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Heavier boys more likely to hit puberty later

(Agencies)
Updated: 2010-02-09 09:15

NEW YORK - Overweight boys may start puberty later than their leaner peers, new research shows.

In girls, the opposite is true: Being heavy increases the likelihood that a girl will start puberty early.

The link between higher body mass index (BMI) -- a standard measure of the relationship between height and weight used to gauge how fat or thin a person is -- and earlier puberty is fairly well established in girls, Dr. Joyce M. Lee of the University of Michigan in Ann Arbor and her team note in the Archives of Pediatrics and Adolescent Medicine. However, much less research has been done on BMI and age at puberty in boys.

To investigate, they looked at 401 boys enrolled in the National Institute of Child Health and Human Development Study of Early Child Care and Youth Development. The boys' height and weights had been measured at several points, beginning when they were two years old and ending when they were eleven and a half.

Lee and her colleagues divided the boys into three groups based on how quickly their BMI increased during that time span. Fourteen percent of boys in the heaviest group had not started puberty by 11 and a half, compared to around 13 percent of boys in the middle group and 8 percent of boys in the lowest BMI group.

Lee said she couldn't provide a cutoff BMI or weight at which the risk of puberty delay increased for boys. However, she explained, an 11 and a half year-old boy of average height for his age (4 feet nine inches) in the heaviest group would weigh 125 pounds, on average, translating to a BMI of about 27, which is considered overweight. A boy in the lowest BMI group of the same height would weigh 77 pounds, for a BMI of almost 17, indicating a healthy weight.

Boys typically start showing pubertal changes shortly after their tenth birthday, Lee told Reuters Health. In boys, there's no clear sign of puberty, like the onset of menstruation in girls, making it a bit more difficult to study male puberty. "Basically when they start puberty it just means that their testicles and penises are starting to enlarge," she explained. In boys, she added, the process can take anywhere from three to five years to run its course.

Right now, Lee said, the leading theory as to why heavier girls start puberty earlier is that the hormone leptin, which rises as a person's fat mass increases, is somehow involved. The same hormonal process happens in boys, she added.

Researchers have also suspected that hormones in fat convert male hormones into female hormones, but there's no evidence to support this idea.

"Essentially, we're pretty much in the dark about how puberty works," Young said.