PURPOSE Although controversial, obesity and underweight may have a negative impact on breast cancer outcome. However, the relationship between body mass index (BMI) and breast cancer outcomes according to tumor subtype and menopausal status remains unclear.

METHODS This study investigated the association between BMI and breast cancer outcome in stage I-III breast cancer patients. The relationships were further evaluated according to tumor subtype and menopausal status.

RESULTS A total of 5919 patients, 3475 (58.7%) hormone receptor (HR)(+) human epidermal growth factor receptor 2 (HER2)(-), 608 (10.3%) HR(+)HER2(+), 621 (10.5%) HR(-)HER2(+), and 1079 (18.2%) HR(-)HER2(-) were included. Underweight and obesity had a negative impact on relapse-free survival but did not affect overall survival. Importantly, the prognostic role of BMI was different according to tumor subtype and menopausal status. In HR(+)HER2(-) patients, underweight was associated with poor relapse-free survival and overall survival in pre-menopausal women. In contrast, obesity had negative impact on relapse-free survival and overall survival in HR(+)HER2(-) post-menopausal patients. Underweight may have a negative prognostic role in HR(+)HER2(+) patients. However, BMI did not impact the outcome of HR(-)HER2(+) and HR(-)HER2(-) patients.

CONCLUSIONS The impact of BMI on breast cancer outcome was dependent on tumor subtype and menopausal status. In HR(+)HER2(-) patients, underweight and obesity had a negative prognostic role in pre-menopausal and post-menopausal women, respectively. These findings in Asian population should be further evaluated and compared in Western population.