In the short term, it negatively impacts fitness.
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But in the context of exercise, ketones could have a dark side: Workout fatigue is caused by high levels of acid, and more ketones makes the body more acidic, explains study author Ted Weiss, Ph.D., professor of nutrition at Saint Louis University in Missouri. So it follows that keto dieters will tire out more quickly during anaerobic exercises like sprinting, jumping rope, climbing a hill, or heavy lifting. (The study didn’t look at aerobic fitness.)
Your body can’t work at maximum effort on a short-term keto diet, though you might adapt to it after a few months. But even in the long term, the diet hurts your body's ability to burn carbs as fuel, so it'll still stunt your anaerobic fitness, Weiss says.