Thinning Protein Has Sex Appeal

WASHINGTON, D.C.--A protein that seems to regulate hunger and weight loss may also trigger an entirely different kind of appetite: one's sex drive. Findings presented here yesterday at the Society for Neuroscience's annual meeting strengthen the notion that leptin--the subject of a flurry of research since scientists reported 2 years ago that it made obese mice thinner--helps regulate sexual maturity and reproduction.

Endocrinologist Jeffrey Flier and colleagues at Beth Israel Hospital in Boston injected small amounts of leptin into mice after weaning. Normal mice reach sexual maturity at about 30 days old, but the leptin-treated mice blossomed about 3 days earlier, Flier says. In a second experiment, the researchers measured blood levels of leptin in eight boys involved in a study on puberty. They found that leptin levels increased sharply--to two or three times their normal level--at the same time the hormone testosterone first becomes measurable in the blood.

The findings point to a role for leptin in the onset of puberty. They also dovetail with work Flier's group published this summer, which showed that leptin reverses loss of fertility and other starvation-induced hormonal changes in mice, says Arthur Campfield, who studies metabolic diseases at Hoffmann-La Roche in Nutley, New Jersey. The work bolsters the increasingly popular idea that leptin's main role in the body is not to prevent overeating; rather, it is to help protect against starvation by regulating the complex neuroendocrine system that governs reproduction and metabolism, Campfield says. "It's really the dominant thing it's designed to do," he says. This view of the protein makes both biological and evolutionary sense, Campfield says, because starvation is a greater threat to most organisms than are the long-term health effects of obesity.