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First, weight; next, puberty

Heavier girls tend to develop sooner, but obesity is the real problem, experts say.

March 12, 2007|Mary Beckman | Special to The Times

Also, the ages of puberty's onset and menarche differ depending on the race of the girls and where they live, with nonwhites in the U.S. starting both younger and girls from developing countries starting later. And compared with the pre-Industrial Age, menarche has been arriving earlier and earlier. The average age of first menstruation in 1877 was almost 15 years; by the mid-1990s, the age had dropped to about 13 years. Most researchers say changes in the average age of menarche have leveled off, while the age for breast budding continues to decrease.

"Most people think the changes are due to an improved energy and nutritional balance. That is, more and better food, less physical activity," says evolutionary biologist Stephen Austad of the University of Texas Health Science Center in San Antonio. Athletic and anorexic girls experience the opposite of overweight girls -- their periods are delayed or stop if they lose too much weight.

Menarche's connection to nutrition might ensure conditions are favorable for reproduction. "There's a risk to carry a pregnancy if there's not enough food around," says Dr. Paul Kaplowitz of Children's National Medical Center in Washington, D.C.

Studies of immigration in Europe support this idea. Kids who have been adopted from less-developed countries tend to go into puberty earlier than the peers they left, says Biro, probably because their new homes provide more abundant food.


The weight link

The girls in the recent study have probably never been underfed. "Is the trend continuing now because of better nutrition or over-nutrition?" says Kaplowitz.

Researchers speculate that a hormone that helps to maintain a stable body weight could contribute to onset of puberty and menarche. The hormone leptin is made by fat tissue, and greater amounts of leptin in overweight girls might signal reproductive readiness. Lee says she can't tell from her work whether overweight 3-year-old girls who slim down will experience puberty later rather than earlier, but given the health consequences of obesity, it can't hurt to reduce the weight gain. She stresses that breast budding in girls between 8 and 9 years old is still normal and not cause for alarm. "The issue can be so sensationalized. Puberty is one of those issues people care about," she says.

Kaplowitz says parents should pay attention to how rapidly their kids progress, rather than the age at which they reach milestones.

Tests for unrelated genetic problems, as well as for hormone levels in the blood, help clarify whether the girls are indeed too young. For those who are maturing too early, treatments are available, but they're expensive. The majority of the kids brought in by parents concerned with early puberty are perfectly normal, he says. "Parents tend to panic."

Finally, Biro points out that the link between sex hormone exposure and breast cancer can be cast in doubt. "The risk is associated with age of menarche," he says, but the studies were conducted when breast budding and menarche matched up. The next generation of women will reveal whether the important factor is the start of puberty or menstruation.

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