Weighing the Evidence on Exercise – NYTimes.com.

How exercise affects body weight is one of the more intriguing and vexing issues in physiology. Exercise burns calories, no one doubts that, and so it should, in theory, produce weight loss, a fact that has prompted countless people to undertake exercise programs to shed pounds. Without significantly changing their diets, few succeed. “Anecdotally, all of us have been cornered by people claiming to have spent hours each week walking, running, stair-stepping, etc., and are displeased with the results on the scale or in the mirror,” wrote Barry Braun, an associate professor of kinesiology at the University of Massachusetts at Amherst, in the American College of Sports Medicine’s February newsletter.

But a growing body of science suggests that exercise does have an important role in weight loss. That role, however, is different from what many people expect and probably wish. The newest science suggests that exercise alone will not make you thin, but it may determine whether you stay thin, if you can achieve that state.