The obesity epidemic may be producing a generation of late-blooming males.
Overweight boys tend to start puberty at a later age than their less-heavy peers, according to one of the first studies to chart the effects of childhood obesity on male sexual development.
Previous studies have focused primarily on girls - and came to an opposite conclusion. "Heavier girls tend to develop earlier, rather than later," said the lead author of the new study, Joyce Lee of C.S. Mott Children's Hospital in Ann Arbor, Mich.
Dr. Lee and her research colleagues aren't sure why excess weight seems to speed up the sexual maturation process in females while slowing it down in males. But one thing is obvious: "This tells us that boys and girls are truly different and there is a lot we don't understand about the hormonal mechanism governing puberty," Dr. Lee said.
For their study, the researchers analyzed data on 401 boys from diverse socioeconomic backgrounds from across the United States. Their growth and development was regularly measured between 2 and 12 years of age.
Puberty is a multistaged process lasting three to five years. About half of boys start going through a period of penis and testicular growth at roughly age 10, Dr. Lee said. But many of the overweight boys in the study still showed no genital changes by 11½ years of age, according to the findings published in the journal Archives of Pediatric & Adolescent Medicine.
Dr. Lee noted that fat is biologically active and can influence the level of certain hormones circulating in the body.
One possibility is that substances released by the fat act to convert the male hormone androgen into the female hormone estrogen, which could possibly hasten puberty in girls but delay it in boys.
But Dr. Lee stressed that more studies must be conducted before researchers have answers.
What's also uncertain is the long-term health and emotional consequences of having an altered sexual development.
"I think we can say that obesity is a stigma in itself, and when you add on to the teenage boy the fact that he is not developing like his peers, it is another stigma that could cause anxiety for the child and the parents."