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People who exercise, then reach for more snacks might not lose weight, but they will alter their body chemistry in a way that burns up more fat, a researcher reports.

Dr. Jorge Calles, an endocrinologist at the University of Vermont, offered what he called the first carefully controlled study to show that fat burning increases significantly with exercise, even if people eat too much.People get their energy by burning a combination of fat and carbohydrates, Calles said. In people who are naturally lean, the ratio of fat burned to carbohydrates burned is higher than in people who are overweight.

Calles found that exercise shifts that ratio so people get a higher percentage of their energy from burning fat.

He reported his findings Tuesday at the Seventh International Congress on Obesity.

Calles said the study supports the idea that exercising can help people lose weight. That might seem obvious, but it has been the subject of debate among obesity specialists, he said.

"There's no question about the cardiovascular benefits and no question exercise is helpful for people with diabetes," Calles said. "But for treatment of obesity, there is a debate.

"We are beginning to show that indeed exercise can be beneficial."

Dr. Richard Atkinson, an obesity specialist at the University of Wisconsin, said that individual variations in fat burning help explain why some people seem to be able to eat a lot without gaining weight, while others put on weight much more easily.

Calles' findings are an important part of the argument that exercise can help people lose weight, said Atkinson.

Calles' study involved 20 people. Some were overfed 1,000 calories per day and were not allowed to exercise, while others were overfed and assigned to do just enough exercise to burn up the extra 1,000 calories.

In those who were overfed without exercise, the amount of fat burned dropped from the equivalent of 518 calories per day to 97 calories per day. The burning of carbohydrates rose.

In those who were overfed and who exercised, fat burning rose from the equivalent of 406 calories per day to 685, while the burning of carbohydrates fell.

In a separate study, John Blundell and colleagues at the University of Leeds in England found that exercise can lead to appetite suppression and weight loss. But they also found that people who exercise have to watch the fat content of their diets, because high-fat foods can overwhelm the beneficial effects of exercise.