Much has been made of late about exercise and its relation to weight loss. There was the Time cover story The Myth About Weight Loss and a recent article on the topic in the New York Times, which recommended working out in the fat burning zone.

You see the phrase on all types of cardio machines: the fat burning zone, aka the "weight loss zone," means working out at a lower intensity to burn fat as fuel. But these workouts also burn considerably fewer calories. I was confused about how we should all be spending our cardio time, so I took my questions to exercise physiologist Jason Karp, who also happens to be an avid runner.

When I asked him if we should all be scaling our workouts back and aiming for the fat burning zone, Karp explained that it's not the type of fuel your body is burning, but the overall calories that count. "It doesn't matter if you're using fat or carbohydrates. In order to lose fat from your waistline you don't have to be using fat as fuel during workout," he said. He offered up the example of sprinters; they don't train in the fat burning zone, but they're mighty trim in the torso. Losing weight comes down to math — you want to be in a calorie deficit to shed pounds and body fat. Higher intensity exercise puts you at a higher burn rate. To burn the most calories in a cardio workout, skip past the fat burning zone and work out with more intensity. To really burn up calories, Karp recommends intervals. To learn why, read more.

Essentially, interval workouts give you the most bang for your sweat buck. Karp explains that with "intense exercise you're pushing your body more out of homeostasis," that is, the equilibrium of energy the body prefers to maintain. During the few hours after an interval workout, the body must go through a recovery period, and a lot needs to happen to return to the body to pre-exercise levels. "You've got to drop the heart rate, synthesize glycogen for the muscles, remove lactate from muscle and take it to the liver," says Karp. All these actions require energy, which raises the metabolic rate for many hours after exercising. Thirty minutes of exercise at a low intensity doesn't place nearly as much stress on the body as intervals, so you burn fewer calories both during and after the workout.

Before you hit the track and start running intervals, Karp recommends building a strong cardiovascular fitness base and working with a trainer for guidance. Even if you're not ready for sprints, play with your speed during your next cardio session. Slow and steady might win the race, but intervals will burn more calories.

Source: Flickr User pistoldrawn