How long do you have to exercise before you start burning fat?

Even the most inconstant gym-goer is bound to hear someone on a treadmill theorizing that the beginning of a workout doesn't dig into your fat stores. The truth is, two thirds of the energy your body uses just to perform basic functions like breathing comes from the breakdown of fat. That means you don't have to do more than watch Friends reruns to burn fat—although going to Spinning class will speed up the process. "The longer you exercise, the more fat you'll burn," explains Larry Durstine, professor of exercise science at the University of South Carolina. "But you burn fat throughout a workout, from start to finish." To optimize the fat-burning during exercise, remember the 50/50 rule: "Exercising at 50 percent of your maximum aerobic capacity, your body is burning 50 percent carbohydrates and 50 percent fat. As you push yourself closer to max [beyond 70 percent of maximum capacity], carbs will have a greater contribution than fat" into the calorie-burning furnace.