A group of men undergoing 'watchful waiting' for prostate cancer saw a decrease in marker protein when taking genistein, a soy extract.
Previous research has suggested that a group of substances called isoflavones, found in soy products, might be helpful in preventing or treating prostate cancer. Doctors at the University of California, Davis, tried giving a dietary supplement, genistein, which is one of these isoflavones, to a group of men who were assigned to 'watchful waiting' for prostate cancer.

The men were being monitored for tumors causing no symptoms which were localized to the prostate gland. They were part of a larger group of 62 men with prostate cancer and the others were undergoing surgery, radiation or hormone treatment. All received the genistein supplement. Of the 16 men undergoing watchful waiting, three dropped out of the trial, and eight of those remaining showed a drop in their prostate specific antigen (PSA) level on genistein. The remaining five had an increase in PSA as did nearly all of the men on other forms of treatment. PSA is a protein secreted into the blood by a prostate tumor, and levels are taken as a 'marker' of the progression of the disease. This was just a small study, but it does suggest that maybe genistein has a favorable effect on prostate cancer - at least in certain cases.