In the short term, it negatively impacts fitness.
The science:
Athletes who ate a ketogenic diet for four days covered 15 percent less distance in a sprinting exercise compared to those who ate more carbs, according to a new study in the Journal of Sports Medicine and Physical Fitness.
Expert insight:
People following keto diets usually eat only about 20 to 30 grams of carbs per day. This forces you to use fat for fuel, a process called ketosis. In this state, the body breaks down fat and converts it into acidic substances called ketones. Many advocates of the diet say these compounds improve cognition and lower body fat. 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.)
The bottom line:
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.