Exercise has countless health benefits, but weight loss may not be one of them. A challenging new study shows that many people gain weight after starting a physical activity program, and what’s worse, much of that gain is excess fat rather than muscle. But it was this same study that first identified a simple strategy that could actually increase people’s chances of losing weight through exercise. We all know that the basics of weight loss are pretty simple. As long as you burn more calories than you take in each day, you’ll lose more and more weight over time. Theoretically, reducing calorie intake through dieting or increasing calorie expenditure through exercise can achieve these ideal conditions. In reality, however, most people try everything and still fail to achieve or maintain the desired weight loss. Exercise is particularly problematic in this regard. A recent review of research on exercise and weight control found that in most studies, people lost only one-third of the weight they expected to lose after exercise when converted to the number of calories burned during exercise. Many of the studies also reported that even when the same exercise program was implemented, there was a huge difference between the changes in waist circumference among different people, with some losing weight and others gaining more. But scientists still don’t have a clue as to why exercise helps some people shed pounds but not others, and it’s not clear whether there are any early indicators that predict how effective regular exercise will be for a specific person. A new study was published last month in The Journal of Strength and Conditioning Research. In the study, scientists at Arizona State University in Phoenix recruited 81 healthy adult women who were habitually sedentary. All of them were overweight, as judged by their body mass index (B.M.I.), but some of them were more significantly overweight. None of them had exercised regularly in the past year. The scientists told the women that they would be participating in a fitness study with the goal of increasing aerobic endurance and asked them not to make any changes to their previous eating habits. At the beginning of the study, each volunteer came to the physiology lab and was measured for a variety of health and fitness indicators such as weight, B.M.I., body fat percentage, and current endurance level. Subsequently, the women began a supervised exercise program and, according to the study’s senior author, Glenn Gaesser, a professor of nutrition and health promotion at Arizona State University, the program was designed to be intense, but within the range of what is acceptable for most people. The women were required to complete a 30-minute walking workout on a laboratory treadmill three times a week at a pace that was maintained at 80 percent of their maximum endurance. The exercise program lasted 12 weeks, during which time the scientists reviewed the subjects’ fitness metrics once a month, just as they had done at the beginning. At the end of week 12, all of the women’s aerobic capacity was significantly better than at the start, but a number of them were fatter. Nearly 70 percent of the women gained body fat mass throughout the study, with several gaining up to 4.5 kilograms, and most of that was fat rather than muscle gain. Still, a few women lost an equal or even greater amount of fat, and a significant portion of the women’s weight remained the same as at the start of the study. At this point, the researchers went back and looked at the data obtained on the first day of the study, wanting to determine if there were any significant differences between the women who gained or lost weight later. Dr. Geisel said, “Some previous studies on dieting have shown” that women who are “heavier at the beginning of a weight loss program are more likely to lose weight during exercise.” But in this study, the researchers found no such correlation between the weights of the women at the beginning and end of the study. In fact, the scientists found no correlation between any of the women’s health and fitness parameters at the start of the study and the effectiveness of the exercise program for them. But as they looked more deeply into the data, they found an interesting indication: women who lost weight after four weeks of workouts tended to be more likely to continue losing weight in subsequent workouts, which was not the case in others. “The practical guideline for this is that if you want to rely on exercise to lose weight, “you should go to the bathroom and weigh yourself after a month of working out,” says Dr. Geisel. If at that point you still don’t see any weight loss or instead get heavier, “you need to pay close attention to your diet and other activities.” The study didn’t track the subjects’ eating and exercise habits outside of the lab, but Dr. Geisel thinks it’s likely that women who gained weight after a workout were eating more and exercising less outside of a treadmill workout, “although that shouldn’t be intentional either.” Of course, the study was fairly short and didn’t involve men, though there have been a number of previous studies that have shown that men as often as women experience an increase in body fat mass after exercise instead. While in some ways, these findings make the weight-loss bonanza feel like a grim situation, they also point in the direction of hope. According to Dr. Geisel, it is entirely possible to lose weight with exercise, strict self-discipline, and a scale that sits in your bathroom. What’s more, after four months of exercise, these women in the study were all in much better shape. Dr. Geisel says, “Fitness is more about getting healthier than it is about simply trying to change those numbers on the scale.”