Uncovering the role of leptin.

Uncovering the role of leptin.

Nobody said losing weight is easy, but keeping it off is usually harder: more than half of all successful dieters gain back the weight they lost-sometimes with interest-within 3 to 5 years. While no one knows why this happens, researchers at New York's Columbia University may have found one piece of the puzzle, in determining how reduced levels of the hormone leptin-a key player in the complex system of weight regulation-triggers the body to regain those hard-lost pounds.

Leptin, which is made in fat tissue, tells the brain how much energy (a.k.a. calories) is stored in the body. When you lose weight, you have less leptin than your body is accustomed to. That signals the body to burn calories more slowly and increase food intake. The new findings suggest that the brain interprets a new, lower body weight as a signal of leptin deficiency-and works to increase leptin levels by setting off metabolic changes that promote weight regain.

In the study, researchers measured the natural leptin levels of ten people-seven of them obese-then put them on liquid diets that caused them to lose 10 percent of their body weight. For five weeks afterward, the subjects continued on a liquid diet, with calories adjusted to their new weight's requirements-plus daily leptin injections to restore their leptin to pre-weight-loss levels. During the study period, none of the subjects gained back any weight-and most even continued to lose a little more: in effect, the leptin reversed the metabolic shifts that would normally have fought the body's new, slimmer self.

Without the leptin, the pounds would likely have crept back, explains lead researcher Michael Rosenbaum, M.D., because a body that has lost weight is more efficient at using calories than the the same-size body of a nondieter. "People who lose 10 percent of their body weight, say from 167 down to 150 pounds, will need to eat 300 to 400 calories less per day, or increase their activity by 300 to 400 calories more per day, than someone who is ‘naturally' at 150 pounds."

Can leptin, then, help us stay slimmer? "Leptin is not a weight-loss drug," Rosenbaum emphasizes; and currently, it's only available for research. But someday it may help dieters maintain their new svelte selves by making the body sense that its fat stores are adequate. "This is only a short-term study," he adds. "Much longer studies are needed to determine whether these findings will persist."

-Jill Weisenberger, M.S., R.D., C.D.E

Illustration by Heidi Younger