Isoflavones comprise a class of organic compounds, often naturally occurring, related to the isoflavonoids.[1] Many act as phytoestrogens in mammals. Some are termed antioxidants because of their ability to trap singlet oxygen.[2] Some isoflavones, in particular soy isoflavones, when studied in populations eating soy protein, have indicated that there is a lower incidence of breast cancer and other common cancers because of its role in influencing sex hormone metabolism and biological activity through intracellular enzymes, protein synthesis, growth factor actions, malignant cell proliferations, differentation and angiogenesis.[2] Isoflavones are produced almost exclusively by the members of the Fabaceae (i.e., Leguminosae, or bean) family.
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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 nutritional 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.
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 soybean (Glycine max L.), 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.[3] 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 peanut (genistein).
In plant tissue, they most often occur as glycosides or their respective malonates or acetyl conjugates, rendering them even more water-soluble (see isoflavone-7-O-beta-glucoside 6"-O-malonyltransferase). The latter forms are unstable and are transformed, e.g. by decarboxylation. Often when leguminose plants are challenged with viral or fungal infections, the water-soluble transport forms are hydrolyzed to the respective aglycones at the target site.[4]
Because of their diversity, isoflavones can have diverse effects on human and animal health.[5] Isoflavones such as genistein and daidzein, were able to prevent the growth of estrogen-receptor positive and negative breast cancer cells in vitro.[2] Although isoflavones were able to prevent growth of breast cancer cells, early evidence that plants produced estrogen-like compounds was first manifested in the infertility among sheep that ate large amounts of clover in Australia.[6] Similarly, California Quails fed on the leaves of high-isoflavone desert annuals during periods of food shortage had reduced fertility.[7]There multiple cases of documented adverse reactions to isoflavones in men. One report, published in 2008, documented a case of gynecomastia in a 60-year-old man. The man reported erectile dysfunction and decreased libido, though no changes in testicular size. Laboratory assessment showed estrone and estradiol levels to be increased. He described a daily intake of 3 quarts of soy milk. After he discontinued drinking soy milk, his breast tenderness resolved.[8]There are increasing number of reports of isoflavone's adverse estrogen like effects on males. A 2011 report documented a 19-year-old male who faced loss of libido and erectile dysfunction after engaging in a soy-heavy diet.[9] However, a 2010 meta-analysis of fifteen placebo-controlled studies said that "neither soy foods nor isoflavone supplements alter measures of bioavailable testosterone concentrations in men."[10] Furthermore, Isoflavones supplementation has no effect on sperm concentration, count or motility, and show no changes in testicular or ejaculate volume.[11][12]
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.[5]
Studies on mice indicate that isoflavones may cause thymic and immune system abnormalities and reduction in immune system activity.[13]
There may be a link between soy and health problems in certain animals.[14]
10. Kaur N and Murphy B. 2010. Cloning, characterization, and functional analysis of cowpea isoflavone synthase homologs. Plant Molecular Biology and Biotechnology 1(1): 6-13
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