If you've ever tried to exercise after having not eaten in a while, you know the results can be disastrous. That's because your body has no ready source of fuel to power working muscles. Yet you don't have to slam a sandwich: Caffeine can also help give your body energy for a workout. "Studies show that caffeine can increase levels of blood fats," allowing your body to use fat instead of carbs for fuel during exercise, says Burke. This lets your body save glycogen stores – your muscles' primary fuel source – for later in your workout when you need them.