By David N. Leff

Very thin, young girls encounter another side effect of their extremely slender physiques: Absence of body fat delays their puberty. (See related story on p. 1.)

Gymnasts and ballet dancers are examples of females whose reproductive years begin later than the average age of menarche.

“In the U.S., “ said molecular geneticist Farid Chehab, at the University of California, San Francisco, “onset of puberty long averaged 13 years of age. Recent studies show,“ he added, “owing to the high fat intake of young girls — what with junk food — it’s now coming on a few months earlier.“

It’s now a bell-shaped curve, Chehab pointed out, “peaking at 12.5 years of age, with extremes ranging from 8.5 to 16.5.

“Needless to say,“ he continued, “the food chain and human reproduction must have evolved very close to each other in prehistory. You need a certain amount of calories to sustain pregnancy. So when a girls starts to reproduce, it’s a signal from her stored body fat that tells the brain how much stored adipose tissue she has, and then switches on reproduction.“

The hormone that sends this signal, leptin by name, is synthesized in fat cells, and secreted into the blood en route to the brain.

Chehab is first author of an article in the current issue of Science, dated Jan. 3, 1997. Its title: “Early onset of reproductive function in normal female mice treated with leptin.“

“We took young female mice with no external signs of puberty,“ Chehab related, “and, by treating them with leptin, tricked their brains into believing that they were fat. Their brains thereupon triggered earlier reproduction in those rodents than in control animals, to whom we gave a saline placebo.“

That precocious murine puberty ranged from one to nine days ahead of controls.

Mice, Chehab explained, are born with sealed vaginas. Four or five days after it opens, they have their first estrous cycle, which corresponds to human menstruation.“

Female mice accept copulation only at the peak of estrus, when they are in heat. The male certifies its impregnation by inserting a proteinaceous barrier, the copulatory plug, into the vaginal entry. This plug signaled the researchers the precise day when an animal’s reproductive life had begun.

A few generations ago, the ideal feminine body image favored plumpness, even portliness, and early marriage.

Today, Chehab observed, “Take marathon-running women, who run long distances. It’s known that they lose fat and stop menstruating. So conceivably, leptin treatment could help them. Alternatively, to restore menstruation, they should run less and eat more.“

Leptin is a satiety factor,“ Chehab concluded. “Conceptually, the hormone might help such girls to correct their reproductive disturbances.“