat diet led to an equal performance (vs. high-carb) in time to exhaustion at 90% of the athletes’ max oxygen uptake and actually increased time to exhaustion at 60% of their max (moderate intensity) — this was accompanied by a lower rate of carbohydrate oxidation, suggesting fat burning capacity improved.
There are plenty of low-carb interventions in the literature, but the two I’ve cited above indicate a general finding among the (positive) studies on high fat diets for athletes — they support the concept that moderate intensity exercise capacity can improve, along with fat-burning capacity.
Other studies, however, aren’t as supportive. Let’s take a look at a few.
Perhaps the most-cited (and controversial) study was conducted by Louise Burke and colleagues in 2017. Groups of elite race walkers were placed on 3 diets for 3 weeks: a high-carb diet, a periodized low-carb diet (alternating high- and low-carb availability) and a low-carb high-fat diet. They ate each diet while maintaining their highly demanding training schedules.
No surprise — fat oxidation capacity increased in the low-carb group during a 2-hour race walk. However, an interesting result was observed. When the race walkers were studied at exercise intensities similar to “real life competition” (race pace), the groups eating the high and periodized carb diets reduced their oxygen cost — efficiency improved.
On the other hand, low-carb race walkers saw no improvement in their exercise economy, nor did they see an improvement in their 10k race walk time (which improved in the other two groups).
Performance didn’t decline in this study per se, but it didn’t improve either. And isn’t that the goal of training?
One criticism of this study was that the race walkers weren’t allowed to fully adapt to their diet. Was 3 weeks enough? The authors argue that given the enhanced fat oxidation capacity after low-carb, adaptation did indeed occur.
A second study tested the effects of a 10-week ketogenic diet in 5 endurance athletes (4 men, 1 woman). In line with the other studies, each athlete was able to better use fat as a fuel source by the end of the trial — peak fat oxidation increased by 41% — and could do so at a higher percent of their VO2 max (48% before vs. 63% after).
But time to exhaustion declined, on average, by about 2%. Peak power output and VO2 peak also declined — overall indicating that performance reductions were observed. This isn’t surprising since all variables were measured during the same exercise test.
An interesting outcome from this study however, should be underscored. When measuring objective experiences, athletes reported an initial decline in energy, but they reported energy levels returned to normal thereafter, whether at rest or while exercising. Despite this “adaptation”, athletes did report that the higher-intensity exercise bouts were rather difficult.
The ketogenic diet seemed to improve overall well-being. Improved recovery, reduced inflammation, and improved skin conditions were some of the positive outcomes reported. A ketogenic diet was a regimen many athletes said they’d be likely to continue.
Despite performance decrements and some negative experiences, athletes were keen to pursue a modified low-carbohydrate, high-fat eating style moving forward due to the unexpected health benefits they experienced
This brings me to the gist of things surrounding the topic of diet and performance. Don’t get me wrong — performance matters — perhaps above all. For elites, hell, even sub-elite athletes or couch to 5k runners, improved outcomes are the reason we train, diet, supplement, and biohack. Everyone wants to get better.
But not at the expense of health (for most). It is vital to find a balance between lifestyle and performance optimization, and sometimes trade-offs are necessary.
What may be optimal for health may not be optimal for performance, and vice-versa. This isn’t to say that you have to make a sacrifice. If you want to eat a ketogenic diet, as an athlete, looking at the nuanced data is important, but so is testing things out for yourself. Given the highly individual nature of responses, many athletes might find this diet to benefit the sport they’re in, others may ditch the diet after a few weeks.
I obviously haven’t included every low-carb or ketogenic diet study in athletes in this post. However, I believe the ones I have chosen are poignant illustrations of the role ketogenic diets may play in performance.
One thing is for certain, the data will continue to mount for keto and athletes just as it is for keto in other realms of health. It’s fun to be witness to an area of science where dogma is being “challenged” and interesting discussions (arguments?) are being had.
Burke LM, Hawley JA, Wong SH, Jeukendrup AE. Carbohydrates for training and competition. J Sports Sci. 2011;29 Suppl 1:S17–27.
Volek JS, Freidenreich DJ, Saenz C, et al. Metabolic characteristics of keto-adapted ultra-endurance runners. Metab Clin Exp. 2016;65(3):100–10.
Zinn C, Wood M, Williden M, Chatterton S, Maunder E. Ketogenic diet benefits body composition and well-being but not performance in a pilot case study of New Zealand endurance athletes. J Int Soc Sports Nutr. 2017;14:22.
Burke LM, Ross ML, Garvican-lewis LA, et al. Low carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers. J Physiol (Lond). 2017;595(9):2785–2807.
Lambert, E.V., Speechly, D.P., Dennis, S.C. et al. Eur J Appl Physiol (1994) 69: 287