Serum Isoflavone Concentrations and Equol-Producing Intestinal Flora in Prostate Cancer in Japanese-American in Hawaii

Abstract

Studies have reported that soy isoflavones and the ability to produce equol may play a role in the prevention of prostate cancer (PCa). To further investigate this relationship, we evaluated Japanese-Americans (JA) and Caucasians (CA) in Hawaii with and without PCa, with regards to dietary soy intake, serum isoflavone levels, and the presence of equol-producing Slackia sp. in the intestine. The serum levels of isoflavone were determined in 65 JA (34 PCa, 31 controls) and 68 CA (34 PCa, 34 controls). All completed a lifetime dietary soy consumption survey. The serum levels of genistein and o-DMA were significantly higher in JA (p = 0.04). Daidzein and isoflavone levels trended higher in JA, but no differences were noted in dihydrodaidzein (DHD) and equol levels. There were no differences in serum isoflavone levels between PCa patients and controls for both JA and CA. Only 8.6% stool samples were positive for equol-producing Slackia sp., a rate much lower than seen in Japan. JA reported a higher intake of soy than CA. The high soy consumption and isoflavone levels in JA may be related to the lower incidence of PCa relative to CA in Hawaii. The near absence of equol producers in Hawaii JA may contribute to their higher incidence of PCa compared to Japan Japanese. These results suggest that both soy isoflavone levels and equol production may be inversely associated with PCa risk.