Body fat linked to late puberty in boys

Boys can take a lot of ribbing from their peers for not being macho enough. A new study now indicates that it can take longer to begin transforming into a man if a boy starts out fat.

Back in 2007, pediatric endocrinologist Joyce Lee and her colleagues linked obesity in girls with an earlier-than-normal onset of puberty. Curious as to whether this trend was gender-specific, the University of Michigan physician and her coworkers pored over data that had been collected almost annually throughout a decade on 400 boys taking part in a federal Study of Early Child Care and Youth Development.

Being fat significantly increased the likelihood a boy exhibited zero signs of entering puberty by age 11.5, Lee’s group now reports in the February Archives of Pediatrics and Adolescent Medicine. Among boys that had been slim since early childhood, all but 7.7 percent had entered puberty by that age. In contrast, 14 percent of those who started chubby and went on to become fat still hadn’t entered puberty by 11.5 years old.

A handful of studies over the past few decades has offered hints that obesity might slow a boy’s reproductive development, Lee notes. But that research often involved tiny cohorts or boys predominantly undergoing screening because of their weight problems. The new analysis not only appears to be the first to investigate the issue in a fairly large and normal range of boys, but also a group that was followed from the age of 2.

That’s important, Lee says, because it helps identify which came first — “the chicken or the egg?” Obesity or slowed gonadal development?

In boys, identifying when puberty begins can be much tougher than in girls. Females undergo several landmark changes — breast development, the emergence of pubic hair and the beginning of monthly periods. For boys, Lee says, initial stages of puberty are largely characterized by a growth in the size of their gonads. And recognizing those changes, in their early stages, can be a judgment call for anyone not well trained in what to expect, Lee notes.

Right now, although she suspects that endocrine perturbations underlie the exaggerated incidence of delayed sexual maturation in fat boys, which hormone(s) are responsible remains elusive. But it’s something she plans to delve into — along with ascertaining the mean age at which puberty started and ended among participants of this study (who have been studied to at least age 15).

In the mean time, Lee offers a take-home message for parents: Youthful obesity’s biological impacts are not just a heightened risk of diabetes or heart disease once kids grow up. Being fat might alter the rate at which kids mature — with all of the psychosocial fallout that unusually early or late sexual development can bring.

Janet Raloff is the Editor, Digital of Science News Explores, a daily online magazine for middle school students. She started at Science News in 1977 as the environment and policy writer, specializing in toxicology. To her never-ending surprise, her daughter became a toxicologist.

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