If untrained guys eat a high-fat diet for 7 weeks, their bodies get better at burning fat from the blood during exercise compared to eating a high-carb 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.

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.

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.

In teen boys who are healthy, doing six weeks of regular cardio or short, intense sprints improves their fitness, but doesn't change their standard cholesterol numbers—meaning those blood tests might miss some of the good changes exercise is doing inside their bodies.

Working out before eating breakfast burns about 3 grams more fat than working out after eating, because your body has to use stored fat for energy when you haven’t eaten.