Isoflavones are a type of naturally occurringisoflavonoids,[1] many of which act asphytoestrogens inmammals.[2] Isoflavones occur in many plant species, but are especially high in soybeans.[1]
Although isoflavones and closely related phytoestrogens are sold asdietary supplements, there is little scientific evidence for either the safety of long-term supplementation or of health benefits from these compounds.[1] Some studies indicate that isoflavone supplementation may help lower the risk of hormone-related cancers.[1][3]
Isoflavone is an isomer offlavone, which ischromone substituted with a phenyl group in the 2-position.[1] In isoflavone, the phenyl group is in the 3-position.[1][4]
Substituted isoflavone derivatives are related to the parent by the replacement of two or three hydrogen atoms with hydroxyl groups.[1]
Isoflavone differs fromflavone (2-phenyl-4H-1-benzopyr-4-one) in location of thephenyl group.
Isoflavones are produced via a branch of the generalphenylpropanoid pathway that producesflavonoid compounds in higher plants.Soybeans are the most common source of isoflavones in human food; the major isoflavones insoybean aregenistein anddaidzein.[1] The phenylpropanoid pathway begins from theamino acidphenylalanine, and an intermediate of the pathway,naringenin, is sequentially converted into the isoflavone genistein by two legume-specificenzymes, isoflavone synthase, and adehydratase. Similarly, another intermediatenaringenin 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 asphytoalexin compounds to ward off disease-causingpathogenicfungi and othermicrobes. In addition, soybean uses isoflavones to stimulatesoil-microberhizobium to form nitrogen-fixingroot nodules.
Most members of the familyFabaceae contain significant quantities of isoflavones.[1] Analysis of levels in various species has found the highest levels of genistein anddaidzein inpsoralea (Psoralea corylifolia). Various legumes includingsoybean (Glycine max L.),[5]green bean (Phaseolus vulgaris L.), alfalfa sprout (Medicago sativa L.),mung bean sprout (Vigna radiata L.),cowpea (Vigna unguiculata L.),kudzu root (Pueraria lobata L.), and red clover blossom and red clover sprout (Trifolium pratense L.) have been studied for their estrogenic activity.[6] Highly processed foods made from legumes, such astofu, retain most of their isoflavone content, and fermentedmiso, which has increased levels.[1]
Soy milk has a much higher concentration of isoflavones than soy sauce, but fermented soybeans show considerably higher concentrations, withtempeh having the highest isoflavone content.[1][7]
Other dietary sources of isoflavones includechick pea (biochanin A),alfalfa (formononetin), andpeanut (genistein). Isoflavones are also found in foods of animal origin such as dairy products,[8] meat, eggs and seafood,[9] but the overall contribution to total intake is low. In countries using thechorleywood bread process, such as in the UK, bread is a source of isoflavones from soy.[10]
Inplant tissue, they most often occur asglycosides or their respectivemalonates or acetyl conjugates,[5] rendering them even more water-soluble (seeisoflavone-7-O-beta-glucoside 6"-O-malonyltransferase). The latter forms are unstable and are transformed, e.g. bydecarboxylation. Often when leguminose plants are challenged with viral or fungal infections, the water-soluble transport forms arehydrolyzed to the respectiveaglycones at the target site.[11]
The consumption of isoflavones-rich food ordietary supplements is under preliminary research for its potential association with lower rates ofpostmenopausal cancer andosteoporosis in women.[1][3][12] Use of soy isoflavonedietary supplements may be associated with reduction ofhot flashes in postmenopausal women.[1][3] Soy isoflavones can act assubstrates forthyroid peroxidase, therebycompetitively inhibitingthyroid hormone production.[13]
Despite the frequent use of isoflavone supplements, there are insufficient data on safety and adverse effects.[1] Isoflavones haveGRAS status in the United States.[14] In a risk assessment of isoflavone supplements for post-menopausal women, theEuropean Food Safety Authority found noadverse effects with intakes up to 150 mg/d, although it criticized the lack of data.[15]
Isoflavones and theirglycosides | |
|---|---|
| Isoflavones | |
| O-methylated isoflavones | |
| Glycosides | |
| Prenylated isoflavones | |
| Pyranoisoflavones | |
| Derivatives | |
| Synthetic | |
