The ketogenic diet, a fat-forward meal plan that limits followers to about 20 grams of daily carbs, may help some people shed a little weight (in the short term), but it might not be good news for their athletic pursuits.

A small Saint Louis University study published in the Journal of Sports Medicine and Physical Fitness chronicled 16 men and women who ate a low-carb, ketogenic-style diet (i.e., getting less than 10% of one’s energy from carbs) for 4 days, then measured their anaerobic exercise performance on a Wingate cycling test. The second phase of the study conducted the same test after participants spent 4 days on a higher-carb diet (both diets had the same number of calories). The results: Participants performed worse after the low-carb diet.

This study shows that the anaerobic energy system, used for everything from sprints to lifting weights to walking up stairs, might be compromised by a diet that essentially starves the body of carbs. Carbohydrates require less oxygen than fat to power your muscles, so carbs are a more efficient energy source for shorter bouts of high-intensity exercise.