Body fat: the hormone factor.

How can one person pig out and still stay slim, while another diets conscientiously, only to remain chubby? Exercise often plays a major role. But evolving research suggests that the number and size of fat cells, or adipocytes, a person carries may also influence the propensity for obesity. A new animal study offers a hormonal explanation for the wide adipocyte variations from one individual to the next. And, if confirmed in humans, the finding suggests the possibility of identifying some obesity-prone people at an early age, and perhaps treating them to thwart their adipocyte heritage.

Rats are born slim because the adipocytes that will eventually store their excess calories have not yet matured into a state that will accept lipids (fats). In recent cell-culture studies, biochemist Ginette Serrero identified the hormone that blocks this transformation of adipocyte precursors into fat cells. Now, she and Dianne Mills have confirmed that this hormone, called epidermal growth factor (EGF), also blocks adipocyte maturation -- and weight gain -- in rats.

Working at the W. Alton Jones Cell Science Center in Lake Placid, N.Y., the researchers selected 80 rats, each 24 hours old, for use in 10-day tests. All animals received a daily injection of saline or saline laced with enough EGF to deliver 0.1 to 1 microgram of the hormone per kilogram of body weight.

Compared with the saline-only rats, the EGF-treated animals showed a dose-dependent reduction in abdominal fat-storage "pads," Serrero and Mills report in the May 1 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES. Fat pads of rats receiving the most EGF weighed only half as much as those of untreated animals, contained only 25 percent as many mature adipocytes and accumulated only 20 percent as much lipid.

People who develop "early-onset obesity," which usually shows up by adolescence, suffer from excess adipose tissue, containing both more and larger fat cells than usual, explains Serrero. Her work with genetically obese mice has shown that they produce insufficient EGF. If humans predisposed to excess weight gain suffer from too little EGF, she says, then EGF assays might identify this condition. Moreover, she notes, "EGF or something like that might someday be used like a therapy" to head off obesity in these individuals.