Body size, modifying factors, and postmenopausal breast cancer risk in a multiethnic population: the San Francisco Bay Area Breast Cancer Study.

Data on body size and postmenopausal breast cancer in Hispanic and African American women are inconsistent, possibly due to the influence of modifying factors. We examined associations between adiposity and risk of breast cancer defined by hormone receptor status in a population-based case-control study conducted from 1995-2004 in the San Francisco Bay Area. Multivariate adjusted odds ratios and 95% confidence intervals were calculated using unconditional logistic regression. Associations with body size were limited to women not currently using menopausal hormone therapy (801 cases, 1336 controls). High young-adult body mass index (BMI) was inversely associated with postmenopausal breast cancer risk, regardless of hormone receptor status, whereas high current BMI and high adult weight gain were associated with two-fold increased risk of estrogen receptor and progesterone receptor positive breast cancer, but only in women with a low young-adult BMI (≤22.4 kg/m(2)) or those with ≥15 years since menopause. Odds ratios were stronger among non-Hispanic Whites than Hispanics and African Americans. Waist circumference and waist-to-height ratio increased breast cancer risk in Hispanics and African Americans only, independent of BMI. These findings emphasize the importance of considering tumor hormone receptor status and other modifying factors in studies of racially/ethnically diverse populations.