Intestinal Bacterial Metabolism of Isoflavones and Lignans

Dietary plant sources, such as vegetables, fruit, and cereals contain in addition to nutrients a large number of physiological active compounds. Many of these orally consumed compounds are transformed by the intestinal bacteria, which can result in either biological activation or deactivation of these substances. There are many plant-derived substances. In this section, two of these compounds of recent interest, the bacterial metabolism of the phytoestrogen compounds isoflavones and lignans are discussed.

Isoflavones have weak estrogenic and antiestrogenic activities. Soybeans contain the highest levels of isoflavones in the human food chain. Other plant foods that contain isoflavones are pinto beans, navy beans, and chick peas, which have approximately two orders of magnitude lower levels. For populations consuming soy-based foods, the amount of isoflavones eaten daily is between 30 and 150 mgs. For daidzein, one of three major soy isoflavones, the intestinal bacteria can convert the parent compound into several end products. Among the end products are o-desmethylangolensin, equol, cis-4-equol, and dihydrodaidzein (32). There is a large individual variation in the ability of intestinal bacteria to metabolize daidzein. Studies have shown that production of equol does not occur in 30-40% of people fed soy isoflavones, and the remainder are active equol producers (33). The conversion of daidzein to equol can be of physiological importance since equol is a more potent estrogenic substance. The factors that control the extent of bacterial conversion of isoflavones in the intestine are unknown.

Genistein, the isoflavone with the highest concentration in soy, is converted by intestinal bacteria to dihydrogenistein, and p-ethyl phenol. These reactions most likely lower or destroy the estrogenic activity of genistein. Glycetein, the third most prevalent isoflavone contained in the soybean, is bacterially converted to 5-hydroxy-, and 5-methoxy-o-desmethylangolensin. There are other bacterial metabolites of isoflavones, and new end products are still being isolated.

Lignans are found in relatively high concentrations in flaxseed, whole-grain products, vegetables, and sesame seeds (32). Lignans also exhibit weak estrogen and anti-estrogen activity, although these activities are lower than those found in isoflavones (32).

The plant lignan precursors secoisolariciresinol and matainresinol are converted by the intestinal microflora to enterodiol and enterolactone, respectively (32). The physiological importance of these bacterial conversions are not clear.

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