Epidemiological data indicate that diet is an important factor in explaining the 5-8 fold lower incidence of breast cancer in Oriental countries compared with the Western world, and recently soy protein intake was found to be associated with a decreased risk for breast cancer in premenopausal Oriental women. Our overriding hypothesis is that "Soy protein is beneficial in the prevention and/or treatment of breast cancer, because of the presence of non-steroidal estrogens of the isoflavone class that have demonstrated anticancer actions." We have shown that soy protein will inhibit tumor growth in experimental animal models of breast cancer, in a dose-dependent fashion, and that the effect is correlated with soy isoflavone intake. This proposal seeks to clarify our understanding of the absorption, metabolism and physiology of soy isoflavones in humans. To establish the degree of intestinal metabolism and absorption, unconjugated and conjugated forms of the principal soy isoflavones, daidzein and genistein, will be administered to healthy premenopausal women, and urine, serum, bile, and feces, will be collected at timed intervals for analysis of isoflavone concentrations by mass spectrometry. By site-specific delivery, under conditions of high and low carbohydrate diets, the metabolism and bioavailability of different forms of soy isoflavones will be compared. We have shown that chronic soya ingestion affects the hormonal regulation of the menstrual cycle of women. To determine whether these effects are due to isoflavones, are dose-dependent and are related to the circulating concentrations and urinary excretion of isoflavones, soy proteins differing in isoflavone composition will be given to healthy premenopausal women, and urine, serum and feces collected. To determine whether in common with endogenous estrogens, local uptake and secretion of isoflavones occur in the human breast, concentrations of isoflavone will be measured in breast cyst fluid and nipple aspirates of non-lactating premenopausal women before and following soy isoflavone ingestion. The isoflavone content and composition of soybean varieties indigenous to different regions of the world will be determined and their stability to temperatures used in cooking will be examined. In response to a critical need for a routine method for measuring isoflavones in biological samples we will develop a specific immunoassay using strategies successfully employed for steroid hormone assays. We will also refine existing mass spectrometric methods to permit the determination of isoflavones in tissues. The results of these studies will permit the most appropriate design of future dietary intervention studies using soy protein.