*Geek Box: Isoflavones & Equol
While soy is generally recognised for the phytoestrogen activity of its main isoflavones, it is the metabolism of daidzein to equol that may be of particular interest for the purported health effects of soy foods. Before they can be fully absorbed, soy isoflavones are metabolised by gut bacteria; this is a step that is now recognised as a critical stage in metabolism and ultimate bioactivity of all (poly)phenolic compounds.
It is important to note that equol is not a phytoestrogen, yet is is often misclassified as such in the literature. Equal is exclusively a product of bacterial metabolism of daidzein, and does not appear in urinary excretion [its elimination pathway] unless soy foods are produced in the diet. Equol is a non-steroidal oestrogen, which can be drugs [e.g., tamoxifen] or naturally occurring compounds that may exert oestrogenic activity.
In this regard, while genistein and daidzein have affinity for binding to the ERβ receptor, the affinity for equol is much greater, and equol exerts greater antioxidant activity compared to the precursor isoflavones. In this regard, it appears that between genistein and daidzein, genistein has significantly greater affinity for the ER-beta receptor [greater than tamoxifen]. Thus, the purported activity of daidzein may be predicated upon conversion to equol.
Due to the modifying effect of the gut microbiota, the concept of an “equol producer” has been identified, although the definition appears to be relatively arbitrary. It may be a combination of genetics and the background diet, however, East Asian populations exhibit greater equol-production capacity compared to Western populations. It has been suggested that equol provides more of a plausible mechanistic explanation for the association between soy foods consumption and lower risk of chronic diseases, compared to the precursor soy isoflavones. If this is the case, the distinction between “equol producers” and “non-producers” may provide an explanation for certain of the inconsistencies in the literature. Future studies should seek to quantify equol producing capacity as a critical potential effect modifier.