Discovering the Health Benefits of Isoflavones

Soyfoods are the richest, dietary source of isoflavones. These compounds are being studied intensively because they exert physiological effects which may help reduce risk for certain disease.

Isoflavones in Soyfoods

Phytochemicals are plant compounds which exert biological effects in the animals or humans that consume them. One type of phytochemical is the isoflavones. These are found in varying amounts in legumes, but the only significant source in the human diet is soybeans. Isoflavones are phytoestrogens, and have a chemical structure similar to that of estrogen. When consumed by animals or humans isoflavones exert weak estrogenic effects. The two primary isoflavones in soybeans are genistein and daidzein, and their glycosides. In fact, most isoflavones occur in soybeans as the glycoside forms, genistein and daidzein.

Foods made from soybeans have varying amounts of the isoflavones, depending on how they have been processed. Soyfoods such as tofu, soy milk, soy flour, and soy nuts have isoflavone concentrations of 1.3 to 3.8% mg/g or about 37 to 108 mg per ounce. Concentrations are lower in foods made from a combination of soy and grains. Soy sauce and soybean oil have virtually no isoflavones. Soy products which are used as additives, such as isolated soy protein and soy protein concentrate, likewise, have varying amounts of isoflavones, depending on how they were processed. For example, some research shows using alcohol in the extraction process may result in lower isoflavone levels.

Soybean isoflavones are metabolized by gut microflora. Stomach acid may also help in the process. The sugar portion is removed from the glycosides, forming the active compounds genistein and daidzein, which are then absorbed from the intestine. However, the isoflavones can also be degraded by microflora to other metabolites. Daidzein is more bioavailable than genistein, because if has a longer half-life in the intestine. The genistein is degraded about twice as fast, therefore less is absorbed. However, some of the metabolites of these compounds may also have biological significance.

Soyfoods have been a dietary staple in the Orient for hundreds of years. In most Asian countries the consumption of isoflavones is estimated as 25 to 45 mg per day. Japan has the highest consumption level, estimated at up to 200 mg per day. In western countries typical consumption is less that 5 mg of isoflavones per day.

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Physiological Effects of Soy Lsoflavones

Research in several areas of healthcare has shown that consumption of soyfoods may play a role in lowering risk for disease. Soy isoflavones are being studied intensively to clarify the physiological effects they exert. In some cases, the isoflavones may be one of the key factors in soybeans which have disese-fighting potential.

Soy isoflavones may decrease risk for heart disease by helping to lower blood cholesterol levels. Soy protein has been shown to have a cholesterol-lowering effect, which some believe may be partially due to the isoflavones associated with the proteins. Recently, a study with rhesus monkeys showed that soy protein with isoflavones significantly lowered cholesterol levels compared to soy protein without isoflavones.

Epidemiological studies have indicated that populations which regularly consume soyfoods have lower incidences of breast, colon, and prostate cancers. Their incidence is particularly low for the hormone-dependent cancers. This fact has led researchers to investigate the possible effects of phytoestrogens on cancer risk. Women who have excessively high estrogen levels are at risk for beast cancer. Phytoestrogens have weak estrogenic effects, and can bind to estrogen receptors in place of the more potent estradiol. Theoretically they may act as antiestrogens in these cases, and thereby decrease cancer risk.

Soy isoflavones are being investigated for other apparent anticancer effects. In vitro work has shown that genistein inhibits tyrosine kinases, and thereby inhibits cell proliferation and angiogenesis. This means that a tumor would not be able to create new blood vessels, and would therefore not be able to grow. Other in vitro studies have shown that genistein inhibits the growth of human prostate cancer cells and human breast cancer cells. Genistein has been found to have antioxidant properties which may be partially responsible for its anticarcinogenic effects.

The possible role of isoflavones in osteoporosis prevention is also being studied. Consumption of soy protein with isoflavones has been shown to prevent bone loss in a rat model of osteoporosis. It was not clear whether the protective effect was from the isoflavones or the protein. However, another study in a rat model showed the same effect from genistein alone. Ipriflavone, a drug which is metabolized to daidzein, has been found to inhibit bone resorption in postmenopausal women.

Soy foods which contain isoflavones may help in the treatment of menopause symptoms. In women who are producing little estrogen, phytoestrogens may produce enough estrogenic activity to relieve symptoms such as hot flashes. A recent study found that women who were fed 45 grams of soy flour per day had a 40% reduction in the incidence of hot flashes. From an epidemiological point of view, it is interesting that in Japan, where soy consumption is very high, menopause symptoms of any kind are rarely reported.

It is important to keep in mind that our knowledge of the long-term effects of isoflavones is based on their content in soyfoods. These foods have been consumed for hundreds of years, and are known to be safe. However, some labs are now extracting isoflavones to sell as supplements in "health food" stores. These are completely unregulated at this point, and the safety of such compounds if used in unregulated amounts is unknown. It is still best to obtain isoflavones by enjoying a variety of soyfoods.

Conclusion: Consumption of soyfoods containing isoflavones is associated with potentially lower risk for several chronic diseases. Current research efforts are aimed at clarifying the possible role of the soy isoflavones in reducing risk for heart disease, cancer, and osteoporosis, as well as in relieving the symptoms of menopause.

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