Isoflavones are a group of polyphenolic substances in plants. Legumes, particularly soybeans, are a very good source of isoflavones. Isoflavones are often grouped as flavonoids, but they are not true flavonoids structurally. The main isoflavones in the human diet are daidzin and genistin, which are glycosides of daidzein and genistein, respectively. Upon consumption, the daidzin and genistin are deglycosylated in the small and large intestine, through a combination of endogenously produced β-glucosidases from enterocytes and exogenously produced β-glucosidases from bacteria in the colon. The resultant daidzein and genistein may be absorbed into enterocytes where they are conjugated through a series of phase II conjugating enzymes, before entering the hepatic portal circulation. In the colon, the resulting daidzein and genistein are metabolised to a number of conjugates including equol and O-desmethylangolensin (O-DMA). The isoflavones that are absorbed are therefore a homogeneous group of metabolites.
Isoflavones may have beneficial effects on glucose homeostasis and in this way protect from cardiovascular disease. For example, in one study1 a group of researchers analysed the isoflavone intakes of a group of pregnant women. The results showed that those women with the highest urinary concentrations of total isoflavones had a significantly lower risk of having elevated levels of plasma glucose compared to those with the lowest urinary concentrations of total isoflavones. Those with the highest urinary concentration of total isoflavones also had a significantly lower risk of having insulin resistance compared to those with the lowest urinary concentrations. However there was no association between urinary isoflavone concentrations and cholesterol or lipid levels. Therefore, assuming that urinary concentrations are reflective of isoflavone intakes, higher intakes of isoflavones are associated with a reduced risk of blood glucose abnormalities in pregnant women and this may have cardioprotective effects.
Isoflavones belong to a group of plant chemicals called polyphenols. Polyphenols are secondary plant metabolites that have structures that makes them effective antioxidants in humans. High intakes of plant antioxidants may be protective of insulin resistance because of their ability to decrease oxidative stress and inflammation, both of which are associated with reductions in insulin sensitivity. Further, polyphenols are thought to be protective of detrimental changes to blood glucose level because they delay the absorption of glucose from the intestine to the blood. This may occur because polyphenols can interfere with starch and disaccharide digesting enzymes. Isoflavone intake is also representivity of legume intake, as legumes are the primary source of isoflavones in the diet. Legumes have beneficial effects of blood glucose levels because they possess high amount of fibre, starch inhibiting enzymes and high protein concentrations that have all been shown to lower postprandial blood glucose levels.
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