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, 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.