People who exercise don’t necessarily lose weight, the New York Times reports: a September study in the British Journal of Sports Medicine was just one of many to highlight this, tracking 58 obese people who did 12 weeks of supervised aerobic training, leaving their diets the same. The group lost an average of just over seven pounds, with many participants losing barely half that. In fact, few people lose weight with exercise alone. Another study from Denver’s Colorado School of Medicine compared endurance athletes, who are sedentary and lean, with people who are sedentery and obese. In the study, subjects agreed to spend several 24-hour periods in a special lab room (a walk-in calorimeter) that measures how many calories a person burns. The researchers looked at whether the calories used were from fat or carbohydrates, the body’s two main fuel sources; burning fat is best for weight loss, since it comes from body fat stores. People have long believed that exercise revs up the metabolism, burning fat throughout the day. But to their surprise, researchers found that none of their subjects, not even the athletes, experienced this effect: they didn’t use extra body fat after exercising. According to Edward Melanson, lead author of the study, “It all comes down to energy balance,” or calories in and out. People “are only burning 200 or 300 calories” in a 30-minute exercise session, he adds. “You replace that with one bottle of Gatorade.”