Young adulthood body mass index, adult weight gain and breast cancer risk: the PROCAS Study (United Kingdom)

Background: We tested the hypothesis that body mass index (BMI) aged 20 years modifies the association of adult weight gain and breast cancer risk. Methods: We recruited women (aged 47 to 73 years) into the PROCAS (Manchester, United Kingdom: 2009-2013) Study. In 47,042 women, we determined BMI at baseline and (by recall) at age 20; and derived weight changes. We estimated hazard ratios (HRs) and 95% confidence intervals (CIs) for new breast cancer using Cox models and explored relationships between BMI aged 20 years, subsequent weight changes and breast cancer risk. Results: With median follow-up of 5.6 years, 1142 breast cancers (post-menopausal at entry: 829) occurred. Among post-menopausal women at entry, BMI aged 20 years was inversely associated [HR per-SD:0.87 (95%CI:0.79-0.95) whilst absolute weight gain was associated with breast cancer [HR per-SD:1.23 (95%CI:1.14-1.32)]. For post-menopausal women who had a recall BMI aged 20 years <23.4 kg/m2 (75th percentile), absolute weight gain was associated with breast cancer [HR per-SD:1.31 (95%CIs:1.21-1.42)] but there were no associations for women with a recall BMI aged 20 years of >23.4 kg/m2 (Pinteraction values<0.05). Conclusion: Adult weight gain increased post-menopausal breast cancer risk only amongst women who were less than 23.4 kg/m2 aged 20 years.