Does a keto diet hurt exercise performance?

Short answer

It depends on the type of exercise. Lifting strength did not drop. But performance fell in the first week, and even after adapting, fast sustained efforts and repeated sprints lagged behind a carb-based diet.

Does a keto diet hurt exercise performance?
'Fewer carbs → less fuel → worse performance' — does this formula hold?
A keto diet cuts carbs below 50 g a day and runs on fat as the main fuel. Some say you cannot train without carbs, the muscles' fuel; others say it is fine once your body adapts. We checked it by type of exercise.
The first few days, performance clearly drops
After 4 days, short all-out efforts fell 6-15%
16 people tried 4 days of keto and 4 days of a high-carb diet in turn. After keto, peak power in a 30-second all-out bike test was 7% lower, average power 6% lower, and shuttle-run distance 15% shorter. A 2026 analysis that pooled 33 studies also found that every study testing within 7 days saw performance fall.
After a week, peak endurance comes back
No difference in maximal oxygen uptake or time to exhaustion
An analysis that pooled 10 studies found that after adapting to keto, maximal oxygen uptake and time to exhaustion did not differ from a carb-based diet. In the 2026 analysis, studies testing after more than 1 week of adaptation saw performance hold or improve.
Lifting strength did not drop either
No difference in max squat or bench press
A 2024 analysis of people who lift found no difference between keto and a regular diet in max squat (106 people) or max bench press (119 people). In a 10-week trial of 25 people, strength and power rose by the same amount on both diets.
But fast, sustained race times were a different story
The high-carb groups got faster; keto stayed put
29 world-class race walkers trained harder for 3 weeks. The high-carb groups cut their 10 km times by 6.6% and 5.3%; the keto group did not improve. Burning fat made the same speed cost more oxygen. In a repeat trial of 26, the keto group got 2.3% (86 seconds) slower.
Efforts with short rests suffered too
One all-out effort held up; repeated sprints dropped
A 2026 analysis that pooled 13 studies found that keto and low-carb diets kept the ability to produce one maximal effort. The ability to repeat short sprints, however, dropped. The more an exercise relies on burning carbs fast, the bigger the cost.
Bottom line: lifting is fine; fast, repeated efforts pay a price
The formula splits on the type of exercise
Keto lowered performance during week 1; after adapting, lifting strength and peak endurance held. But fast sustained race times and repeated sprints lagged behind a carb-based diet. For lifting, strength held on keto; for timed races, carbs came out ahead.