Isoflavones comprise a class of organic compounds, often naturally occurring, related to the isoflavonoids. Many act as phytoestrogens in mammals. Some are termed antioxidants because of their ability to trap singlet oxygen. 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. However, the risk reduction in breast cancer from soy isoflavones was only shown in Asian populations (Shanghai Breast Cancer Survival Study). Isoflavones are produced almost exclusively by the members of the Fabaceae (i.e., Leguminosae, or bean) family.
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 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 into 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. 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.
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.
Potential health effects
Isoflavones may affect human and animal health, although research is preliminary. Isoflavones such as genistein and daidzein, may affect the growth of estrogen-receptor positive and negative breast cancer cells in vitro. 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." Furthermore, isoflavone supplementation had no effect on sperm concentration, count or motility, and showed no changes in testicular or ejaculate volume, in one study. 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.
Studies on mice indicate that isoflavones may cause thymic and immune system abnormalities and reduction in immune system activity.
According to one study, soy isoflavone supplements did not decrease breast cancer cell proliferation. Another study showed that soy food consumption did not increase the risk of cancer recurrence or death among survivors of breast cancer, while another found a 36% reduction of breast cancer recurrence in subjects that consumed ≥10 mg soy isoflavones per day.
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