Isoflavones are a class of organic compounds and biomolecules related to the flavonoids . They act as phytoestrogens in mammals. They are also very strong antioxidants. Isoflavones are thought of by many as useful in treating cancer.  Isoflavone (3-phenyl-4H-1-benzopyr-4-one) differs from flavone in the following: in isoflavones, the position of the phenyl group on the 4H-1-benzopyr-4-one skeleton is in position 3 relative to the oxygen of the ring, whereas in flavones it is in position 2. Isoflavones are polyphenolic compounds produced almost exclusively by the members of the Fabaceae/Leguminosae (bean) family.
Views on the inclusion of isoflavones in food differ radically. Supporters cite studies which provide evidence of significant cholesterol-reducing effects and protection against certain types of cancers, as well as other benefits. Critics claim that isoflavones increase the incidence of epithelial hyperplasia, which precedes cancerous tumors, and that they cause goitre and hyperthyroidism. Phytoestrogens may actually promote cancer.
Early evidence that plants produced estrogen-like compounds was observed in the infertility among sheep eating 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.
Isoflavones are produced from a branch of the general phenylpropanoid pathway which produces all 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.
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 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.
Isolated phytoestrogen-like isoflavones are an active research area. A paper in the Proceedings of the National Academy of Sciences studied the effect of the isolated soy isoflavones genistein and daidzein (commonly found in dietary supplements and infant formulas) on adult mice with their ovaries removed. The study found the mice had thymic and immune system abnormalities and reduction in immune system activity. The article suggests further research into human phytoestrogen response is warranted.
"Researchers Daniel Doerge and Daniel Sheehan, two of the FDA's experts on soy, signed a letter of protest, which points to studies that show a link between soy and health problems in certain animals. The two say they tried in vain to stop the FDA approval of soy because it could be misinterpreted as a broader general endorsement beyond benefits for the heart."
- 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
- Multiple hydride reduction pathways in isoflavonoids Auli K Salakka , Tuija H Jokela and Kristiina Wahala Beilstein Journal of Organic Chemistry 2006, 2:16 doi:10.1186/1860-5397-2-16
- "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.