Bone Density and Timing of Puberty in a Longitudinal Study of Girls.

STUDY OBJECTIVE: Primary: To examine the relationship between relative timing of puberty with bone mineral density (BMD) in a group of adolescent girls; Secondary: To determine if family history of breast cancer was associated with bone mineral density.

DESIGN/SETTING/PARTICIPANTS: Longitudinal study of girls recruited between 6 and 7 years of age seen every 6 months for 5 years, and subsequently seen annually. BMD of the lumbar spine was measured by dual-energy X-ray absorptiometry (DXA) at mean age of 12.5 years; age- and race-specific Z-scores (BMDz) were calculated. Age of pubertal onset was determined by the first occurrence of breast stage 2, and participants were categorized into race-specific early, on-time and late puberty onset groups.

MAIN OUTCOME MEASURES: BMDz by timing of pubertal onset, and by family history of breast cancer.

RESULTS: DXA scans were performed on 227 study participants, and a second scan was performed on 114 participants 2 years later. Age of onset of puberty was inversely correlated with BMDz, r = -0.31 (P < .0001). There was no association between BMDz and family history of breast cancer.

CONCLUSIONS: Earlier timing of puberty was associated with higher BMD. The high shared variance of BMD and timing of pubertal onset implies an underlying biologic basis.