Schoemaker MJ, et al. – In this study, researchers assessed the correlation between body mass index (BMI) and premenopausal breast cancer risk, in particular by age at BMI, attained age, risk factors for breast cancer, and tumor characteristics. They noted an association of the increased adiposity with a reduced risk of premenopausal breast cancer at a greater magnitude than previously shown and across the entire distribution of BMI. They observed the strongest associations of risk for BMI in early adulthood. Important preventive potential of understanding the biological mechanisms underlying these associations were noted.

Methods

• This multicenter analysis included pooled individual-level data from 7,58,592 premenopausal women from 19 prospective cohorts to estimate hazard ratios (HRs) of premenopausal breast cancer in association with BMI from ages 18-54 using Cox proportional hazards regression analysis.
• The investigators noted 9.3 years (interquartile range, 4.9-13.5 years) to be median follow-up per participant, with 13,082 incident cases of breast cancer.
• They recruited the participants from January 1, 1963, through December 31, 2013, and data were analyzed from September 1, 2013, through December 31, 2017.
• Body mass index at ages 18-24, 25-34, 35-44, and 45-54 were included in the exposures.
• Main outcomes and measures included the invasive or in situ premenopausal breast cancer.

Results

• Among the 7,58,592 included premenopausal women (median age, 40.6 years; interquartile range, 35.2-45.5 years), inverse linear correlations of BMI with breast cancer risk were found that were stronger for BMI at ages 18-24 years vs for BMI at ages 45-54 years.
• They observed inverse associations even among non-overweight women.
• Between the highest and lowest BMI categories, there was a 4.2-fold risk gradient at ages 18-24.
• Hazard ratios did not appreciably vary by attained age or between strata of other breast cancer risk factors.
• Findings suggested stronger correlations for estrogen receptor–positive and/or progesterone receptor–positive than for hormone receptor–negative breast cancer for BMI at every age group; BMI at ages 25-54 was not consistently associated with triple-negative or hormone receptor–negative breast cancer overall.