Weight control update: Just do it ... and do it and do it and do it

Yes, exercise burns calories. But according to a new study, simply being fit won't boost your metabolism as much as you might expect. University of Vermont researchers had previously sedentary (but not obese) women, ages 18-35, do either six months of resistance or endurance training, gradually increasing the intensity under a trainer's direction.

The resistance exercisers, who worked out on machines, gained muscle strength and lost fat; the endurance exercisers, who jogged and ran, upped their aerobic capacity by 18 percent -- though they showed little change in body composition. But, except for the expected rise in resting metabolic rate due to increased muscle mass, none of the women studied showed a significant change in their daily energy expenditure. "The benefits came primarily from the energy they used while exercising," says Eric Poehlman, Ph.D., a professor of nutrition and medicine at the university.

Though Poehlman had expected that these newly fit women would burn extra calories by being more physically active the rest of the day, none of them spontaneously upped their daily activity levels. Still, his research shows once again that exercise burns calories, and strength training elevates your resting metabolism in proportion to the amount of lean tissue you add.

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