High body mass index (BMI) has been associated with an increased risk for
breast cancer among postmenopausal women. However, the relationship between BMI and
breast cancer risk in premenopausal women has remained unclear. Data from two large prevention trials conducted by the National Surgical Adjuvant Breast and Bowel Project (NSABP) were used to explore the relationship between baseline BMI and
breast cancer risk. The analyses included 12,243 participants with 253 invasive
breast cancer events from the
Breast Cancer Prevention Trial (P-1) and 19,488 participants with 557 events from the Study of
Tamoxifen and
Raloxifene (STAR). Both studies enrolled high-risk women (Gail score ≥ 1.66) with no
breast cancer history. Women in P-1 were pre- and postmenopausal, whereas women in STAR (P-2) were all postmenopausal at entry. Using Cox proportional hazards regression, we found slight but nonsignificant increased risks of invasive
breast cancer among
overweight and obese postmenopausal participants in STAR and P-1. Among premenopausal participants, an increased risk of invasive
breast cancer was significantly associated with higher BMI (P = 0.01). Compared with BMI less than 25, adjusted HRs for premenopausal women were 1.59 for BMI 25 to 29.9 and 1.70 for BMI 30 or more. Our investigation among annually screened, high-risk participants in randomized,
breast cancer chemoprevention trials showed that higher levels of BMI were significantly associated with increased
breast cancer risk in premenopausal women older than 35 years, but not postmenopausal women.