Unlike overweight girls, who tend to enter puberty early, overweight and obese boys in the U.S. may begin puberty later than thin boys, according to one of the first longitudinal studies of weight and timing of puberty in males.

At 11.5 years, boys with the highest body mass index (mean BMI z score=1.84) were 165% more likely to be prepubertal than the thinnest boys (95% CI 1.05 to 6.61; P=0.04), researchers reported online in the Feb. 1 Archives of Pediatrics and Adolescent Medicine.

“This longitudinal study provides further evidence that higher BMI during early and middle childhood is not associated with earlier pubertal onset in boys, contrary to what is seen in girls,” Joyce M. Lee, MD, MPH, of the University of Michigan, and colleagues wrote.

“In fact, higher BMI in earlier childhood may be associated with and precede later onset of puberty among a population-based sample of U.S. boys.”

Rates of obesity among American girls and boys have nearly tripled since the 1960s, prompting concerns about the effect of excess weight on growth and development. Most research has focused on obese girls, who appear to reach puberty earlier than thin girls. A recent cross-sectional study suggested that, unlike their female counterparts, overweight boys may develop later.

To further explore this relationship, Lee and colleagues analyzed the records of 401 boys from diverse socioeconomic backgrounds in ten regions of the U.S., using data from the National Institute of Child Health and Human Development Study of Early Child Care and Youth Development. The participants were full-term, only children born in 1991.

The data included height and weight measurements of the children from ages 2 to 12 years and a visual assessment of whether the children had begun puberty, using Tanner genitalia staging, at 9.5, 10.5, and 11.5 years. Boys were defined as prepubertal if they were Tanner stage 1 at 11.5 years old and were otherwise categorized as pubertal.

Among the participants, 14.4% were overweight (BMI ≥ 85th and

<95th percentiles) and 19.4% were obese (BMI≥95th percentile) at age 11.5. Overall, 49 boys (12.2%) were prepubertal at age 11.5 years by Tanner genitalia staging.

The authors wrote that their findings have important implications for understanding sex differences in physiological mechanisms of puberty.

They noted that puberty is regulated by the gonadotropin-releasing hormone axis for both girls and boys, but it's unclear why such different associations between body fat and the timing of pubertal onset would exist between the sexes.

"Given the recent childhood obesity epidemic, additional studies are needed to further investigate the epidemiological link between body fat and pubertal initiation and progression in boys as well as the physiological mechanisms responsible," they concluded.

The authors were unable to analyze the data based on race, because most of the children in the study were white. They also noted that BMI is a surrogate measure of overall body fat, and that study has found that the relationship between body fat and BMI varies depending on race. They also recommended that future studies use multiple methods of determining whether children have entered puberty.

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Primary source: Archives of Pediatrics and Adolescent Medicine
Source reference: Lee J, et al “Body mass index and timing of pubertal initiation in boys” Arch Pediatr Adolesc Med 2010; 164: 139-44.