A high body mass index (BMI) may be associated with a reduced risk of developing premenopausal breast cancer, according to a study published in JAMA Oncology.

Previous findings have demonstrated that adiposity — often assessed using BMI — modulates the risk of breast cancer differently before and after menopause. A high BMI has been observed to increase the risk of breast cancer post menopause, but the inverse relationship between high BMI and risk of premenopausal breast cancer is not fully understood.

For this study, researchers analyzed data from 19 prospective cohorts that encompassed 758,592 women aged 18 to 54 years to determine the relationship between premenopausal breast cancer risk and BMI. After a median follow-up of 9.3 years, 13,082 cases of breast cancer were reported.

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Analysis revealed a stronger inverse linear association of breast cancer risk with BMI at ages 18 to 24 with a 23% reduction in risk per 5.0 kg/m2 difference compared with just a 12% reduction observed for BMI at ages 45 to 54 years. A 4.2-fold risk gradient was observed between the highest BMI category and the lowest (BMI ≥35.0 vs <17.0) at ages 18 to 24 years.

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Associations between reduced risk and increased BMI were stronger among women with estrogen receptor-positive and/or progesterone receptor-positive tumors compared with hormone receptor-negative breast cancer.

Results of the study suggest that the risk of premenopausal breast cancer may be reduced by increased BMI with greater magnitude, and across all BMI gradients. The authors concluded, “Understanding the biological mechanisms underlying these associations could have important preventive potential.”


The Premenopausal Breast Cancer Collaborative Group. Association of body mass index and age with subsequent breast cancer risk in premenopausal women [published online June 21, 2018]. JAMA Oncol. doi: 10.1001/jamaoncol.2018.1771