How Exercise Burns Fat

Despite your indoor cycling teacher’s claim that a ride “torches fat,” exercise doesn’t reallymeltit. All day long, your body uses calories from the food you eat as energy. Fat and carbohydrates provide most of the fuel, and the dominant power source depends on how hard you’re working. Think of a hybrid car: at low speeds, it’s propelled by electricity; put the pedal to the metal, and it switches to gas. Your body operates in a similar way.

“When you’re taking it easy, you have plenty of oxygen to metabolize fat—so you burn the highest percentage of fat when you’re resting or doing a low-intensity activity,” saysWayne Westcott, PhD, a professor of exercise science at Quincy College in Quincy, MA.

Oxygen becomes limited when you kick things into high gear, making it tougher for your body to metabolize fatty acids—and that’s when carbs takes the lead. Hit Autobahn level (akasprint), and you enter the anaerobic (or “no oxygen”) zone. “At an all-out effort, your body primarily uses carbs for fuel,” says Westcott. With this in mind, it would seem that working out at a low intensity (i.e., in the “fat-burning zone”) maximizes fat loss—but experts disagree. You actually crunch through more fat calories (and total calories overall) when you push yourself hard.