Context: Age at menarche (AgeM) is earlier in African American (AA) than in European American (EA) girls. Neither the physiologic cause nor the health implications of this difference is known.

Objective: We tested the hypotheses that higher insulin among AA vs EA precipitates an earlier elevation of estradiol (E2), an associated earlier AgeM, and greater gain in body fat.

Setting: University research laboratory and General Clinical Research Center.

Participants: Subjects were 137 girls (57 AA and 79 EA) aged 7–15 yrs.

Design: Longitudinal. Annual evaluations were conducted for body composition by dual-energy X-ray absorptiometry, acute insulin response to glucose (AIRg) by intravenous glucose tolerance test, and reproductive-endocrine profile.

Main outcome measures: Multiple linear regression modeling and mixed model analyses were used to identify independent predictors of AgeM and E2 concentration at menarche.

Results: AgeM was significantly earlier in AA vs EA (10.8 vs 11.6 yrs). Neither E2 nor insulin was a significant independent predictor of AgeM. AIRg was a significant predictor of E2 concentration. AA had higher E2 than EA (p<0.01), and girls with higher AIRg had higher E2. Total fat increased with age in both EA and AA. However, among EA, the increase in fat mass was similar both before and after menarche (9.4%/yr before vs 10.0%/yr after), whereas among AA, fat deposition nearly doubled after menarche (8.4%/yr before vs 14.9%/yr after).

Conclusion: Results did not support a direct cause-and-effect relationship between higher insulin, higher E2, and earlier AgeM in AA girls. However, the data suggested that higher insulin was associated with higher E2. Further, reproductive maturation appeared to be associated with an acceleration of fat deposition among AA girls.