Although overweight girls are more likely to start their periods earlier than their peers who are at or below normal weight, early menstruation is not by itself a risk factor for later obesity, according to a long-term study released this week.
In fact, it's more likely that excess body fat jump-starts puberty than the other way around. Additionally, girls who are overweight before their first periods are almost eight times more likely than their slimmer peers to be overweight as adults.
"It has been long known that if you are overweight as you grow up, you are more likely to begin puberty early," said Aviva Must, Ph.D., associate professor of Public Health and Family Medicine at Tufts University School of Medicine in Boston and lead author of the study, published in the September issue of the journal Pediatrics. "Girls who are overweight are more likely to have early menarche, or start their period, before age 12. I have been concerned that a widespread belief was forming that the timing of menarche was itself linked to later weight status."
This belief, which may have led physicians to focus on changing the timing of puberty as a way to combat adult obesity, was discounted by Dr. Must's findings. "These findings are significant because they show us where our efforts should focus: childhood obesity," said Dr. Must, who is also a scientist at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts. She spoke today in New York City at the American Medical Association and National PTA media briefing, Back to School: Child and Adolescent Health.
The new paper used data from the Newton Girls Study, which followed a group of about 700 girls from a small city near Boston. The girls were recruited in 1965 and studied from before their first period through their 20th period. The girls or their mothers provided monthly reports, allowing researchers to accurately pinpoint their age at menarche. Most studies rely on subjects recalling the age at which they got their first period, which may be remembered with error, Dr. Must said.
Dr. Must and her coauthors contacted the original participants of the Newton Girls Study 30 years later. About 450 of the women, with an average age of 42, participated in the follow-up. The women provided their current weight and height and researchers computed their body mass index (BMI), a weight measure that is adjusted for height and can be used to determine whether a person is overweight or obese. About 28 percent of the women were overweight and 9 percent were obese. In addition, 153 of the women came to the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts for a laboratory analysis of their body fat percentage.
The women whose BMIs indicated they were overweight before their first periods were 7.7 times more likely to be overweight as adults than those who were at or below normal weight at the beginning of the original study. This effect was even more pronounced when level of fat was evaluated for those women who came into the lab for more accurate body fat measurements. Neither the girls' age at menarche nor the interaction between their weight and the age at menarche accurately predicted whether or not they would have more fat as adults.
Parents can learn something from this study whether their daughters are overweight or not, said Dr. Must. "The parents of an overweight child may expect that their child is going to mature earlier," she said. "She may be taller than her classmates before menarche, and she may have her first menstrual period and start developing breasts before her leaner girlfriends. That is not a cause for concern; it is part of normal growth and development for an overweight child. For the parents of a girl who is not overweight and who gets her first period early, it doesn't mean she's at increased risk for being overweight as an adult."