After months of eating mostly fat and very little carbs, elite endurance athletes can burn fat much faster during long races, helping them save their body’s limited sugar stores for when they need a final burst of speed.

Even when eating almost no carbs after a race, athletes who’ve been on a high-fat, low-carb diet for months can still rebuild their muscle sugar stores just as well as those eating lots of carbs.

You can’t just switch to a low-carb, high-fat diet and expect to perform better in endurance sports right away—it takes months for your body to adjust and start burning fat efficiently.

If untrained guys eat a high-fat diet for 7 weeks, their muscles use less stored sugar during medium-intensity exercise compared to eating a high-carb diet — it’s like saving their fuel — but their legs still take up the same amount of glucose from the blood.

When endurance athletes do high-intensity interval training at very hard effort (over 85% of their maximum oxygen capacity), they burn fat at extremely high rates - about 1.58 grams per minute on average, with some burning over 1.85 grams per minute. These are the highest fat-burning rates ever measured in people.

For male endurance athletes who are dehydrated after long workouts, drinking a little or a lot of fluid with carbs doesn’t make a difference in how fast their muscles refill energy stores in the first 4 hours after exercise.