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Isoflavones comprise a class of organic compounds, often naturally occurring, related to the flavonoids.[1] Many act as phytoestrogens in mammals. Being polyphenols, they are antioxidants.citation needed Some isoflavones and isoflavone-rich foods possess activity against cancer, including certain types of breast and prostate cancer.[2] Isoflavones are produced almost exclusively by the members of the Fabaceae/Leguminosae (bean) family.
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Organic chemistry and biosynthesis
Isoflavones of nutritional interest are substituted derivatives of isoflavone, being related to the parent by the replacement of two or three hydrogen atoms with hydroxyl groups. The parent isoflavone is of no nutrional interest.
Isoflavone differs from flavone (2-phenyl-4H-1-benzopyr-4-one) in location of the phenyl group.
Isoflavones are produced via a branch of the general phenylpropanoid pathway that produces flavonoid compounds in higher plants. Soybeans are the most common source of isoflavones in human food; 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
Most members of the Fabaceae family contain significant quantities of isoflavones. Analysis of levels in various species has found that the highest levels of genistein and daidzein 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, retain most of their isoflavone content, with the exception of fermented miso, which has increased levels.
Other dietary sources of isoflavones include chick pea (biochanin A), alfalfa (formononetin and coumestrol), and peanut (genistein).
Health effects
- Further information: Phytoestrogens#Health Risks and Benefits
Because of their diversity, isoflavones can have diverse effects on human and animal health.[2] Early evidence that plants produced estrogen-like compounds was manifested in the infertility among sheep that eat large amounts of clover in Australia.citation needed Similarly, Californian Quails fed on the leaves of high-isoflavone desert annuals during periods of food shortage had reduced fertility.[3] Studies using chemically pure isoflavones or plant materials with known concentrations of these compounds have indicated both positive and negative effects of isoflavones on disease progression and fertility. (Reviewed in [2])
Studies on mice indicate that isoflavones may cause thymic and immune system abnormalities and reduction in immune system activity.[4]
There may be link between soy and health problems in certain animals.[5]
References
- ^ 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
- ^ a b c Dixon, RA (2004). "Phytoestrogens". Annu Rev Plant Biol. 55: 225–61. doi:. PMID 15377220.
- ^ Phytoestrogens:Adverse effect on reproduction in California quail PMID 1246602
- ^ A paper in the Proceedings of the National Academy of Sciences:"The phytoestrogen genistein induces thymic and immune changes: A human health concern?" Proceedings of the National Academy of Sciences May 28, 2002
- ^ FDA Scientists Against Soy. The anti-soy website "Soy Online Service" has the original letter in PDF. [1]
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