Just last month 2007, the American Heart Association and the American College of Sports Medicine published joint guidelines for physical activity and health. 'It is reasonable to assume that persons with relatively high daily energy expenditures would be less likely to gain weight over time compared with those who have low energy expenditures. So far, data to support this hypothesis is not particularly compelling.' In other words, despite half a century of efforts to prove otherwise, scientists still can't say exercise will help keep the pounds off.