Obesity and Insulin-Like Growth Factor-I in African Americans and Whites

To the Editors: We were surprised by the results reported by Henderson et al. (1) showing a different relationship between body mass index (BMI) and insulin-like growth factor-I (IGF-I) for African Americans and Whites. They found essentially no relationship between BMI and IGF-I for African Americans but a clear decline in IGF-I levels with BMI for Whites. We measured IGF-I in plasma collected from 981 premenopausal women ages 35 to 49 years who had been randomly selected from the membership of a large urban health plan in Washington, District of Columbia (576 African Americans and 405 Whites). Measurement of IGF-I was by extraction using RIA kits from Nichols Institute Diagnostics. On average, African Americans have lower IGF-I than Whites [162 ng/mL (SE, 2.5) compared with 171 ng/mL (SE, 2.9)], but means for each BMI category are nearly identical for the two ethnic groups (Table 1). Regression analyses show that age-adjusted BMI (modeled continuously) is strongly and inversely related to IGF-I in both ethnic groups, and the β coefficients for BMI are nearly identical [African Americans: −2.4 (SE, 0.29); Whites: −2.5 (SE, 0.42)].