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Purpose: The role of exogenous estrogen use in explaining racial/ethnic differences in insulin-like growth factor-I (IGF-I) levels in relation to cancer risk is not clear. We investigated whether the relationship between race and circulating bioactive IGF-I proteins is mediated by exogenous estrogen and the extent to which exogenous estrogen explained the race–IGF-I relationship in postmenopausal women.

Methods: This cross-sectional study included 636 white and 133 black postmenopausal women enrolled in an ancillary study of the Women's Health Initiative Observational Study between February 1995 and July 1998. The race–IGF-I relationship was analyzed using ordinal regression, and quartiles of molar ratios of IGF-I/IGF binding protein-3 were used as a proxy of bioactive IGF-I outcomes. To assess exogenous estrogen as a mediator of the race–IGF-I relationship, we used the Baron-Kenny method and an …