This study investigated the association between breast cancer outcome and being underweight among Korean women. Data was obtained from both a large nationwide database of 24,698 breast cancer patients and a single-institution database including 4,345 breast cancer patients. The associations were calculated taking into account known prognostic factors such as age, hormone receptor status, tumor size, histologic grade, lymph node metastasis, and lymphovascular invasion. Based on the nationwide database, underweight patients had significantly lower overall survival and breast cancer-specific survival compared to patients of normal weight, results which were not found for obese patients. Based on the single-institution data, underweight women also had a significantly higher risk of local recurrence of breast cancer and a markedly higher risk of distant metastasis. The authors conclude that being underweight should be considered to be a high risk factor for death and recurrence after breast cancer surgery, especially for Asian women.

Implications for breast cancer risk and survival

Few studies have specifically addressed the impact of being underweight on breast cancer survival. The current study features large sample sizes and adjustment for known prognostic factors, giving them validity for the population studied. Whether the results apply to non-Asian-American U.S. women is not clear, given substantial differences in diet and lifestyle between the two populations, as well as possibly influential genetic differences. The focus of U.S. studies has almost always been on the impact of being overweight on breast cancer risk; underweight women (BMI less than 18) tend to be grouped with normal weight women in such studies. Nevertheless, numerous studies have found that being overweight is protective against breast cancer before menopause, but increases the risk after menopause for U.S. and European women.

Underweight girls are more likely to develop breast cancer later in life. It is the tall, thin girls who go on to have the highest rates of premenopausal beast cancer. This suggests that girls should not be deprived of needed calories in an effort to delay puberty or to avoid later obesity. A study of Dutch World War II famine survivors found that they had higher subsequent risk of breast cancer. While starvation reduced growth, and resulted in later first period and earlier menopause (all of which would be expected to reduce the risk of breast cancer), levels of various estrogens and growth factors appeared to rebound after the famine ended, overshooting normal levels and resulting in a net increase in the risk of breast cancer. This suggests that adopting an extremely low-calorie diet in an effort to reduce the risk of cancer or its recurrence could backfire if the diet is subsequently abandoned.