Metabolism of isoflavones

Formation of aglycones

Isoflavones occur in foods in the form of glucosides which means that the isoflavones are bound to sugar (conjugated isoflavones). These glycosides are very water soluble. These conjugated isoflavones have to undergo further changes. When ingested, these conjugated isoflavones undergo hydrolysis by ß-glucosidases in the intestine, releasing the principal bioactive aglycones (daidzein, genistein and glycitein). These aglycones may be absorbed and further metabolized to many specific metabolites such as equol.

Influence of diet on isoflavones metabolism

Further metabolism of aglycones seems to be strongly influenced by the diet. A high carbohydrate environment, which causes increased intestinal fermentation, results in more phytoestrogens being transformed in equol. This may be relevant because the potency of equol is higher than that of its plant precursor, daidzein. Also, the intestinal microflora has an effect on the metabolism of isoflavones. When intestinal flora is low (antibiotics, germfree animals, newborn babies) metabolism falls down too. When the dietary intake of fat is high, intestinal microflora has difficulty in synthesizing equol from isoflavones.



Like endogenous estrogens (estradiol), isoflavones are metabolized in the intestines and liver. Absorption happens along the entire length of the intestine and they are secreted in bile and urine. Excretion of isoflavones metabolites can vary strongly between individuals. This may be influenced by the fact that each person has his own specific intestinal microflora population.

Once absorbed equol shows less affinity to be bound to serum proteins and therefore has a greater availability than estradiol. When soy is consumed on a regular basis (50 mg isoflavones/day), plasma isoflavone levels far exceed normal estradiol concentrations. This observation led to the hypothesis that isoflavone would be biologically active, conferring health benefits that could explain the relatively low incidence of hormone-dependent diseases in countries in which soy is a dietary staple.

More literature about isoflavones metabolism

Absorption in humans of isoflavones from soy and red clover is similar. J Nutr 2002 Aug;132(8):2199-201

Isoflavones and Functional Foods Alter the Dominant Intestinal Microbiota in Postmenopausal Women. Journal of Nutrition 135:2786-2792, December 2005

Factors Affecting the Bioavailability of Soy Isoflavones in Humans after Ingestion of Physiologically Relevant Levels from Different Soy Foods. J. Nutr. 136:45-51, January 2006

Bioavailability of Isoflavones after Ingestion of Soy Beverages in Healthy Adults. J. Nutr. 136:2291-2296, September 2006

Method of defining equol-producer status and its frequency among vegetarians. J Nutr. 2006 Aug;136(8):2188-93

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