The aims will be tested by conducting a randomized, double-blind study, with two arms and 100 women in each arm. Premenopausal women will be recruited and randomly allocated to take one of two different dietary supplements daily for 2 years. The two test diets soy protein drink and caseinate protein drink. Multiple blood, urine, and breast fluid samples will be obtained before and during the dietary intervention period and analyzed for ovarian hormones. At baseline and after the intervention period, breast density, and bone density will be assessed by radiological techniques at baseline and 1 yr and 2 yr after protein drink supplementation. The efficacy of the dietary intervention will be determined by comparing mean changes of ovarian steroids, and density of the breast and bone tissues over the 2 year dietary intervention period in the two diet groups with adjustment for baseline values and individual patient characteristics of interest. We predict that 2 years of soya dietary supplements will reduce breast density, which will be explained by individual hormonal changes.
