Case-Control Study of Breast Cancer in Asian-American Women

The main objective of this population-based case-control study of breast cancer in Asian-Americans is to investigate the causes of their increased breast cancer rates compared to rates in Asia. The study design and sample size will allow us to test the main hypothesis that high intakes of soy products, vegetables and fruits, dietary fiber, and specific micronutrients reduce the risk of breast cancer and that high intakes of fat increase the risk of breast cancer. We will also examine the association of breast cancer with physical activity, body size, body fat distribution, and weight change.

The proposed three-year investigation will include interviews with 350 Asian-Americans (Chinese, Filipino, Japanese), ages 25 to 74, diagnosed with histologically-confirmed (i.e., confirmed by examination of biopsied breast tissue), primary invasive breast cancer identified in Los Angeles County. An equal number of Asian Americans without breast cancer, matched to cases on age, ethnicity, and neighborhood (the "control" group) will also be interviewed. A standardized in-person interview will be conducted and will include a comprehensive diet history covering usual diet during the year before diagnosis for breast cancer cases and the year before interview for controls.

While this study is a complete, self-standing study, we are very fortunate in being able to conduct this study at this time for two reasons. First, it will allow us to compare our results on physical activity and use of exogenous hormones to those obtained in the Los Angeles component of the nationwide CARE study (which is not covering the Asian-American community or dietary factors). Second, this study will build upon and expand our study on Asian-Americans conducted in the 1980s by adding the important 55-74 age group, information on physical activity, and specifics of dietary factors. We can begin to answer some of the very important new questions which have been raised in the area of physical activity, diet, and cancer during the last decade of research. Thus, with our relatively modest grant, we will be able to gain the advantages of much more costly research conducted with other funds.

In summary, this study will help to provide a better understanding of the role of soy and other modifiable risk factors (i.e., other dietary components, physical activity, body size) in the etiology of breast cancer. Achieving these objectives will have substantial public health significance not only for Asian-Americans, but for all racial/ethnic groups, and will have important implications in the prevention of this disease.

This population-based case-control study of breast cancer in Asian-Americans was designed to investigate the causes of their increased breast cancer rates as compared to their rates in Asia. Specifically, our primary objective is to test the hypothesis that a diet rich in soy products reduces the risk of breast cancer. We will also investigate the role of other dietary factors (e.g. vegetables and fruits, dietary fiber and specific micronutrients, total and specific components of fat) and non-dietary factors including physical activity, body mass characteristics and menstrual and reproductive factors in the etiology of breast cancer in Asian-Americans.

Data collection has been completed which included in-person interviews with 350 Asian-Americans (Chinese, Filipino, Japanese), ages 25 to 74, diagnosed with histologically-confirmed, primary breast cancer identified in Los Angeles County and an equal number of Asian-American women without breast cancer ("controls"), matched to cases on age, ethnicity, and neighborhood. All questionnaire data have been coded, edited and computerized.

Preliminary analysis shows that the mean intake of soy (in milligrams of isoflavones per day) among Asian-American control women in Los Angeles County was 11.3 mg per day; this level of intake in isoflavones is one-fifth to one-half of the intake that has been reported in recent studies in Asia. In Los Angeles County, the intake of soy was highest among Chinese, intermediate among Japanese, and lowest among Filipinos. In all Asians combined, fresh tofu eaten alone or in mixed dishes represented over half of the isoflavones consumed but there were ethnic differences in the consumption of specific soy foods. In case-control comparisons, our findings show a 30% reduction in risk in association with the highest level of isoflavone intake after adjustment for other factors which might also affect risk. When we examined risk patterns in relation to intake of individual soy foods, statistically significant reductions in risk were observed in association with the highest intake level of fresh tofu, eaten alone or in mixed dishes. We are continuing to analyze these data to evaluate the soy-breast cancer association by migration history and menopausal status and to investigate the interrelationship between soy intake and other dietary and non-dietary risk factors in this population.