The keto diet is a high fat, very low carb diet, where carbohydrate intake is typically limited to 30-50g per day. The keto diet has two big downsides for athletes: 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 [ 2 2 3
2]. A keto-adapted athlete will simply have less glucose available to fuel a long ride than a carbohydrate-fed competitor, and research to date suggests that this impairs performance, even if carbohydrate is temporarily restored pre-ride [
Another reason why keto is inferior to higher carbohydrate diets for endurance performance is that ATP (energy) production from fatty acids requires more oxygen than ATP production from glucose [ 2 3
3]. Higher oxygen demands during exercise equate to lower efficiency, thus limiting the potential performance benefits for athletes following a strict ketogenic diet.
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 McSwiney, F. T., Doyle, L., Plews, D. J., & Zinn, C. (2019). Impact Of Ketogenic Diet On Athletes: Current Insights. Open access journal of sports medicine, 10, 171–183. https://doi.org/10.2147/OAJSM.S180409
 Burke, L. M., Sharma, A. P., Heikura, I. A., Forbes, S. F., Holloway, M., McKay, A., Bone, J. L., Leckey, J. J., Welvaert, M., & Ross, M. L. (2020). Crisis of confidence averted: Impairment of exercise economy and performance in elite race walkers by ketogenic low carbohydrate, high fat (LCHF) diet is reproducible. PloS one, 15(6), e0234027. https://doi.org/10.1371/journal.pone.0234027