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Keto, sports and human performance

By January 28, 2021February 9th, 2021Exercise, Nutrition

Low-carb continues to get rave reviews for increasing energy and performance, and improving health. Ketosis too! But is it for you?

Traditional sports nutrition promotes glucose as the body’s primary fuel, with fat taking a back seat. These guidelines are slowly fading away as many athletes today are relying more and more on fat for energy and significantly improving their performances.

High-carb sports nutritionists suggest that ketosis and sports do not mix — a false notion. Humans have been big fat burners from the beginning. Today, relying more on fat for energy can significantly improve performance and health — athletes can even grow physiologically younger! The key is finding the best approach for your body’s needs.

During fasting, and periods of carbohydrate restriction, the liver produces ketone bodies in large amounts to serve as an important energy source. While smaller levels of ketones are generated most of the time and used by the heart muscle and other body areas, increasing fat-burning can also increase ketones for use as an energy source to help keep blood sugar stable and, especially during exercise, prevent depletion of glycogen stores.

High levels of ketones — nutritional ketosis — can improve mitochondrial function, where fat-burning takes place. It also reduces both oxidative stress and inflammation. The result is that physical and mental performance can improve, often significantly.

There are seemingly unlimited numbers of athletes to highlight who have excelled by cutting back on carbohydrates and filling up the difference with fat. Increased endurance energy and slimming down are just two reasons why.

Triathlete Dan Plews was the overall age group winner at age 36 at the Kona World Championships (2018) in a record time of 8:24. Dan is also an exercise physiologist and MAF founding coach. We have authored scientific papers together on the benefits of low-carb and ketosis during exercise. Professional triathlete and medical doctor Amanda Stevens is another athlete who switched gears on eating. Her case was published in a scientific coaching journal, and is reviewed below.

In addition, professor Jeff Volek led a study of 20 elite ultra runners aged 21-45. Half ate a very low carb diet and the other half traditional high-carb fare. The low-carb eaters burned more than twice as much fat for energy as the high-carb athletes during maximum exertion.

The Name Game

Relative to popular trends of high carb/low fat, low- or very-low carb eating is associated with high healthy fat because the reduced calories from carbohydrates must be made up with either dietary fat and/or protein. This is best done by increasing fat while maintaining moderate but adequate protein intakes. However, this terminology is deceptive as humans ate this way for millions of years, so it’s more normal and compatible to our natural physiology than it is “high-fat.”   

Many ask whether higher intensity, shorter competitive events are compatible with low-carb and ketogenic eating. The short answer is yes, they could be. The caveat, of course, it whether this way of eating matches your needs. The Two-Week Test, or Two Week Keto Test can help you determine these needs. Just remember — even very short bursts of high-intensity training and racing can result in increased fat-burning.

An important factor, whether in scientific studies or in your own testing, is to allow metabolic adaptation to low-carb eating, a shift that could take two to four weeks, sometimes months, to completely adapt. So evaluating performances during the first week or two, or sometimes even a month of making dietary and metabolic adjustments, can show temporarily impaired performance. (This was a problem with older published studies that demonstrated impaired performance in low-carb diets.)

In addition, low-carb and very-low carb may or may not induce ketosis, although the latter usually does when carbohydrate intake is below 50 grams per day.

A Carb-Fat 180

The graph below of a female professional Ironman athlete shows the dietary macronutrient changes that occurred during the transition from her popular high-carb to ketogenic eating. As you can see this is complete 180. The health and performance results were not unusual.

Over a six-week period, daily carbohydrate was gradually decreased from 73 (475 g) to 12 percent (78 g) of total calories, while fat content increased from 14 (40 g) to 75 percent (217 g), and protein levels remained constant at 13 percent (85 g). Within two months, the athlete reported increased perception of daily energy during and between training sessions, less perceived hunger and fatigue, and reduced need for daytime naps. Cycling power increased by 20 watts and run pace increased (12–15 seconds/km) at the same MAF heart rate (141 beat min). With the exception of water, nutrition was no longer required during long duration training.

Race Nutrition

Another benefit associated with the ability to increase one’s fat burning engine is that it provides us all with a built-in source of nearly unlimited race energy. Even lean athletes have sufficient stored fat to do this — both in training and racing. The result is that we usually require much less nutrient intake during a long event like the Ironman. The chart below shows the nutrient intake during this athlete’s three Ironman events compared to her previous years’ races. The caloric differences are made up by burning stored body fat.

Reducing or eliminating (in shorter races) the need to consume food or drink other than water can be a significant benefit to many athletes to avoid significant gut stress, which can impair race performance.

What about shorter events?

In a study (Cipryan, et al.) of two groups of runners, one that switched from a common high-carb diet to one of very low carb/high fat over four weeks, while the other did not change, the low-carb eaters increased fat-burning significantly during high-intensity interval training (HIIT). In addition, performance was not impaired after consuming a VLCHF diet relative to a group consuming their mixed Western-based diet, preserving high-intensity performance.

Aging Athletes

Athletes performing high-intensity activities increase oxygen free radicals which can damage the body and speed aging. Some of us age faster or slower than others. While genetics play a role, lifestyle influences the process significantly (including controlling many genes that influence aging). Both exercise quality (higher vs. lower intensity) and food (higher vs. lower carbohydrates) play major roles in this process. Fat-burning plays a key role, supported by a healthy lifestyle.

A key aspect of MAF is that higher fat. lower carb eating increases fat-burning at rest, during submax exercise, and at higher intensities. This may simply be due to more fat being available to fuel muscles during physical activity. These actions are opposite those produced by moderate- and high-carbohydrate diets because they can significantly impair fat-burning and promote fat storing.

Exactly how much carbohydrate and fat one needs depends on the individual’s level of carbohydrate intolerance (insulin resistance), one goal of the Two-Week Test LINK.

  • Younger people tend to metabolize carbohydrates better, being less carbohydrate intolerant.
  • However, in the last 50 years this trend has changed in many younger individuals as indicated by a dramatic increased prevalence of overfat individuals of all ages. For adolescents, this is also associated with the development of adult diseases like diabetes, hypertension and non-alcoholic fatty liver.

No matter the age, no one should ever eat junk food, and anyone who does not burn fat well can benefit from very-low carb, high fat — and a ketogenic — lifestyle that not only can improve health, but athletic performance too.



Cipryan L, et al. Effects of a 4-Week Very Low-Carbohydrate Diet on High-Intensity Interval Training Responses. J Sports Sci Med. 2018;17(2):259-268.

Lima-Silva A, et al. Relationship between training status and maximal fat oxidation. J Sports Sci Med. 2010;9.

Maffetone P, Laursen P. Case study: Reductions in training load and dietary carbohydrates help restore health and improve performance in an Ironman triathlete.

Prins PJ, et al. High Rates of Fat Oxidation Induced by a Low-Carbohydrate, High-Fat Diet, Do Not Impair 5-km Running Performance in Competitive Recreational Athletes. J Sports Sci Med. 2019;18(4).

Volek JS, et al. Metabolic characteristics of keto-adapted ultra-endurance runners. Metabolism, 2015. DOI: 10.1016/j.metabol.2015.10.028