Is weight simply a matter of willpower? You might think so, given the number of dieters who add on exercise, subtract food, and expect excess fat to melt away. But it does not always happen that way. Older athletes notice the fat that creeps on year after year seems harder to lose. And others who have slimmed down complain how easily they regain lost body fat. The Endocrine Society recently took a close look at these issues affecting so many of us. They describe fat gain as a “disorder of the body’s energy balance system,” not just a passive accumulation of excess calories. They highlight many factors other than food and exercise that influence body fatness, including genetics, the environment, and evolution. No surprise, genetic factors alone fail to explain the rapid increase in obesity during the past 40 years. We need to learn more about the combined impact of genes plus environmental toxins, highly processed foods, a sedentary lifestyle, antibiotics, and other factors. Exercise does play a role in weight management, but less than you might think. Exercise alone is largely ineffective as a means to lose weight, even though it contributes to a calorie deficit. For some, it triggers the urge to eat more. So you want to be sure your reason to exercise is to enhance health, not lose weight.