Can working out help us to drop pounds after all?

A provocative new study involving overweight men and women suggests that it probably can, undercutting a widespread notion that exercise, by itself, is worthless for weight loss.

But the findings also indicate that, to benefit, we may need to exercise quite a bit.

In theory, exercise should contribute substantially to weight loss. It burns calories. If we do not replace them, our bodies should achieve negative energy balance, use stored fat for fuel and shed pounds.

But life and our metabolisms are not predictable or fair, as multiple exercise studies involving people and animals show. In these experiments, participants lose less weight than would be expected, given the energy they expend during exercise.

The studies generally have concluded that the exercisers had compensated for the energy they had expended during exercise, either by eating more or moving less throughout the day. These compensations were often unwitting but effective.