To analyze the differences in the plasma levels of insulin-like growth factor-I (IGF-I), IGF-binding protein-3 (IGFBP-3), and their ratio between black and white men while controlling for factors that could confound the relationship between IGF levels and race. Furthermore, we analyzed the association between age, height, prostate-specific antigen level, digital rectal examination status, and current smoking status on IGF levels separately in black and white men. Greater levels of IGF-I and lower levels of IGFBP-3 have been associated with an increased risk of prostate cancer in many studies.

A cross-sectional analysis was performed on 171 white and 130 black men aged 40 to 80 years. Multiple linear regression analysis was used to model the data separately for blacks and whites. A chi-square global test was used to test for racial differences in regression curves.

Our results indicated that black men have lower levels of IGFBP-3 and IGF-I and a greater IGF-I/IGFBP-3 ratio than white men across all ages younger than 70, with and without an adjustment for height. We found racial differences in the effect of age and height on levels of IGF-I, IGFBP-3, and the molar ratio. Age had an inverse correlation with IGF-I and IGFBP-3 levels in whites, but no such relationship in blacks.

The differences in IGF levels between blacks and whites may explain some of the racial disparity in prostate cancer risk. Because age and height affect IGF levels differently in black and white men, future analysis exploring the determinants of IGF levels may need to be stratified by race.