Exercise is a convenient way to justify a last-minute decision to indulge in dessert. What's more, afterburn -- the assumption that your body continues to burn calories post-workout -- was an even better way to rationalize those breaches of will power or even not dieting at all. Emphasis on was.

According to an April 2009 study published in the journal Exercise and Sport Science Reviews, exercise has little to no effect on the body's ability to burn calories up to 24 hours after a work out. Although most people are inclined to do double-duty by exercising and eating right, there are some who believe working out gives them a free pass to eat whatever they want while still losing weight. Well, the free ride is over.

"People will say, 'I exercise because it gives me license to eat whatever I want.' My research says no, that's not the case," says Edward Melanson, lead author of the study and an exercise physiologist and associate professor of medicine at the University of Colorado in Denver. "Your body doesn't magically burn that extra fat."After a decade of research, Melanson speaks with firm conviction that afterburn is a myth, but the idea is so pervasive that even he admitted that he was surprised by the results.

"I hate to be the one to have bleak news about exercise, but it's the reality," he says. "If you're overweight and using exercise to lose weight, then you have to be conscious of the calories you're eating. It seems simple, but the perception out there is, 'No, I exercise and I'm burning all this fat,' but you can't."

Melanson's research findings also hold true for resistance training, which has long been thought to be a catalyst for afterburn. And even though lean muscle may burn more calories than fat, the actual number of excess calories burned is much fewer than most people think.

"For every five pounds of lean muscle, only 35 calories per 24 hours are burned, which is rather negligible," says Pete McCall, a spokesman for the American Council on Exercise. As a trainer, McCall sees firsthand how influential this sort of misconception is. "I see people go straight from the health club to the Starbucks, and they'll get the frappuccino and a scone."

While the results of this research may be common sense for some regular exercise-goers, it may be quite the eye-opener for those who have struggled with weight loss. But really, it's simple math. If you burn 400 calories during one exercise session and then eat an extra 400 calories later that day, you're not going to experience any weight loss, so you need to budget your calories appropriately to lose a pound a week, which is approximately 3,500 calories.

The most important thing to remember is that this study should by no means negate the benefits of exercise, regardless of weight loss.

"I'm an exercise physiologist. I'm certainly not going to tell people not to exercise. It gives you a buffer to burn those calories," says Melanson. "There are many, many, many reasons to exercise, but if you're exercising to lose weight, you need to maintain an energy-balance."