Leptin has become a hot area for obesity research since the discovery of a mutation in the mouse leptin gene that increases the animals' appetite while lowering their metabolic rate. New findings, however, dampen the prospect that this hormone-like signal may explain differences in body fat among people.

The research was done at the Human Nutrition Research Center on Aging at Tufts, Boston. The center is funded by the Agricultural Research Service, USDA’s chief scientific agency.

The researchers found no relationship between the amount of leptin circulating in the blood of 61 men and women and the total number of calories they burned each day or their metabolic rate while resting or after eating. The study volunteers ranged in age from 18 to 81, and none were obese.

The researchers concluded in the September issue of Obesity Research that leptin doesn't influence energy regulation in adults by increasing their energy expenditure. In a study by others, young children with higher leptin levels reportedly burned more calories during physical activity. But the recent study indicates adults apparently lose their responsiveness to this signal.

Maintaining a stable body weight is a matter of burning as many calories as we consume. In people whose weight control mechanism is working properly, the body's metabolic rate automatically revs up after periods of overeating and slows down after periods of undereating to maintain this balance. Similarly, appetite automatically adjusts by decreasing or increasing. This process of energy regulation is controlled by a sequence of metabolic signals. But the details of that sequence are still sketchy.

To better understand leptin's role, Susan B. Roberts, who heads energy metabolism studies at the Boston center, and her colleagues examined how leptin might affect metabolic rate in adults. Because leptin is produced by fat cells, the volunteers who had more body fat also had higher blood leptin levels. But that didn't prompt them to burn more calories.

Scientific contact: Susan B. Roberts, Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University, Boston, Mass., phone (617) 556-3237, fax (617) 556-3344, roberts_em@hnrc.tufts.edu.