It has been believed for many years that athletes need sugar for high performance. The reasoning behind this recommendation is that sugar quickly delivers energy in the form of ATP, which is required for muscle contraction. It takes much longer to supply ATP from the oxidation of fatty acids.
On the other hand, our carbohydrate storing capacity is very small. Only a few thousand kcals can be stored as glycogen in muscle and liver, whereas fat stores comprise at least 100,000 kcals, even in very lean athletes. When it comes to endurance, glycogen stores soon diminish, but fat stores are almost indefinite.
So much for the theory, but what happens if we put it in practice?
Studies in humans actually gave mixed results. The difficulty is that full adaptation to a ketogenic, low carb high/fat diet can take several weeks. Most studies were done with athletes being in ketosis only for one or two weeks. After such a short time, the metabolism is not adapted, and the body cannot efficiently use ketone bodies as fuel. As a result, the performance of these athletes drops.
Animal studies have the advantage that food intake can be easily controlled and monitored for a long time. In humans, adherence to the prescribed diet is often unpredictable and impacts the results.
A recent study with mice assessed the exercise capacity after eight weeks on a ketogenic diet. As a control, mice of the same age and strain were kept on a standard, high-carb diet for eight weeks. After eight weeks, mice from the keto and standard diet groups were put on a treadmill and the time until exhaustion was measured. Mice in the keto group could run for an average of 289 min, whereas mice in the standard diet group only run for an average of 243 min.
Biochemical analysis revealed that blood glucose levels were higher in the keto group compared to the standard diet group. Fatty acid levels were, however, lower in the keto group, suggesting that the keto-adapted mice used fat rather than glucose as fuel.
As rodents, the natural diet of mice is very different from the human one. It is, therefore, questionable how the results of such a study can be translated to humans. On the other hand, the biochemical mechanisms of energy production from fat and sugar are pretty much the same in both species, and these results suggest that human athletes might also benefit from a ketogenic diet, provided that they are fully adapted.
Would you also like to benefit from a ketogenic diet? Check out our Ultimate Beginner’s Guide!