Purpose: The association between body mass index and breast cancer outcome is controversial. Furthermore, the impact of underweight on breast cancer recurrence and death has not been adequately addressed.

Patients and Methods: We investigated this issue using a large nationwide database of 24,698 Korean breast cancer patients. The association between body weight status and breast cancer recurrence was further explored using a single-institution database containing information on 4,345 patients.

Results: The results from the nationwide database showed significantly lower overall survival (OS) and breast cancer-specific survival (BCSS) in underweight patients compared with survival in patients of normal weight after adjusting for known prognostic factors such as age, tumor size, lymph node metastasis, hormone receptor status, histologic grade, and lymphovascular invasion (hazard ratio [HR], 1.48; 95% CI, 1.15 to 1.90 for OS; HR, 1.49; 95% CI, 1.15 to 1.93 for BCSS), which were not observed in obese patients. In an analysis of recurrence data from the single institution, underweight women had a significantly higher risk of both distant metastasis and local recurrence of breast cancer (HR, 1.93; 95% CI, 1.04 to 3.58 and HR, 5.13; 95% CI, 2.66 to 9.90, respectively).

Conclusion: Our study suggests that underweight should be considered to be a high risk factor for death and recurrence after breast cancer surgery, especially in Asian breast cancer patients.