A Mediation Analysis of Obesity and Adiponectin Association with Postmenopausal Breast Cancer Risk: A Nested Cohort Study in the International Breast Cancer Intervention Study II (IBIS-II) Prevention Trial.

Obesity is a risk factor for postmenopausal breast cancer (BC), and evidence suggests a role for adiponectin in the relationship between obesity and BC. We investigated whether adiponectin or other biomarkers mediate the effect of body mass index (BMI) on postmenopausal BC risk in a cohort study nested in the IBIS-II Prevention Trial. We measured adiponectin, leptin, IGF-I, IGFBP-1, high-sensitivity C-reactive protein, glycemia, insulin, HOMA-IR index, and SHBG in baseline and 12-month serum samples from 123 cases and 302 matched controls in the placebo arm of the IBIS-II Prevention trial. We conducted the main mediation analysis considering baseline BMI as an exposure and the 12-month adiponectin increase as a mediator after adjustment for the Tyrer-Cuzick score and the lipid-lowering medications/supplements use. In the multivariable Cox model, both the 12-month adiponectin increase (HR, 0.60; 95%CI, 0.36-1.00) and BMI were associated with BC risk (HR, 1.05; 95%CI, 1.00-1.09), with a 40% reduction in women with a 12-month increase in adiponectin. A significantly higher cumulative hazard of BC events was observed in obese women (BMI > 30) with decreased adiponectin (