FITNESS : Exercise alone won't provide desired weight loss. Your metabolism's response to workouts may be key to optimizing exercise.

For every 100 calories we might expect to burn as a result of working out, most of us will actually net fewer than 71 calories burned, according to an eye opening new study of how physical activity affects our metabolisms.

The study finds that our bodies tend to automatically compensate for at least a quarter of the calories we expend during exercise, undermining our best efforts to drop pounds by working out.

The results also show that carrying extra pounds compounds calorie compensation, making weight loss through exercise even more elusive for those who are already overweight.

But the study suggests, too, that calorie compensation varies from person to person, and that learning how your metabolism responds to workouts may be kept to optimizing exercise for weight control.

In theory - or in a kindlier, alternative universe - exercise would aid substantially in weight loss. When we move, our muscles contract, requiring more fuel than at rest, while other organs and biological systems likewise expend extra energy.

Thanks to past studies, we know approximately how much energy these processes demand. Walking a mile, for instance, burns about 100 calories, depending on one's weight and walking speed.

Until recently, most people, including exercise scientists, assumed that this process would be addictive - that is, stroll a single mile, burn 100 calories. Stroll two, burn 200, and so on, in logical mathematical fashion.

If we do not then replace those calories with extra food, we should wind up burning more calories than we consume that day and start dropping pounds. But that rational outcome rarely happens.

Most people who begin a new exercise program lose less weight than would be expected based on the number of calories they burn during their workouts, even if they strictly monitor their diets.

So some scientists started speculating that energy expenditure might be less elastic than we had thought. In other words, it might have limits. That possibility gained traction in 2012, with the publication of an influential study of African hunter-gatherers. It showed that, although the tribespeople regularly walked or jogged for hours, they burned about the same number of total daily calories as relatively sedentary Western men and women.

Somehow, the study authors realized, the active tribespeople's bodies were compensating, dialling back over all calorie burning, so that they avoided starvation as they stalked their food.

Other small studies since have reinforced the finding that more activity does not necessarily result in greater daily calorie expenditure.

But few large-scale experiments have tried to pin down just how much our bodies compensate for the calories burned while moving, since measuring metabolic activity in people is complex and expensive.

The publishing continues in the future to Part 2. The World Students Society thanks author Gretchen Reynolds.


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