Why So Many of Us Don’t Lose Weight When We Exercise

People hoping to lose weight with exercise often wind up being their own worst enemies, according to the latest, large-scale study of workouts, weight loss, and their frustrating interaction. The study, which carefully tracked how much people ate and moved after starting to exercise, found that many of them failed to lose or even gained weight while exercising because they also reflexively changed their lives in other, subtle ways. But a few people in the study did drop pounds, and their success could have lessons for the rest of us.

In a just and cogent universe, of course, the exercise would make us thin. Physical activity consumes calories, and if we burn calories without replacing them or reducing our overall energy expenditure, we enter negative energy balance. In that condition, we utilize our internal energy stores, which most of us would call our flab, and shed weight.

But human metabolisms are not always just and cogent, and multiple past studies have shown that most men and women who begin new exercise routines drop only about 30 percent or 40 percent as much weight as would be expected, given how many additional calories they are expending with exercise.

Why exercise underwhelms for weight reduction remains an open question, though. Scientists studying the issue agree that most of us compensate for the calories lost to exercise by eating more, moving less, or both. Our resting metabolic rates may also decline if we start to lose pounds. All of this shifts us back toward positive energy balance, otherwise known as weight gain.

So, for the new study, which was published last month in the American Journal of Clinical Nutrition, researchers with the Pennington Biomedical Research Center in Baton Rouge, La., and other institutions decided to exhort a large group of inactive people into exercising and closely track how their waistlines and daily habits changed.

Interestingly, the researchers also found that those exercisers who had compensated the most and lost the least weight tended to be those who had reported at the start that they thought some good health habits gave people license for other, insalubrious ones.

“In effect, they felt that it’s O.K. to trade behaviors,” says Timothy Church, an adjunct professor at Pennington who led the new study. “It’s the ‘if I jog now, I deserve that doughnut’ idea.”

In consequence, they lost little if any weight with exercise.

But the study produced other, more encouraging data, he says. For one thing, almost everyone’s resting metabolic rates remained unchanged; slowed metabolisms would encourage pounds to creep back. And those few exercisers who avoided an extra cookie or handful of crackers did lose weight.

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