SAN DIEGO, March 3 -- The so-called "fat" hormone apparently hates exercise, according to a study being presented today at the 40th Annual Conference on Cardiovascular Disease Epidemiology and Prevention.
Researchers say lean, physically active people have low levels of a hormone called leptin, which is produced by the body's fat cells and is believed to be a major culprit in causing obesity. The study finds that when couch potatoes start exercising regularly, their leptin levels decrease.
Researchers at Harvard School of Public Health in Boston found that regular vigorous physical activity dramatically lowered leptin levels in the blood of both normal weight and overweight men, which may also protect their hearts. The average leptin level was 5.8 nanograms/millimeter. Each 20 MET increase in physical activity (the equivalent of about three hours of jogging per week per week) was associated with a 10 percent lower level of leptin.
"We don't know yet if a high level of leptin is an independent risk factor for heart disease," says Nain-Feng Chu, M.D., Dr.P.H., who headed the study. "But we do know that obesity raises a person's risk of a heart attack and stroke, and our findings provide evidence that physical activity may reduce the chronic disease risk in men through changes in leptin levels."
The researchers looked at the dietary and lifestyle habits of 268 men ages 47 to 83. All of them were free of cardiovascular disease, diabetes and cancer. Each man in the study completed a questionnaire on what types of food he ate, whether or not he smoked, how much he exercised and whether or not he drank alcohol and if so, how much.
Men whose leptin levels were the highest -- 14.4 ng/ml-- weighed more, didn't exercise and ate more foods high in saturated fat and cholesterol, compared men whose leptin levels were lowest -- 3 ng/ml.
"And, men whose bodies produce more insulin also tend to have increased leptin levels," Chu says. Leptin's association with insulin production could mean that it plays a role in the development of diabetes, a disease in which the body fails to properly regulate glucose and insulin.
Molecular biologists first identified leptin in 1994, and the Harvard study -- the first of its kind began later that same year. Since then, leptin has been the subject of intense speculation among scientists.
"Five years ago, it was believed that leptin research might offer a miracle solution to obesity and the many health problems associated with being overweight," says Eric B. Rimm, Sc.D., associate professor of epidemiology and nutrition at Harvard School of Public Health, and a co-author of the study. "By turning the gene that manufactures leptin off and on in laboratory mice, it was possible to either increase or block their leptin production and influence obesity levels.
Although regulating levels of leptin in the human bloodstream isn't nearly as simple as it is in mice, and its total role in obesity must still be determined, two facts are clear, according to Rimm.
"As you get fatter, your body makes more leptin," he says. "But when you exercise, the amount of leptin in the blood decreases. Most of the time, you also lose weight, which means your risk of heart disease decreases too." Co-authors of the study are Meir Stampfer, M.D., Dr.P.H.; Donna Spiegelman, S.D. and Gokhan S. Hotamisligil, M.D., Ph.D.
Media Advisory: Dr. Chu can be reached at (617) 731-4618. Dr. Rimm can be reached at (617) 432-1843. (Please do not publish phone numbers.)