First, weight; next, puberty
Girls seem to be growing up faster these days, and not just because they dress to show more skin. Compared with their mothers, they actually have more skin to show -- and that added fat seems to be altering their rate of development.
Pediatric experts had noticed that girls appeared to be developing breasts (the first outward sign of puberty) at earlier ages -- and that they tended to gain weight around puberty. But no one knew which came first: earlier development or weight gain. By tracking girls’ weights from early ages, researchers have found that the extra pounds come first.
Because both early puberty and weight gain can affect girls’ health, the results add a new dimension to the importance of diet and exercise for girls.
In a study published in the March issue of the journal Pediatrics, a team of scientists monitored hundreds of girls from age 3 to 12 and found the heavier ones hit puberty earlier than their slimmer peers. Of the heaviest 15% of girls, 50% had started developing breasts by age 9. Because girls generally start puberty between the ages of 8 and 14, the researchers considered this “early,” though normal, puberty.
In fact, the girls’ weight as early as age 3 could predict a younger entrance into puberty, says lead study author Dr. Joyce Lee, a pediatric endocrinologist at the University of Michigan. “I thought, ‘Wow, these girls are still toddlers,’ ” she says.
In addition, the faster girls gained weight between 3 years of age and first grade, the more likely they were to begin to develop breasts by fourth grade.
The shift toward earlier puberty has worried parents and pediatricians because some studies show that girls whose bodies mature younger tend to misbehave in school and suffer from anxiety or depression, Lee says. In addition, longer exposures to sex hormones are thought to put women at greater risk for breast cancer.
But it’s the obesity that’s more problematic. Health problems such as diabetes, high blood pressure and heart disease due to obesity are better documented, experts say. “Parents should try to prevent excessive weight gain in their children,” Lee says.
*
What defines puberty?
Confusion over weight’s effects on puberty has been fueled in part by the various definitions of puberty in previous studies.
Some have used the development of breasts, others the appearance of pubic hair and still others the start of menstruation. The age at which all of these changes occur has shifted since the Industrial Age, but only changes in breast development have paralleled the obesity epidemic, presenting a challenge for researchers seeking to untangle the connection.
Researchers now largely define budding breasts as the first outward sign of puberty. In the 1960s, a child development expert named Dr. James Tanner created the scale still in use today to determine the extent of a girl’s progression into puberty. In his study of almost 200 British girls published in 1969, he found they start puberty at about 11 years. In 1997, Dr. Marcia Herman-Giddens, a doctor of public health at the University of North Carolina in Chapel Hill, found that this age had dropped to about 10 years.
The problem with trying to establish the onset of puberty by breast growth, researchers say, has been that extra weight makes breasts look a little larger than they are, and kids have been getting fatter over the last several decades. Researchers can tell the difference between breast tissue and fat by feeling the breast, but many studies have relied simply on an eye check. So researchers have sometimes mistaken plumpness for puberty.
Many parents think that the appearance of pubic hair signifies puberty, but it doesn’t. Pubic hair, along with underarm hair and body odor, is caused by hormones pouring from the adrenal glands, which aren’t directly involved in sexual maturation. “Pubic hair before 8 years of age is extremely common,” Kaplowitz says. “We see it in about 20% of black girls and 8% of white.”
Menarche is much easier to date than breast development, but has been more difficult to link to obesity. “If I asked when a woman had her first period, she could probably tell me to the year and month,” says Dr. Frank Biro, an adolescent medicine physician at Cincinnati Children’s Hospital Medical Center. He says studies in which people have to recall how old they were at the time of menarche are more accurate than asking a parent when his or her daughter started budding breasts.
But although menarche used to follow on the heels of breast development predictably, that’s no longer the case. For women born in the 1940s and 1950s, the age of puberty onset appears to correlate with menarche, but for women born in the 1970s, that connection is gone. For that reason, using menarche as an indication of puberty would not reveal a connection between obesity and sexual maturation.
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.
