For endurance athletes, switching to a high-fat diet for six days and then loading up on carbs doesn’t seem to boost performance in a one-hour cycling test compared to sticking with a high-carb diet.

When endurance athletes eat a high-fat diet for 6 days, their bodies burn less carbs during a long bike ride compared to when they eat a high-carb diet — showing their metabolism switches to using more fat for fuel.

If endurance athletes eat a high-fat, low-carb diet for just 6 days, their bodies burn way more fat during a long bike ride — about 50 grams more over 4 hours — compared to when they eat lots of carbs, even if they're used to carb-heavy diets.

If a male cyclist eats a super high-fat diet for 6 days and then loads up on carbs for 1 day, it won’t slow them down in a 100-km race compared to eating lots of carbs all along.

Even after endurance athletes eat lots of carbs for a day, their bodies still burn more fat than usual if they recently spent a week on a high-fat diet.

If elite male cyclists eat a super high-fat diet for six days, their bodies get better at burning fat for fuel — even when they switch back to eating lots of carbs for a day.