Abstract
Isoflavones are found in leguminous plants, especially soybeans. They have a structural similarity to natural estrogens, which enables them to bind to estrogen receptors and elicit biological activities similar to natural estrogens. They have been suggested to be beneficial for the prevention and therapy of hormone-dependent diseases. After soy products are consumed, the bacteria of the intestinal microflora metabolize isoflavones to metabolites with altered absorption, bioavailability, and estrogenic characteristics. Variations in the effect of soy products have been correlated with the isoflavone metabolites found in plasma and urine samples of the individuals consuming soy products. The beneficial effects of the soy isoflavone daidzin, the glycoside of daidzein, have been reported in individuals producing equol, a reduction product of daidzein produced by specific colonic bacteria in individuals called equol producers. These individuals comprise 30% and 60% of populations consuming Western and soy-rich Asian diets, respectively. Since the higher percentage of equol producers in populations consuming soy-rich diets is correlated with a lower incidence of hormone-dependent diseases, considerable efforts have been made to detect the specific colonic bacteria involved in the metabolism of daidzein to the more estrogenic compound, equol, which should facilitate the investigation of the metabolic activities related to this compound.
Highlights
The community of microorganisms residing in the human gastrointestinal tract forms a complex ecosystem populated with over 1011–1012 bacterial cells per gram of contents of the large intestine [1,2,3].The colon harbors more than 400 bacterial species, the majority of which are anaerobes; they are present both in the colonic contents and attached to the mucosa [4]
Chang and Nair [58,59] showed that incubation of daidzein and genistein with fecal bacteria under anaerobic conditions resulted in the conversion of genistein to dihydrogenistein and daidzein to dihydrodaidzein and equol, with equol being the major metabolite detected in the cultures
We showed that fecal bacterial samples from five individuals consuming soy diets varied in the metabolism of daidzein
Summary
The community of microorganisms residing in the human gastrointestinal tract forms a complex ecosystem populated with over 1011–1012 bacterial cells per gram of contents of the large intestine [1,2,3]. Some of the metabolites found in a host are exclusively the result of the activities of bacteria in the colonic microbial community. This includes the digestion of fibers not metabolized by host enzymes into short-chain fatty acids, and the metabolism of isoflavones, which is performed by a number of bacteria [2,3,5]. Inter-subject differences in response to isoflavones may be due to specific differences in gut microflora species [7,8,9,10,11,12,13,14,15,16] In this mini-review, the role of colonic bacteria in the metabolism of the isoflavone daidzin will be discussed
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