The effects of exercise on the appetite and energy systems, however, are by no means consistent. In one study presented last year at the annual conference of the American College of Sports Medicine, when healthy young men ran for an hour and a half on a treadmill at a fairly high intensity, their blood concentrations of acylated ghrelin fell, and food held little appeal for the rest of that day. Exercise blunted their appetites. A study that Braun oversaw and that was published last year by The American Journal of Physiology had a slightly different outcome. In it, 18 overweight men and women walked on treadmills in multiple sessions while either eating enough that day to replace the calories burned during exercise or not. Afterward, the men displayed little or no changes in their energy-regulating hormones or their appetites, much as in the other study. But the women uniformly had increased blood concentrations of acylated ghrelin and decreased concentrations of insulin after the sessions in which they had eaten less than they had burned. Their bodies were directing them to replace the lost calories. In physiological terms, the results “are consistent with the paradigm that mechanisms to maintain body fat are more effective in women,” Braun and his colleagues wrote. In practical terms, the results are scientific proof that life is unfair. Female bodies, inspired almost certainly “by a biological need to maintain energy stores for reproduction,” Braun says, fight hard to hold on to every ounce of fat. Exercise for many women (and for some men) increases the desire to eat.

Thankfully there has lately been some more encouraging news about exercise and weight loss, including for women. In a study published late last month in The Journal of the American Medical Association, researchers from Harvard University looked at the weight-change histories of more than 34,000 participants in a women’s health study. The women began the study middle-aged (at an average of about 54 years) and were followed for 13 years. During that time, the women gained, on average, six pounds. Some packed on considerably more. But a small subset gained far less, coming close to maintaining the body size with which they started the study. Those were the women who reported exercising almost every day for an hour or so. The exercise involved was not strenuous. “It was the equivalent of brisk walking,” says I-Min Lee, a researcher at Harvard Medical School and Brigham and Women’s Hospital and the lead author of the study. But it was consistently engaged in over the years. “It wasn’t something the women started and stopped,” Lee says. “It was something they’d been doing for years.” The women who exercised also tended to have lower body weights to start with. All began the study with a body-mass index below 25, the high end of normal weight. “We didn’t look at this, but it’s probably safe to speculate that it’s easier and more pleasant to exercise if you’re not already heavy,” Lee says.

Image Credit... James Welling for The New York Times

On the other hand, if you can somehow pry off the pounds, exercise may be the most important element in keeping the weight off. “When you look at the results in the National Weight Control Registry,” Braun says, “you see over and over that exercise is one constant among people who’ve maintained their weight loss.” About 90 percent of the people in the registry who have shed pounds and kept them at bay worked out, a result also seen in recent studies. In one representative experiment from last year, 97 healthy, slightly overweight women were put on an 800-calorie diet until they lost an average of about 27 pounds each. Some of the women were then assigned to a walking program, some were put on a weight-training regimen and others were assigned no exercise; all returned to their old eating habits. Those who stuck with either of the exercise programs regained less weight than those who didn’t exercise and, even more striking, did not regain weight around their middles. The women who didn’t exercise regained their weight and preferentially packed on these new pounds around their abdomens. It’s well known that abdominal fat is particularly unhealthful, contributing significantly to metabolic disruptions and heart disease.