The low risk of prostate cancer in Asia is thought to be due to dietary factors, including soy consumption. Soy is a concentrated source of plant estrogens (phytoestrogens), and studies have shown that men who excrete high levels of soy phytoestrogens in their urine have low risk of prostate cancer. These studies suggest that phytoestrogens may be the cancer-preventive components of soy. One way that soy phytoestrogens are thought to be cancer preventive is by lowering the levels of natural sex hormones, such as estrogen and testosterone, which have both been associated with prostate cancer risk. Studies in women have shown that soy phytoestrogens reduce estrogen levels, and studies in animals show that phytoestrogens reduce synthesis of testosterone and other potent male hormones that stimulate prostate cancer growth. Despite the interest in soy phytoestrogens for the prevention of prostate cancer, there have been no studies in men to evaluate the effects of these substances on sex hormones and on prostate tissue itself.

The main purpose of this project is to evaluate the effects of soy phytoestrogen consumption on hormones and prostate tissue cell growth in men at high risk of prostate cancer. The idea behind this proposal is that soy phytoestrogens may prevent prostate cancer by lowering natural hormones. Although there have been a number of studies investigating the hormonal effects of soy phytoestrogens in pre- and postmenopausal women, no such studies in men have been reported.

The specific aims of this study are to compare the effects of consumption of phytoestrogen-containing soy protein, phytoestrogen-free soy protein, and milk protein on risk factors for prostate cancer (natural hormones, prostate specific antigen, prostate tissue markers of cell growth, and hormone action) in men at high risk for prostate cancer. Comparing the three groups will enable us to distinguish the specific effects of soy phytoestrogens from effects caused by other soy components.

A dietary study will be performed in which 90 men at high risk of prostate cancer will be divided into three groups, each of which will receive one of three dietary supplements for 6 months: (1) soy powder containing phytoestrogens; (2) phytoestrogen-free soy powder; and (3) phytoestrogen-free milk powder. Urine and blood will be collected at 0, 3, and 6 months for evaluation of serum hormones (male and female hormones associated with prostate cancer risk) and prostate specific antigen, urinary estrogens thought to be carcinogenic, and urinary phytoestrogens. Before and after the intervention, prostate biopsies will be performed to evaluate cell growth and androgen action in the prostate tissue itself.

There is a great deal of data to suggest that androgens and estrogens play a role in prostate cancer development. Soy phytoestrogens have been shown to alter sex hormones in women in a potentially beneficial direction, yet such studies in men have not been reported. Studies of the hormonal effects of soy phytoestrogens in men will contribute to our knowledge of the way in which soy phytoestrogens may be cancer preventive and may lead to dietary recommendations for prevention of prostate cancer.