Does A Low Carb, High Fat Diet Impair Or Improve Endurance Performance?

With the inaugural EndurElite Research Corner article, we want to do something a little unconventional – because who actually enjoys pointless rhetoric, amirite? We’re going to review an article that, at face value, appears to perpetuate some generally accepted knowledge (carbs are good for performance) and play devil’s advocate.

First, of course, what did the researchers do? What were the results? Three diets were compared over a 3-week training camp. One was a high-carbohydrate diet (60% carbohydrate, 20% fat, 16% protein), one was a periodized high-carbohydrate (60% carbohydrate, 20% fat, 17% protein), and one was a low-carb, high-fat ketogenic diet (4% carbohydrate, 78% fat, 17% protein). The difference between the high-carb and periodized carb diets were that the periodized carb group utilized some “train low” or “sleep low” strategies. Purposefully, all diets were slightly hypocaloric (below maintenance level Calories).

The study was conducted in “elite race walkers.” If you don’t know… yes, that’s a thing – it’s even in the Olympics. While I have no firsthand experience with race walking, it doesn’t appear as grueling as running or other sports with an “as fast as possible” goal (no offense race walkers of the world!). Rather, it looks a bit like a speedy saunter. In any case, they were athletic individuals, with an average peak oxygen consumption in the low 60’s. However, groups were a little bit different at baseline such that high-carb diet participants were less accomplished and younger than periodized high-carb diet participants were slightly less accomplished and younger than keto participants. Remember that.

The results of the study, as the title suggests, found the high-carb diet to be better than the periodized high-carb diet to be better than the ketogenic diet as determined by a 10-kilometer time trial. Keto participants also reported higher perceived exertion, had higher exercising heart rates, and had a lower volume of exercise over 3 weeks, but increased their peak oxygen consumption by 4.8 mL/kg/min to 71.1, while high-carb increased by 4.6 to 66.2 and periodized high-carb increased by 2.1 to 67.0. Irrespective of group, participants lost an average of 1.4 kg body fat.

The devil is in the details.

The ketogenic diet requires 2-4 weeks for side effects to remit. The discussed study implemented the diet for only 3 weeks duration. The duration for complete keto-adaptation is not well defined, but based on other literature, it is likely longer than 4 weeks.2,3,4 Evidence for an absence of keto-adaptation is apparent by comparing the present study to previous studies.

In elite ultra-runners and triathletes, Volek observed no differences in perceived exertion between athletes following a ketogenic diet for several months and high-carbohydrate athletes.2 Most ketogenic dieting studies find a decrease in blood glucose, yet none were observed here. A ketogenic diet is renowned for reducing body fat. However, the authors suggest no differences between groups for body fat (group data not provided). Therefore, it seems unlikely that the race walkers were keto-adapted, so they never stood a chance at improving compared to groups already adapted to their respective diets.

Training volume was quite a bit different between groups as well, which is to be expected during a period of keto-adaptation. The high-carb group had a total training volume of 351 km, the periodized high-carb group 377 km, while the ketogenic group trained 331 km. This is likely a result of diet, yet it does not account for volume following keto-adaptation. It is expected that higher training volumes contribute to more improved performance.

Remember the differences in training status of the athletes between groups when the study began? When do athletes normally enter a training camp? Typically coming out of the offseason or to peak for an upcoming event. The ketogenic diet participants were already “peaking” or near capacity – they either remained in better condition during their offseason/regular season or are flat out better athletes. This means that they had less room to improve. For example, the mean 20 km race walk time for the ketogenic diet group was about 1.5 minutes faster than the time required to qualify for the 2016 Olympic Games. However, the high-carb group was 1.2 minutes slower! Do you think it’s appropriate to compare Olympic-caliber to non-Olympic-caliber athletes? It’s not in science. It’s no coincidence the least-trained group improved.

Finally, and this is for the science nerds, we’re going to look at variance. The variance in performance at baseline is pretty high, suggesting some athletes stayed in shape, while other slacked off. However, after 3 weeks of dialing everything in, variance is greatly reduced – everybody was back in shape. Except for the ketogenic group. Either they maintained an improved training status, which is unlikely because their baseline variance is just as high (so they must just be better athletes!), or they had a more variable response to the training than the carb groups. Of course, they had a more variable response!

They’re right smack dab in the middle of adapting to a radical new diet! Some participants were able to adapt quickly, and they improved their time comparably to the carbohydrate groups (~162 seconds). However, others were not yet adapted, and their time suffered greatly.

The study presented does not demonstrate that a low-carb diet is absolutely inferior to a high-carb diet. It does, however, demonstrate that a low-carb diet is inferior to a high-carb diet after 3 weeks adaptation to a low-carb diet in elite race walkers. Take everything in context. A lot of athletes are experiencing success with low-carb and ketogenic diets. What do you think? Going to stick to a carbohydrate-based diet? Willing to experiment? Tell us in the comments!


  1. Burke, L. M., Ross, M. L., Garvican‐Lewis, L. A., Welvaert, M., Heikura, I. A., Forbes, S. G., ... & Hawley, J. A. (2017). Low carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers. The Journal of physiology595(9), 2785-2807.
  2. Volek, J. S., Freidenreich, D. J., Saenz, C., Kunces, L. J., Creighton, B. C., Bartley, J. M., ... & Lee, E. C. (2016). Metabolic characteristics of keto-adapted ultra-endurance runners. Metabolism65(3), 100-110.
  3. Phinney, S. D., Bistrian, B. R., Evans, W. J., Gervino, E., & Blackburn, G. L. (1983). The human metabolic response to chronic ketosis without caloric restriction: preservation of submaximal exercise capability with reduced carbohydrate oxidation. Metabolism32(8), 769-776.
  4. Phinney, S. D., Bistrian, B. R., Wolfe, R. R., & Blackburn, G. L. (1983). The human metabolic response to chronic ketosis without caloric restriction: physical and biochemical adaptation. Metabolism32(8), 757-768.