Leaf

Isoflavones, including biochanin A, daidzein, formononetin and genistein; caffeic acid derivatives; and coumestrol (trace) (Clifton-Bligh et al 2001, He et al 1996).

In the plant, the isoflavones are attached to a sugar molecule, usually glucose. The chemical term for any compound attached to a sugar is 'glycoside'. The free isoflavone form is known as an 'aglycone'. The active aglycone is liberated in the gut. Isoflavones are readily absorbed from the gut; they circulate freely in the blood and

are excreted in the urine; 50% of ingested isoflavones are eliminated within 12 hours (Joannou etal 1995).

The glycoside and aglycone forms of the four main oestrogenic isoflavones are as follows:

Glycoside

Aglycone

Ononin

Formononetin

Daidzin

Daidzein

Sissotrin

Biochanin

Genistin

Genistein

Clinical note— Phyto-oestrogens and isoflavones

Phyto-oestrogens are plant-based compounds that are structurally similar to oestradiol. The term phyto-oestrogen encompasses isoflavone compounds, such as genistein and daidzein, found predominantly in soya and red clover, and the lignans, such as matairesinol and secoisolariciresinol, found in many fruits, cereals and in linseed. Phyto-oestrogens have been investigated for their potential to reduce the risk of hormone-dependent diseases such as breast and prostate cancers and osteoporosis. The metabolism of isoflavones and lignans is complex and involves gut microbial processes. Isoflavones are present predominantly as glucosides; however, their bioavailability requires initial hydrolysis of the sugar moiety by intestinal [beta]-glucosidases. After absorption, phyto-oestrogens are reconjugated predominantly to glucuronic acid and to a lesser degree to sulfuric acid. There is further metabolism of isoflavones (to equol and O-desmethyl-angolensin) and lignans (to enterodiol and enterolactone) by gut bacteria. In humans, even those on controlled diets, there is large interindividual variation in the metabolism of isoflavones and lignans, particularly in the production of the gut bacterial metabolite equol (from daidzein). Dietary factors and gut microflora directly influence the absorption and metabolism of phyto-oestrogens and is likely to influence the clinical benefits of supplementation with phyto-oestrogens (Rowland et al 2003).

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