The role of obesity in breast cancer development has been known for decades, but better and larger studies over time have uncovered more details about the complex association. There is strong evidence that obesity and weight gain during adulthood are associated with increased risk of postmenopausal breast cancer, particularly among women not using menopausal hormone therapy.1 On the other hand, obesity during adolescence and young adulthood is associated with reduced premenopausal breast cancer risk.1 Over time we also learned that what we have historically known about these associations only applies to hormone receptor–positive tumors (ie, estrogen receptor [ER]-positive and/or progesterone receptor–positive tumors).1 The literature on obesity and breast cancer prognosis is more recent and smaller, but currently an active research area. There is growing evidence that obesity is associated with higher breast cancer–specific and overall mortality in both women who are premenopausal and postmenopausal,2 with associations stronger among those with morbid obesity (body mass index [BMI; calculated as weight in kilograms divided by height in meters squared] ≥40). Furthermore, associations may vary by race/ethnicity, depending on the adiposity measure used. Fewer studies have evaluated associations by hormone receptor status, and most had limited statistical power, particularly for estrogen receptor (ER)-negative tumors. Overall, the evidence for higher mortality associated with obesity tends to be consistent for ER-positive tumors, but inconsistent for ER-negative and triple-negative breast cancer.3
Bandera EV, John EM. Obesity, Body Composition, and Breast Cancer: An Evolving Science. JAMA Oncol. 2018;4(6):804–805. doi:10.1001/jamaoncol.2018.0125
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