Schoemaker MJ, et al. - Researchers examined the link between body mass index (BMI) and premenopausal breast cancer risk, taking into account age at BMI, attained age, risk factors for breast cancer, and tumor features. An attenuated risk of premenopausal breast cancer was observed in relation to higher adiposity at a greater degree than shown before and across the entire distribution of BMI. For BMI, strongest associations of risk were observed in early adulthood. Possible important preventive potential of understanding the biological mechanisms underlying these associations was acknowledged.

Methods

• Pooled individual-level data from 758,592 premenopausal women from 19 prospective cohorts was used in this multicenter analysis.
• Researchers used Cox proportional hazards regression analysis to estimate hazard ratios (HRs) of premenopausal breast cancer in association with BMI from ages 18 through 54 years.
• They performed median follow-up of 9.3 years (interquartile range, 4.9-13.5 years) per participant, with 13,082 incident cases of breast cancer.
• Recruitment of participants was carried out from January 1, 1963 through December 31, 2013, and data analysis was performed from September 1, 2013, through December 31, 2017.
• They assessed body mass index at ages 18 to 24, 25 to 34, 35 to 44, and 45 to 54 years.
• Main outcomes and measures included invasive or in situ premenopausal breast cancer

Results

• This study included a total of 758,592 premenopausal women (median age, 40.6 years; interquartile range, 35.2-45.5 years).
• Findings revealed inverse linear associations of BMI with breast cancer risk; these associations were stronger for BMI at ages 18 to 24 years (HR per 5 kg/m² [5.0-U] difference, 0.77; 95% CI, 0.73-0.80) than for BMI at ages 45 to 54 years (HR per 5.0-U difference, 0.88; 95% CI, 0.86-0.91); even among nonoverweight women, inverse associations were seen.
• At ages 18 to 24 years, a 4.2-fold risk gradient was noted between the highest and lowest BMI categories (BMI≥35.0 vs <17.0) (HR, 0.24; 95% CI, 0.14-0.40).
• No appreciable variations were seen in the hazard ratios by attained age or between strata of other breast cancer risk factors.
• For BMI at every age group, they observed stronger associations for estrogen receptor–positive and/or progesterone receptor–positive vs for hormone receptor–negative breast cancer (eg, for BMI at age 18 to 24 years: HR per 5.0-U difference for estrogen receptor–positive and progesterone receptor–positive tumors, 0.76 [95% CI, 0.70-0.81] vs hormone receptor–negative tumors, 0.85 [95% CI: 0.76-0.95]); there was no consistent association of BMI at ages 25 to 54 years with triple-negative or hormone receptor–negative breast cancer overall.