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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.