Gut Bacterial Metabolism of the Soy Isoflavone Daidzein: Exploring the Relevance to Human Health

Abstract

The indigenous intestinal microflora are involved in a variety of processes within the human body, and are important for maintaining host health. As such, interindividual differences in the ability to harbor certain intestinal bacteria might be associated with interindividual differences in health and/or disease susceptibility. In the last decade there has been considerable interest in phytoestrogen intakes in relation to human health. Daidzein, an isoflavone phytoestrogen found in soy, is metabolized to equol and O-desmethylangolensin (O-DMA) by intestinal bacteria. The specific bacterium/bacteria responsible for equol and O-DMA production in humans have yet to be identified definitively, but in vitro and animal studies have suggested that equol and O-DMA are more biologically active than their precursor daidzein. Interestingly, substantial interindividual differences in daidzein metabolism exist; following soy or daidzein consumption, approximately 30%-50% of the human population produce equol, and approximately 80%-90% produce O-DMA. Observational and intervention studies in humans have suggested that the ability to produce equol and O-DMA may be associated with reduced risk of certain diseases including breast and prostate cancers. However, relatively few studies have been conducted to date. In this review, we discuss the available evidence for a relationship between daidzeinmetabolizing phenotypes and human health, and suggest potential mechanisms for some of the reported relationships.

Urinary isoflavonoid excretion and soy consumption in three generations of Japanese women in Hawaii

“…This small investigation among three generations of Japanese-Americans observed a higher consumption of traditional Asian soy foods and a slightly higher equol production among older Japanese women, but no evidence that soy intake during early and adult life affected the ability to take up and excrete isoflavonoids after a standardized dose of soymilk. Although there is evidence that intestinal bacteria are responsible for the differences in isoflavone metabolism (Atkinson et al, , 2005, not much is known about the determinants of gut bacteria and equol excretion status (Lampe et al, 1998;Rowland et al, 2000;Blair et al, 2003;Franke et al, 2004) or the possible effects of equol production on health outcomes.…”

“…The extent of the metabolism seems to be highly variable among individuals and may be influenced by other components of the diet (Lampe et al, 1999;Rowland et al, 2000). Approximately 30-50% of the human population produces equol (Setchell et al, 2002;Atkinson et al, 2005), but it appears that a greater proportion of Asian than Western populations have the ability to produce equol (Setchell et al, 2002;Akaza et al, 2004;Ozasa et al, 2004), possibly because they were exposed to soy early in life. Urinary isoflavonoid excretion serves as an excellent biomarker for soy intake owing to the rapid absorption and excretion of isoflavonoids specific to soy foods (Maskarinec et al, 1998).…”

“…Urinary isoflavonoid excretion serves as an excellent biomarker for soy intake owing to the rapid absorption and excretion of isoflavonoids specific to soy foods (Maskarinec et al, 1998). Observational and intervention studies in humans have suggested that the ability to produce equol may be associated with a reduced risk for breast and prostate cancer (Setchell et al, 2002;Atkinson et al, 2005). Women who consumed soy at least once per week during adolescence had a lower risk of breast cancer than women with less than monthly intakes during adolescence in two studies (Shu et al, 2001;Wu et al, 2002).…”

“…We hypothesized that women exposed to soy early in life, such as Japanese migrants, excrete higher levels of urinary isoflavonoids and are more likely to produce equol after consuming a standardized amount of soy than women of later generations who were exposed to a more Westernized diet. It has been proposed that intestinal bacteria are responsible for the differences in isoflavone metabolism (Atkinson et al, 2005) and that the development of gut bacteria may be influenced by habitual nutrition (Lampe et al, 1998;Rowland et al, 2000;Setchell et al, 2002).…”

Premenstrual symptoms interference and equol production status in Japanese collegiate athletes: A cross‐sectional study

“…Equol is a metabolite of a soy isoflavone, which is metabolized from the soy isoflavone daidzein by specific intestinal bacteria 16. Only 30–60% of people can produce equol from daidzein after eating soy 17. The rate of equol producers is higher in vegetarians18 and Asian populations and is considered to be associated with the amount of soy intake 19.…”