Abstract A77: Association between obesity over the lifetime, adult weight gain, and breast cancer risk in Mexican-American women

Background: Obesity is an epidemic in the United States, and nearly one-third of American adults over the age of 20 are considered obese. Studies of obesity and breast cancer risk have been conducted primarily in non-Hispanic whites. While obesity decreases the risk of premenopausal breast cancer, it is positively associated with risk of postmenopausal disease. Studies also suggest that a change in weight leading to adult obesity increases the risk of postmenopausal breast cancer. There have been few studies of body mass index (BMI) or weight gain and risk of breast cancer in minorities, and the results have been inconsistent. The effect of obesity on breast cancer risk in Hispanic women is of great public health significance. Hispanics are the fastest-growing minority in the United States, and obesity rates among Hispanic women are high and continuing to rise. Hispanics have a lower incidence but higher mortality rate of breast cancer than non-Hispanic whites. Elucidating the role of obesity in breast cancer development in Hispanic women will provide more understanding of the etiology of this disease and perhaps explain some of the disparities in incidence and mortality. We hypothesized that in a study of Mexican-American women obesity would decrease the risk of premenopausal breast cancer but increase the risk in postmenopausal women. Further, we hypothesized that weight gain would increase the risk of breast cancer. Methods: Using two ongoing cohort studies of Mexican-American women, we identified 155 women with incident breast cancer and 333 age-matched, healthy controls. In-person interviews were conducted on all study subjects, and medical records were abstracted for the cases to verify all cancer-related data. Self-reported weights at ages 15, 30, and at diagnosis (or at baseline for age-matched controls) were collected, and weight gain from age 15 to diagnosis/baseline was calculated. Logistic regression was used to calculate odds ratios. Exploratory analyses of effect measure modification by both menopausal status and hormone replacement therapy (HRT) use were conducted. Results: There were no significant associations between BMI at any of the time points and breast cancer in Mexican-American women, regardless of menopausal status. However, there was a 7% reduction in the risk of breast cancer for every 5 kg of weight gained from age 15 to diagnosis/baseline (OR=0.93, 95% CI: 0.87-1.00). These results did not vary by menopausal status, but there was a non-significant decrease in risk in never-HRT users (OR=0.89, 95% CI: 0.78-1.01) and no association in ever-HRT users. Conclusions: Obesity at three time points across the lifetime was not associated with breast cancer risk in Mexican-American women, while adult weight gain reduced the risk independently of menopausal status. These results are contradictory of those in non-Hispanic white women and suggest that the etiology of breast cancer may differ by race/ethnicity. Our study sample was small, but we intend to validate the results with a larger sample size. Research supported by Grant KG0901010, Susan G. Komen Breast Cancer Foundation, “A Transdisciplinary Training Program for Public Health Researchers and Practitioners Wanting to Impact Breast Cancer Disparities.” Citation Information: Cancer Prev Res 2010;3(12 Suppl):A77.