Body fat and exercise
Despite your indoor cycling teacher’s claim that a ride “torches fat,” exercise doesn't really melt it. All day long, your body uses energy from the food you eat. 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,” says Wayne 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 take the lead. Hit Autobahn level (aka sprint), 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 when you push yourself hard.
When researchers at the University of Wisconsin–La Crosse measured the energy expenditure of walking, they found that people burned 33 kilojoules per minute, The total energy burn nearly doubled when people ran, to 62 kilojoules per minute.
But here’s the catch: because the high-intensity exercise burned more total energy, it included a greater amount of energy from fat. Bottom line: “You burn more energy from fat in the same amount of time at a higher intensity,” says Westcott. What’s more, when it comes to losing weight and actually decreasing body fat, the source of the energy doesn't really matter.
Fat loss 101
“It’s simple math: When you create an energy deficit, you lose weight,” says John Porcari, PhD, professor of exercise and sports science at the University of Wisconsin–La Crosse, and lead researcher on the study that compared walking and running fat expenditure. That’s because when your body needs more energy than it has, it gets energy from somewhere else—mainly fat. As the fat is metabolized for fuel, the fat cells shrink, (there's the burn!) and you lose weight.
You can create acalorie energy deficit by changing your eating habits, but to burn more fat, exercise is key. “If you only diet, approximately half of the weight loss will come from muscle and half from fat,” says Porcari. “But if you also do aerobic exercise, upwards of 80 per cent of the weight you lose will come from fat.” Add strength training and you’ll hold on to even more of your muscle, resulting in a higher resting metabolic rate and daily energy burn. So while exercise doesn't technically burn fat, it’s the most reliable fire-starter you’ve got—so use it!