Influence of Isoflavone Intake and Equol-producing Intestinal Flora on Prostate Cancer Risk

The age-adjusted incidence rate of prostate cancer (PCa) has been reported to be lower among Asian populations than among Western populations (Curado et al., 2007). Environmental and/or host factors may contribute to this difference in rates. A considerable number of articles have been published assessing dietary habits in association with PCa risk (World Cancer Research Fund/American Institute for Cancer Research, 2007). We previously reviewed them, focusing on the possibility that the traditional Japanese diet may be associated with a decreased risk of PCa (Mori et al., 2009). A traditional Japanese meal consists of a main dish of rice with additional dishes high in soybean products, fish, and other seafood, as well as low in red meat. Isoflavones, polyunsaturated long chain (n-3) fatty acids, and saturated fatty acids were thought to be micronutrients in biological etiology relevant to soybean products, fish, and red meat, respectively (Mori et al., 2009). The most abundant isoflavones are daidzein and genistein, which together comprise 0.1-0.3 mg/g soybean


Introduction
The age-adjusted incidence rate of prostate cancer (PCa) has been reported to be lower among Asian populations than among Western populations (Curado et al., 2007).Environmental and/or host factors may contribute to this difference in rates.A considerable number of articles have been published assessing dietary

Influence of Isoflavone Intake and Equol-producing Intestinal Flora on Prostate Cancer Risk
Yukiko Sugiyama 1 *, Naoya Masumori 2 , Fumimasa Fukuta 2 , Akihiro Yoneta 3 , Tokimasa Hida 3 , Toshiharu Yamashita 3 , Machiko Minatoya 1 , Yoshie Nagata 1 , Mitsuru Mori 1 , Hirokazu Tsuji 4 , Hideyuki Akaza 5 , Taiji Tsukamoto 2 (Kim et al., 2008), while glycitein is less abundant.Recent researches show that equol, which is converted from daidzein by human intestinal flora, is biologically more active than any other isoflavone (Yuan et al, 2007;Kim et al., 2008;Setchell and Clerici, 2010).A number of bacteria in intestinal flora have also been reported to be responsible for this conversion (Setchell and Clerici, 2010).If a person could make equol in response to consuming soy isoflavones, they are classified as an equol-producer, and the health benefits of soy-based diets may be greater in equol-producers than in equol-nonproducers.However, equol production by a specific human intestinal bacterium has been yet been epidemiological evaluated with regard to risk of PCa.
Accordingly, we began a case-control study on PCa to elucidate the role of equol and a specific human intestinal bacterium, Slackia sp.NATTS (Tsuji et al., 2010), which converts daidzein into equol, supported by a grant from Japanese Government.We have collected data of serum isoflavones concentration, NATTS identification in faeces, and semi-quantitative frequency of food intake, from both PCa cases and their controls.In this article, we reviewed

Analytical Epidemiology
Analytical epidemiological studies on the risk of PCa, such as cohort and case-control studies, were identified using the MEDLINE database of the National Library of Medicine, United States.Only studies in English from the database published within the last 5 years of from 2008-2012 were included, because our previous review included publications from 1998from -2007from (Mori et al.,2009)).The keywords "soybean products," "isoflavones," "equol," and "intestinal flora" were added for the search.The statistical significance was set at a 0.05 level.Odds ratios (ORs) and their 95% confidence intervals (95%CI) were cited if shown in the article.Furthermore, we searched recent researches on human intestinal bacteria responsible for converting daidzein into equol.

