Many proponents of the ketogenic diet claim that it not only helps with weight loss but also increases exercise endurance by making the body more efficient at burning fat and ketones for energy. However, an investigation by New Zealand researchers, published in Medicine & Science in Sports & Exercise, hints at a different outcome.

For the study, eight male runners completed two 31-day protocols in randomized order. During one time block, they consumed their normal diets (averaging 43% carbohydrate, 38% fat and 19% protein), and during the other 31-day time frame, they followed a high-fat, low-carb ketogenic diet (averaging 78% fat, 18% protein and just 4% carbs). Before and after each 31-day diet block, the men completed a series of performance and physiological tests.

In the end, diet differences did not affect exercise efficiency at running speeds corresponding to <60% of maximal oxygen consumption—a threshold where the body can more easily rely on fat for energy during exercise. But as the pace picked up—and particularly at intensities >70% of VO2max—runners on the ketogenic diet were less efficient at using oxygen to generate energy. In other words, they required more oxygen to maintain the exercise intensity, since energy from carbohydrates was lacking.

While we need more data from larger studies to show what impact this has on overall performance, here’s sage advice for now: If you’re participating in an activity that requires fairly intense effort, it’s likely a good idea to have some carbs in the tank.