MONDAY, Nov. 4, 2002 (HealthDayNews) -- Leaner boys and heavier girls tend to reach puberty earlier than their peers.
That's the conclusion of a new study in the November issue of Pediatrics from Dr. Youfa Wang, an assistant professor in the University of Illinois' department of human nutrition.
"The findings in boys -- a reverse association between early maturation and obesity -- which is different from those in girls, are new and have not been reported before, to my knowledge," Wang says.
Previous studies have shown that heavier girls tend to mature sexually at a faster rate, a fact borne out by Wang's study. However, he notes, it was surprising to find the opposite was true for boys.
Wang's research found boys who hit puberty earlier were taller and leaner than other boys. Girls who matured faster were heavier -- and taller -- than other girls.
There are many theories as to why heavier girls develop earlier, including the fact that subcutaneous fat tissue acts as a secondary hormonal gland. Since this is the first time leaner boys have been found to develop at a younger age, there are no theories to explain that trend, Wang says.
"Sexual maturity might have different biological influences on growth in weight and height in boys and girls, which may help explain the gender differences we observed for the associations between sexual maturity and fatness," Wang says. "During the growth process in boys, more energy may be devoted toward height than to the development of fat tissue, while early maturing girls are more likely to store extra energy intake as fat tissue."
Data for Wang's study came from boys and girls aged eight to 14 who were part of the Third National Health and Nutrition Examination Survey from 1988 to 1994. They had complete measurements of their weight, height and skin-fold thickness, as well as assessments of their sexual maturity.
Boys' maturity was measured by genitalia stages, while girls were classified according to breast stages. "Overweight" was defined as having a body mass index greater than or equal to the 85th percentile for age, according to year 2000 growth charts from the U.S. Centers for Disease Control and Prevention. "Obesity" was defined as equal to or greater than the 95th percentile.
Only 22.6 percent of males who matured early were overweight, compared to 31.6 percent of those who didn't mature early. Among females, 34.4 percent considered to have matured early were overweight, compared to 23.2 percent who didn't reach puberty sooner than average.
In terms of obesity, few males who matured early were obese -- 6.7 percent versus 14.8 percent of males who didn't mature early. The opposite was true for females. Those considered obese made up 15.6 percent of the girls who matured ahead of their peers, compared to 8.1 percent of those who matured at an average pace, the study says.
There were no exact ages established to define early developers. The researchers rated the children from one to five in terms of stage of development of the genitalia or breasts, then looked at weight and determined who weighed how much at which stage.
"For example, a girl would be classified as maturing early if she was at breast stage 2 and her chronological age was less than the median age for breast stage 2," Wang explains.
Dr. Ora Pescovitz, director of pediatric endocrinology at the Indiana University School of Medicine, says one shortcoming with the study was that it didn't specify the ages at which kids were reaching puberty. "How early is early? It wasn't exactly clear to me," she says.
Pescovitz says the brain is thought to control physical development, so it seems odd that boys and girls should have such diverse sexual maturity patterns. "I don't know why there should be sex differences," she says.
However, she adds, Wang's study is interesting and the findings warrant additional studies.
"I think it's the first to confirm that leanness in men contributes to early puberty. I think it's interesting, but I'm not convinced this is the final analysis," Pescovitz says.
What To Do
To learn more about the sexual development of children, visit the University of Washington or the National Library of Medicine.
