Body mass index and timing of pubertal initiation in boys.

Objective: To examine the association between body mass index (BMI [calculated as weight in kilograms divided by height in meters squared]) and timing of pubertal onset in a population-based sample of US boys.

Design: Longitudinal prospective study.

Setting: Ten US sites that participated in the National Institute of Child Health and Human Development Study of Early Child Care and Youth Development.

Participants: Of 705 boys initially enrolled in the study, information about height and weight measures and pubertal stage by age 11.5 years was available for 401 boys.

Main Exposure: The BMI trajectory created from measured heights and weights at ages 2, 3, 4.5, 7, 9, 9.5, 10.5, and 11.5 years.

Main Outcome Measure: Onset of puberty at age 11.5 years as measured by Tanner genitalia staging.

Results: Boys in the highest BMI trajectory (mean BMI z score at age 11.5 years, 1.84) had a greater relative risk of being prepubertal compared with boys in the lowest BMI trajectory (mean BMI z score at age 11.5 years, -0.76) (adjusted relative risk = 2.63; 95% confidence interval, 1.05-6.61; P = .04).

Conclusions: The relationship between body fat and timing of pubertal onset is not the same in boys as it is in girls. Further studies are needed to better understand the physiological link between body fat and timing of pubertal onset in both sexes.