When people with excess body fat eat very few carbs and more fats, their bodies burn more fat for fuel during intense exercise, as shown by lower respiratory exchange ratio measurements.

If untrained guys eat a high-fat diet for 7 weeks, they burn more fat during exercise than if they eat a high-carb diet.

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.

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.