Discussion
As shown in Table 1, 6 articles were found by the search.Miyanaga et al. (2012) reported a randomized, double-blind placebo-controlled trial of oral isoflavones (60 mg/day) for 12 months.The isoflavone tablet included daidzein, genistein, and glycitein.Of the 89 patients evaluated by central pathological review, the incidence of high-grade prostatic intraepithelial neoplasia (HG-PIN) showed no difference between the isoflavone and placebo groups.However, for the 53 patients aged 65 years or more, the incidence of HG-PIN in the isoflavone group was higher than in the placebo group (28.0% vs. 57.1%,P=0.031).They concluded that this result may support the role of isoflavones for prostate cancer risk reduction.Jackson et al. (2010) evaluated the relationship of spot urinary concentrations of isoflavones with PCa in a hospital-based case-control study in Jamaica, collecting urine samples from 175 cases and 194 controls.As a result, compared with the lowest tertile (non producers of equol), men with the highest tertile of equol concentration in urine showed a decreased risk of total PCa (OR, 0.48; 95%CI, 0.26-0.87)and high-grade PCa (OR, 0.29; 95%CI, 0.13-0.60).They concluded that producers of equol may have a reduced risk of PCa. Park et al. (2009) reported results from a nested case-control study of the Multiethnic Cohort in Hawaii and California in USA, measuring urine samples of 249 incident cases and 404 controls matched for age, race/ ethnicity, and so on.Urinary concentration of daidzein was inversely associated with risk of PCa (OR of the highest quintile vs. the lowest, 0.55; 95%CI, 0.31-0.98).They suggested that high intake of isoflavones, as reflected by urinary excretion of daidzein, may be protective against PCa.Travis et al. (2009)  950 cases and 1,042 matched control participants.As a result, higher concentrations of genistein were associated with lower risk of PCa (OR of the highest quintile vs. the lowest, 0.71; 95%CI, 0.53-0.96).They suggested that higher a intake isoflavones, as reflected by higher concentration of circulating genistein, may reduce the risk of PCa.Kurahashi et al. (2008) conducted a nested casecontrol study within the Japan Public Health Center-based Prospective Study.A total of 14,203 men aged 40-69 years who provided blood samples were followed-up from 1990-2005.After a mean of 12.8 years of followup, 201 newly diagnosed PCa and two matched controls for each case were analyzed.As a result, the highest tertile of plasma equol was associated with a decreased risk of total PCa (OR, 0.60; 95%CI, 0.36-0.99).They concluded that isoflavones, especially equol, may prevent the development of PCa. Park et al. (2008) investigated the relationship of isoflavone intake to PCa risk in a nested case-control study within the Multiethnic Cohort Study in Hawaii and Los Angeles.The analyses included 82,483 men who completed a detailed quantitative food frequency questionnaire from 1993-1996.A total of 4,404 PCa cases were recorded during the average follow-up period of 8 years.As a result of this cohort study, the highest quintile of total isoflavone intake, as well as specific isoflavones, were not associated with risk of PCa, although they slightly reduced risk (OR of total isoflavone, 0.93; 95%CI 0.83-1.04).They concluded that isoflavones may not be significant contributors to reduction of PCa risk in this cohort.As they suggested, it might be thought that levels of soybean or isoflavone consumption in their cohort were too low to detect a protective effect against PCa.
In summary, 5 out of 6 cited articles (Kurahashi et al., 2008;Park et al., 2009;Travis et al., 2009;Jackson et al., 2010;Miyanaga et al., 2012) showed significant association of isoflavones with a decreased risk of PCa, and 2 articles (Kurahashi et al., 2008;Jackson et al., 2010) among them consistently showed that equol-producers carry a significantly reduced risk of PCa.Breinholt and Larsen (1998) found in 1998 for the first time that equol has more potent estrogenic activity than other flavonoids.Moreover, Lund et al. (2004) showed that equol has the ability to specifically bind and sequester 5α-dihydrotestosterone from androgen receptors.They suggested a broad and important usage for equol in the treatment of androgen-mediated pathologies because of this property.In addition, as stated by Setchell et al. (2002), equol is superior to all other isoflavones in its antioxidant activity.
According to the review published by Setchell andClerici in 2010 (2010), 13 human intestinal bacteria that can convert daidzein into equol have been described.We picked up some articles from the review (Setchell and Clerici, 2010), and added some others published during the 5 years from 2008-2012.As shown in Table 2, 5 human intestinal bacteria (Maruo et al., 2008;Yokoyama and Suzuki, 2008;Matthies et al., 2009;Jim et al., 2010;Tsuji et al., 2010) have been identified as responsible for converting daidzein into equol so far.Tsuji et al (2010), one of the co-authors for this review, and his co-workers also isolated an equol-producing bacterium from human faeces, strain NATTS, which was characterized as Gram-positive, non-spore-forming, and rod-shaped.They proposed a novel species of the genus Slackia, NATTS.Tsuji et al. (2012) also showed 3 enzymes and their corresponding genes in conversion of daidzein into equol in the bacterium of the Slackia sp.NATTS.Jin et al. (2010) isolated an equol-producing bacterium, strain DTE T , from human faeces, which was characterized as Gram-positive, obligately anaerobic, non-spore-forming, asaccharolytic and rod-shaped.They proposed a novel species of the genus Slackia, Slackia equolifaciens sp.nov.Matthies et al. (2009) isolated an equol-producing bacterium from human faeces, strain HE8, which was characterized as rod-shaped, Gram-positive, and anaerobic.They proposed a novel species of the genus Slackia, Slackia isoflavoniconvertens. Maruo et al. (2008) isolated an equol-producing bacterium from human faeces, which was characterized as asaccharolytic, obligately anaerobic, non-spore-forming, non-motile, and Grampositive coccobacilli.They proposed a novel species of the genus Adlercreutzia, Adlercreutzia equolifaciens sp.nov.Yokoyama et al. (2008) isolated an equol-producing bacterium from human faeces, strain YY7918, which was characterized as non-sporulating, non-motile, and Grampositive.They proposed a novel species of the genus Eggerthella, Eggerthella sp.YY7918.
In summary, 5 human intestinal bacteria that can convert daidzein into equol were identified in the last 5 years.As these intestinal bacteria need to be epidemiologically evaluated with regard to PCa risk, we began a case-control study including NATTS identification in faeces, as previously mentioned.

Concluding Remarks
Although the incidence rate of PCa is low among Japanese men, it has been increasing, probably because of changes in their dietary habits.If isoflavones, especially equol, can reduce risk of PCa, a possible strategy for reducing risk of PCa may be to increase the proportion of equol-producers by changing intestinal flora to carrying an equol-producing bacterium with dietary alteration or probiotic technology.

Table 1 . Summary of Studies Assessing the Influence of Isoflavones on Risk of Prostate Cancer During 2008-2012
examined plasma concentration of phyto-estrogens in relation to risk of PCa in a case-control study nested in the European Prospective Investigation into Cancer and Nutrition.Concentrations of genistein, daidzein and equol were measured in plasma samples for DOI:http://dx.doi.org/10.7314/APJCP.2013.14.1.1Influence of Isoflavone Intake and Equol-producing Intestinal Flora on Prostate Cancer Risk Asian Pacific Journal of Cancer Prevention, Vol 14, 2013 3