MONDAY, Oct. 29 (HealthDay News) -- Having a very high or low body-mass index or high waist-to-hip ratio raises the risk of death among breast cancer patients, but this association varies some by race and ethnicity, a new study suggests.

Body-mass index (BMI) and waist-to-hip ratio are both measures of body fat, and both affect overall and breast-cancer-specific risk of death, according to the researchers.

The researchers analyzed data from more than 12,000 white, black, Hispanic and Asian-American patients in the California Breast Cancer Survivorship Consortium.

"Overall, we found that patients with breast cancer who were underweight, extremely obese or had high levels of abdominal body fat had the worst survival," Marilyn Kwan, a research scientist in the Kaiser Permanente Northern California Division of Research, said in an American Association for Cancer Research news release.

Compared to normal-weight women, underweight women had a 47 percent increased overall risk of death and extremely obese women had a 43 percent increased risk. Compared to those with the lowest waist-to-hip ratio, women with the highest waist-to-hip ratio (highest level of abdominal fat) had a 30 percent increased overall risk of death and a 36 percent increased risk for breast-cancer-related death.

Further investigation revealed that the association between weight and death risk differed by race and ethnicity. Although this study found an association between the two, it did not prove a cause-and-effect link.

"Among non-Latina white women, being underweight and morbidly obese at breast cancer diagnosis was associated with worse survival, yet this relationship was not found in the other racial/ethnic groups," Kwan said. "Instead, African-American women and Asian-American women with larger waist-to-hip ratios had poorer survival, an observation not seen in non-Latina white women and Latina women."

The findings, scheduled to be presented Monday at a meeting of the American Association for Cancer Research, in San Diego, support the common recommendation to maintain a healthy weight throughout life, Kwan said. She noted, however, that the long-term impact of weight on survival after breast cancer might not be the same in all patients.

Research presented at medical meetings should be viewed as preliminary until published in a peer-reviewed journal.

More information

The American Cancer Society has more about breast cancer.