Isoflavones are flavonoids acting as phytoestrogens that are thought of by many as useful in treating cancer.

Isoflavones are polyphenolic compounds produced almost exclusively by the members of the Leguminosae family (bean-family). They are long known for their estrogen-like effect on mammals. Views on isoflavones differ. Supporters tout studies which provide evidence of significant cholesterol reducing effects and protection against certain types of cancers, as well as other benefits. They are also very strong antioxidants. Critics of the inclusion of isoflavones in food claim that they increase the incidence of epithelial hyperplasia, which precedes cancerous tumors, and that they cause goitre and hyperthyroidism.

History

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Early evidence that plants produced estrogen-like compounds occurred from the observation of infertility among sheep that ate large amounts of clover in Australia. Similarly, Californian Quails were thought to feed on high isoflavone legume seeds during periods of food shortage to reduce their fertility.

Biosynthesis

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Isoflavones are produced from a branch of the general phenylpropanoid pathway that produces all flavonoid compounds in higher plants. Soybeans are the most common source of isoflavones and the major isoflavones in soybean are genistein and daidzein. The phenylpropanoid pathway begins from the amino acid phenylalanine, and an intermediate of the pathway, naringenin, is sequentially converted in to the isoflavone genistein by two legume-specific enzymes isoflavone synthase and a dehydratase. Similarly, another intermediate naringenin chalcone is converted to the isoflavone daidzein by sequential action of three legume-specific enzymes chalcone reductase, type II chalcone isomerase and isoflavone synthase. Plants use isoflavones and their derivatives as phytoalexin compounds to ward off disease causing pathogenic fungi and other microbes. In addition, soybean uses isoflavones to stimulate soil-microbe rhizobium to form nitrogen fixing root nodules.

Occurrence

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Most members of the Fabaceae family contain significant quantities of isoflavones. Analysis of levels in various species found that the highest levels of genistine and daidzein were found in psoralea (Psoralea corylifolia). Various legumes including kudzu (Pueraria lobata), lupine (Lupinus spp), fava bean (Vicia faba), and soy (Glycine max) contained substantial amounts of isoflavones according to this analysis. Highly processed foods made from legumes, such as tofu, retained most of their isoflavone content, with the exception of fermented miso, which actually had increased levels.

Other dietary sources of isoflavones include chick pea (biochanin A), alfalfa sprouts (formononetin and coumestrol) and peanuts (genistein).

References

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Kaufman PB, Duke JA, Brielmann H, Boik J, Hoyt JE (1997) “A comparative survey of leguminous plants as sources of the isoflavones genistein and daidzein: Implications for human nutrition and health” J Altern Compl Med 3:7-12

Flavonoids

Isoflavone | イソフラボン