A case-control study of 200 Japanese men with different stages of prostate cancer—one man in Stage 1, 131 men in Stage 2, 44 men in Stage 3, and 24 men in Stage 4—compared the men's soy isoflavone intake with that of 200 healthy male controls.¹

Men whose diets delivered the most isoflavones (89.9 mg/day) were found to have a 58% lower risk of prostate cancer compared to men consuming the least isoflavones (less than 30.5 mg/day).

Because soyfoods are also rich sources of magnesium, omega-3 and omega-6 fatty acids, researchers made adjustments to see if the potential beneficial effects of these nutrients were responsible for lowering prostate cancer risk. Isoflavones' protective effect was not diminished.

This study confirms a 9-year prospective study of 43,509 Japanese men that found an inverse association between isoflavone intake and localized prostate cancer.( Kurahashi N, et al. Cancer Epidemiol Biomarkers Prev. 2007 Mar;16(3):538-45. Epub 2007 Mar 2.²

Men with the highest intake of isoflavones had half the risk of prostate cancer compared to men with the lowest isoflavone intake. Specifically, men with the highest intake of daidzein had a 50% lower risk, while those with the highest intake of genestein and those consuming the most soyfoods had a 48% lower risk of prostate cancer.

The researchers think soy isoflavones' prostate-protective benefits could be due to their weak estrogenic activity, which may act to reduce testosterone levels and inhibit 5 alpha-reductase, the enzyme that converts testosterone to dihydrotestosterone (DHT), its most potent form.

Traditional soyfoods contain isoflavones, providing 30-40 mg per serving. Soy sauce and soy oil, however, contain no isoflavones. 

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