The keto diet is not advisable for runners, as it may hinder overall performance .
The keto diet is a high fat, low carbohydrate diet that consists of less than 50 grams of carbs per day. Carbohydrate is the primary fuel source for working muscles, and severe restriction appears to have a negative impact on endurance performance .
The keto diet has two big downsides for runners: limited glucose availability and increased oxygen demands. Although fat burning increases following a keto diet, endurance capacity in keto-adapted athletes still appears to be limited by glucose availability . A keto-adapted runner will simply have less glucose available to fuel a long run than a carbohydrate-fed competitor, and research to date suggests that this impairs performance, even if carbohydrate is temporarily restored pre-run .
Despite increased fat burning, endurance capacity in keto-adapted athletes still appears to be limited by glucose availability . Keto may not also offer bigger performance benefits due to the fact that ATP (energy) production from fatty acids requires more oxygen than glucose does [2, 3]. These greater oxygen demands decrease efficiency during exercise and thus limit the potential performance benefits for runners following a strict ketogenic diet.