Abstract 4823: Body mass index (BMI), change in BMI, and postmenopausal breast cancer risk in the Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial (PLCO)

Mid-life body mass index (BMI) has been consistently and positively associated with risk of postmenopausal breast cancer, but BMI at younger ages has been inversely associated with risk. Few studies have looked at the timing of adult BMI change and risk. Our analysis of adult BMI, BMI change, and postmenopausal breast cancer risk is, to our knowledge, the largest study to date. Our study population included 72,007 women, 55-74 years old at baseline, in the PLCO cohort: 3,677 postmenopausal breast cancer cases were ascertained from 1993 to 2007 (median follow-up, 9.9 y; interdecile range, 6.2 -12.9 y). Cox proportional hazards models were used to estimate multivariate relative risks (RRs) and 95% confidence intervals (95% CIs). BMI at age 20 and postmenopausal breast cancer risk were consistently inversely associated across use of menopausal hormone therapy (MHT) (Ptrend = 0.01). As the positive association between BMI at study baseline and breast cancer risk appeared to be limited to women who had never used MHT, we present these results among women who never used MHT. Postmenopausal breast cancer risk increased steadily across categories of increasing BMI at study baseline (Ptrend 25 kg/m2) at both study baseline and at age 20 had a protective effect on postmenopausal breast cancer risk, compared to being overweight or obese at study baseline but having a normal BMI (5 kg/m2 between age 20 and age 50 was 1.82 (95% CI, 1.45-2.30), compared to women with no change in BMI (+/− 1 kg/m2). The RR for the same BMI gain from age 50 to age at baseline was 1.56 (95% CI, 1.29-1.90), compared to women with no change in BMI. In addition, in women who gained >5 kg/m2 between age 20 and age at baseline, postmenopausal breast cancer risk nearly doubled (RR, 1.93; 95% CI, 1.46-2.62), compared to the combined group of women with no change in BMI or with BMI loss (