Is A High Fat Diet Good For Endurance Athletes?

High Fat Diets vs. High Carb Diets: Which Is Better?

Is keto a carb killer? There’s a lot of buzz around ketogenic diets.

Whether you’re an average Joe or an accomplished athlete, chances are you or a buddy have tried a ketogenic diet.

While a good deal of studies have been done for the individual seeking weight loss, not a whole lot of research is available on a ketogenic diet for athletes, which is especially important for competitors whom too often must extrapolate results of research using physiologically dissimilar, less-trained individuals.

You want the study to feature participants with whom you are similar. As with most other topics in sports science, even less data is available for elite athletes. Except for this one!

Research on high fat diets in athletes

are high fat diets a bad idea?

Researchers from University of Connecticut and The Ohio State University recruited 20 elite ultra-endurance athletes (finishing in the top 10% of sanctioned runs of > 50km or triathlons of > 113km) who were already consuming a ketogenic or high-carbohydrate diet.

These athletes cannot be named in the manuscript for privacy reasons, but 55% of the athletes had full sponsorship and 25% were members of Team USA. In other words, these guys were the real deal!

You might be thinking, “but you said we should be similar… I’m not THAT good!!” This might be true, but if you are 10% slower than a professional athlete, that’s a heck of a lot better than being 90% faster than a non-athlete!

What Were the Research Protocols?

Participants recorded their typical diet for 3 days prior to 2 consecutive days of testing.

The first day consisted of a VO2 Max test with several metabolic assessments conducted during the test, such as caloric expenditure. After testing on day 1, athletes ate a standardized dinner prior to an overnight fast in preparation for testing day 2.

On the second day, athletes’ submitted a blood sample and had their body composition and resting metabolism assessed prior to consumption of a meal replacement supplement with macronutrients matching their habitual diet (i.e. keto athletes were given a keto shake with 81% calories as fat).

90 minutes later, they completed a 3-hour run at 64% VO2Max. Measurements of metabolism, muscle biopsies, and blood analytes were conducted periodically throughout the 3 hours as well as a 2-hour recovery period.

A high fat diet increased peak fat oxidation

peak fat oxidation during exercise

Now for what you’ve all been waiting for! The results!

First up is peak fat oxidation (burning) from day one of testing. Prior to this study, the theoretical limit of fat oxidation was 1 gram per minute.

Athletes consuming a ketogenic diet had an average peak fat oxidation of 1.5 grams per minute, and not one of these athletes was below 1.1 gram per minute.

The high-carbohydrate athletes, however, averaged 0.67 grams of fat per minute, and not one athlete was over 0.9 grams per minute.

  • Peak fat oxidation occurred at 55% VO2Max (moderate intensity) in high-carb athletes, but for keto-adapted athletes, peak fat oxidation did not occur until 70% VO2Max (vigorous intensity).
  • VO2Max values were virtually identical between groups at 64 mL/kg/min.

Fat oxidation during the 3-hour submax run on day 2 was, as you may have guessed, greater in the ketogenic athletes, and carbohydrate oxidation was greater for the high-carb athletes.

Interestingly though, the keto athletes jump right up to, and remain at, a fat oxidation rate of 1.2 grams per minute, yet the carbohydrate athletes begin at 0.6 grams per minute and gradually escalate to 0.8 grams per minute.

Even more interesting is what happened with muscle glycogen. It was the same!

Muscle glycogen was not different between groups before, immediately after, or two hours after the run.

However, blood lactate was significantly increased in ketogenic athletes compared to the high-carb athletes.

So is a high fat diet really better for endurance athletes?

diets that are high in fat

There are a number of take-away points from this investigation.

First, although it may be expected, keto-adapted athletes have a much greater capacity to oxidize fat. Body fat, even for lean individuals, is nearly “unlimited,” so this aspect is a positive.

Of course, you can starve yourself and run out eventually, but in the race setting, you very, very likely will not exhaust fat stores, even up to 100 miles.

This is not to mention, muscle glycogen utilization was unencumbered by the diet, so higher intensity activity should not be a problem.

However, the ketogenic athlete would have to supply carbohydrate during exercise and/or before the bout of high-intensity activity if it will be prolonged (this is a teaser, I know, but we will have an entire blog on this topic in the near future).

It is also unclear exactly why ketogenic athletes would have greater lactate in the face of reduced carbohydrate oxidation. It’s possible that lactate could be buffering increasing blood ketones and/or simply a by-product of anaerobic metabolism.

Thus, a ketogenic diet may have some pretty distinct advantages for the ultra-endurance runner or triathlete.

These may or may not come in the form of a physiological advantage, as suggested by the results, but as less rigid refueling strategies during a race because if glucose is not present, fat could still get the job done.

However, the study did not feature a measure of performance, so we can’t say for sure which diet is “superior” based on the results of this lone examination.

At the end of the day, there are hundreds of investigations on carbohydrate-based nutrition strategies versus just a handful of keto studies, so dietary strategies of a high-carb diet are still more defined and clearly understood.


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. Metabolism, 65(3), 100-110.