Schoemaker MJ - This multicenter analysis used pooled individual-level data to examine the relationship of body mass index (BMI) with premenopausal breast cancer risk, by age at BMI, attained age, risk factors for breast cancer, and tumor characteristics. Data reported that increased adiposity was correlated with a reduced risk of premenopausal breast cancer, more so than previously reported, with the strongest associations for BMI at young ages. Findings suggested that understanding the biological mechanisms underlying these associations could have important preventive potential.

Methods

• Using Cox proportional hazards regression analysis, hazard ratios (HRs) of premenopausal breast cancer were evaluated in correlation with BMI from ages 18 through 54 years using data from 758,592 premenopausal women from 19 prospective cohorts.
• Median follow-up was 9.3 years (interquartile range, 4.9-13.5 years) per member, with 13,082 incident cases of breast cancer.
• Participants in the study were enrolled from January 1, 1963, through December 31, 2013, and data were analyzed from September 1, 2013, through December 31, 2017.
• BMI at ages 18 to 24, 25 to 34, 35 to 44, and 45 to 54 years were the main exposures.
• Invasive or in situ premenopausal breast cancer was the main outcome and measure.

Results

• According to the findings, among the 758,592 premenopausal women (median age, 40.6 years; interquartile range, 35.2-45.5 years) in this investigation, inverse linear associations of BMI with breast cancer risk were discovered that 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).
• In addition, the inverse associations were found among women who were not overweight.
• The data showed a 4.2-fold risk gradient between the highest and lowest BMI categories (BMI ≥ 35.0 vs < 17.0) at ages 18 to 24 years (HR, 0.24; 95% CI, 0.14-0.40).
• Findings revealed that hazard ratios did not appreciably differ by attained age or between strata of other breast cancer risk factors.
• It was observed that associations were stronger for estrogen receptor–positive and/or progesterone receptor–positive than for hormone receptor–negative breast cancer for BMI at every age group (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]); BMI at ages 25 to 54 years was not consistently connected with triple-negative or hormone receptor–negative breast cancer overall.