Race/ethnicity varied the associations between an extreme body mass index (BMI) or high waist-to-hip ratio and increased risk for mortality among patients with breast cancer.

Prior research had found that racial/ethnic differences in survival after a breast cancer diagnosis, particularly among non-Latina whites and African Americans. The reasons for these differences were unclear. Researchers had hypothesized that body size at the time of breast cancer diagnosis may have a role.

This study, led by Marilyn L. Kwan, PhD, research scientist at Kaiser Permanente Northern California Division of Research, sought to understand if obesity before a diagnosis of breast cancer is associated with poorer survival for African Americans, Latinas, and Asians. Kwan explained that obesity before a diagnosis of breast cancer in non-Latina whites is associated with poorer survival, and the study sought to examine if that relationship held in the major minority groups.

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Kwan and colleagues collected data from 12,025 female patients with breast cancer from the California Breast Cancer Survivorship Consortium. The data included BMI information for 11,351 women, which included 6,044 non-Latina whites, 1,886 African Americans, 1,451 Asian Americans, 1,864 Latinas, and 106 others.

“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,” Kwan said. Those women who were classified as underweight had a 47% increased risk for overall mortality compared with normal-weight women. Women who were classified as morbidly obese had a 43% increased risk for overall mortality. Those women with the highest waist-to-hip ratio, indicating the highest level of abdominal fat, had a 30% increased risk for overall mortality and 36% increased risk for breast cancer mortality compared with those with the smallest waist-to-hip ratios.

The associations differed by race/ethnicity. Kwan explained, “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. 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.”

For Latina women, the elevated risk for mortality was only among those considered morbidly obese. The study supports the common lifestyle recommendation to maintain a healthy weight throughout life. Further, the long-term impact of weight on survival after breast cancer may not be the same in all patients.

This study was presented at the Fifth American Association for Cancer Research Conference on The Science of Cancer Health Disparities in San Diego.