CHAPEL HILL - Being overweight is closely associated with earlier-than-average puberty in white girls but much less so in black girls, the first large study of the subject shows. While a link between obesity and onset of puberty also exists among black girls, the connection is weak, researchers say, but they don't know why.
Something else must be happening among the latter children, they say. "It might be that genetic or environmental factors are having a stronger effect on black girls in terms of when puberty starts," said Dr. Paul Kaplowitz, associate professor of pediatrics at the Virginia Commonwealth University School of Medicine.
Kaplowitz is lead author of a paper appearing in the August issue of the journal Pediatrics. "Whether early puberty in girls is a problem or not depends on how you look at it," he said. "A lot of people are concerned that early maturing girls might feel isolated and different from their peers. In extreme cases, they can end up rather short because if they start growing early, they may be mostly grown by the time they are 10."
The pediatric endocrinologist carried out the study with several colleagues including Dr. Marcia Herman-Giddens, adjunct professor of maternal and child health at the University of North Carolina at Chapel Hill School of Public Health.
Herman-Giddens made national news four years ago and since then as author of a paper in Pediatrics indicating that U.S. girls of both races appeared to enter puberty earlier than they did in previous decades. Black girls start maturing about a year before whites, her study showed.
A lively debate has since ensued among physicians over whether U.S. girls now mature earlier than they once did. "Dr. Kaplowitz thought that since a higher percentage of African-American girls are overweight, that could be the reason we see that they begin puberty a year earlier than white girls on average," Herman-Giddens said. "That was a good possibility, but we were surprised to find that it wasn't true, and now we'll have to look for other reasons for what's happening in both races."
Hyperinsulinism, for example, an unhealthy condition in which the pancreas is overly stimulated to produce insulin and then becomes insulin-resistant, might play an important role, she said. Hyperinsulinism is more common in black girls and in blacks in general.
The new study, which did not prove that excessive weight causes early puberty but just established a connection, involved analyzing body mass index data in an original sample of more than 17,000 U.S. girls. Body mass index is a combination of height and weight measures.
Researchers concentrated on 6- through 9-year-olds since puberty before those ages is rare and increasingly common afterwards. Data came from the American Academy of Pediatrics' Pediatric Research in Office Settings network, a group of medical practices around the country that pool patient information for analysis, and was the same Herman-Giddens used in her initial study.
Researchers compared the body mass indexes of girls who were pre-pubertal with those who had already begun physical maturation. Seven-year-old white girls weighed an average of 53 pounds if pre-pubertal and 60 pounds if they already had begun maturing, the scientists found. The weight difference was less for black girls.
Herman-Giddens dismissed recent criticism of her 1997 study by a small group of endocrinologists, saying it made no sense to favor a study of 192 white, institutionalized English girls in the 1960s over a much more recent U.S. sample of more than 17,000 subjects of both races.
"Criticisms about whether clinicians could tell if subjects already had begun developing breasts or were just overweight also were invalid, she said. "Dr. Kaplowitz published a paper previously showing that pediatricians' mistakes in distinguishing developing breast tissue from fatty tissue tended to cancel one another out and therefore would not affect study results. Forty percent of the girls had their breast tissue assessed to assure accuracy."
Some research projects focusing on heart disease and other subjects also have generated pubertal data on children as a side issue, Herman Giddens said. "They are all finding that girls are entering puberty earlier, and they are uncovering differences between blacks and whites," she said. "The latest NHANES growth data, which comes from a large national sample, shows that both boys and girls are getting their growth spurts earlier now than they were 30 or 40 years ago."
Growth spurts tend to occur in girls in early puberty and in boys during mid-puberty. "My own bias is that the majority of girls who start puberty between ages 6 and 8 are not at risk of serious psychological or physical problems," Kaplowitz said. "These girls can simply be monitored."
"If environmental conditions are influencing girls to develop earlier than they used to, we ought to try to find out what they are because they are not likely to be healthy," Herman-Giddens said. "Thirty or 40 years ago, the dropping age of puberty was explained by better nutrition and less infectious disease. Now we are beyond that point and have about a quarter of U.S. children who are either overweight or obese, which is definitely unhealthy."
Other authors of the study were Drs. Eric Slora and Richard Wasserman of the Pediatric Research in Office Settings network and biostatistician Steve Pedlow of the University of Chicago.
Note: Kaplowitz can be reached at 808-828-9616, Herman-Giddens at 919-542-2529 or 542-5573. E-mail addresses: firstname.lastname@example.org, email@example.com. UNC News Services Contact: David Williamson, 919-962-8596, UNC School of Public Health Contact: Lisa Katz, 919-966-7467.