Although prostate cancer is the most common cancer among U.S. men, there is good news about the potential for prevention. It’s a slow-growing cancer, typically diagnosed at older ages. Therefore, anything that can slow the growth of these tumors can have a big effect on survival.

For example, research shows that men in Japan are just as likely to have prostate tumors as western men—but far less likely to die from this cancer. One theory is that certain diet or lifestyle factors may slow tumor growth so much that the men die of old age before the cancer ever has a chance to become deadly, or often before the men even know they have prostate tumors. There is evidence that soyfood consumption may be one of those dietary factors. For example, in a study of Chinese men, those who consumed soy more than once a day were less than one-third as likely to develop prostate cancer as men who ate soy less than once a week.

Much of the recent interest in this area has focused on a compound called equol. Equol is synthesized by intestinal bacteria from one particular isoflavone in soybeans called daidzein. (Isoflavones are often referred to as plant estrogens and are found almost exclusively in soyfoods.)

But not everyone has the bacteria to make equol, and this may actually affect risk for prostate cancer. In a study of Asian men, prostate cancer patients were much less likely to be equol producers than healthy controls.. And among Japanese men, risk was found to be inversely related to blood equol levels–that is, the more equol in the blood, the lower the risk.

One way to look at dietary effects on prostate cancer risk is to examine impact of different compounds on prostate specific antigen (PSA) In men with prostate cancer, PSA levels are directly proportional to tumor size—so measuring PSA levels is one way to test the effectiveness of cancer treatment.

Among healthy men—those without cancer—studies have not found any effects of soy or isoflavones on PSA levels. However, in half of the studies done in men with prostate cancer, isoflavones slowed the rise in PSA levels. There is also preliminary data showing that soy isoflavones may lessen the side effects of radiation treatment for prostate cancer.

While the findings are a long way from definitive, the research suggests that soy may prevent the development of prostate cancer and also slow its progression.