Isoflavones are phytochemicals found in soybeans that may explain some of their health benefits. Isoflavones have been suggested to be responsible for the protective effect that soybean foods shown against oestrogen related cancers such as breast cancer. This may relate to the weak oestrogenic effects of the isoflavones, a feature that allows them to interact with the estrogen receptor and block the action of excess oestrogen or oestrogenic chemicals such as alkylphenols. Thus isoflavones may inhibit the cell proliferative effects of oestrogen that is thought to be required to trigger some hormone sensitive cancers. Populations that consume soybeans regularly in their traditional diets, such as found in much of Asia, have far lower rates of breast cancer than Western populations who do not consume soybeans regularly. Isoflavones are sometimes grouped with the flavonoids, but they are not true flavonoids. However they have a similar polyphenolic structures and may be potent reducing agents in humans, as are many flavonoids.
Soybeans contain twelve structurally related isoflavones. These include the parent aglycones daidzein, glycitein and genistein. These compounds can form glycosidic bonds with glucose to form β-glucoside forms of the isoflavones called daizin, glycetin and genistin. Conjugation of these β-glucoside isoflavones can occur with acetyl or malonyl compounds to form 6‘‘-O-acetyl glucosides and 6‘‘-O-malonyl glucosides, respectively. The aglycone forms of the isoflavones are only present in low concentrations in soybeans, with much higher concentrations of the β-glucoside isoflavones and their conjugates being present. However, it is likely that the glucose and associated conjugate malonyl or acetyl group must be cleaved in the small intestine before the isoflavone can be absorbed, and in their respect the aglycone form of the isoflavones can be thought of as the biologically active form. The addition of the glucose group imparts hydrophobicity on the molecule, which may prevent it entering enterocytes.
Studies to date using isolated isoflavones have produced inconsistent results. The reason for this is not clear, but may relate to the use of forms of isoflavones that are not readily absorbed. Certainly evidence in the literature suggests that conjugation of the β-glucoside forms of the isoflavones with acetyl and malonyl groups may lower the bioavailability of the compounds. For example, one study1 administered 100 μmol/kg body weight of either malonylgenistin, genistin, malonyldaidzin or daidzin to rats and assessed the isoflavone metabolites in plasma and urine. The area under the curves for plasma metabolites were 1 to 6 times higher and the maximum plasma and urine concentration of isoflavone metabolites was 1 to 9 times higher following administration of the non-conjugated β-glucoside isoflavones (genistin and daidzin) compared to than those of the malonylglucosides, respectively. Therefore the non-conjugated β-glucoside isoflavones appear to be more bioavailable than their conjugated forms.
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