What are soy isoflavones?
Isoflavones are biologically-active, nonnutritive compounds that are present in relatively large amounts in soybean and soyfoods. Soybeas contain two main types of isoflavones; daidzein and genistein. These compounds are part of a larger group of plant chemicals, called flavenoids, that are common in many fruits, vegetables, and legumes. Soybean are by far the most concentrated source of isoflavones in the human diet.
Isoflavones are produced by the soybean plant as part of their defense mechanism against insects and diseases such as Phytophthora, and in response to environmental stresses such as drought. Isoflavones also play an important role in the growing soybean plant by stimulating nodule formation by nitrogen-fixing Rhizobium bacteria.
Why the current interest in soy isoflavones?
In the past decade or so, the consumption of soyfoods has been linked to human health benefits such as protection against breast cancer and prostate cancer, relieving the discomforts of menopause and the risk of osteoporosis, and lowering cholesterol levels. There is considerable speculation that the soy isoflavones in soybean are responsible for these possible effects on human health.
Scientists have determined that that chemical structure of soy isoflavones is very similar to that of our own hormone estrogen. It was not surprising then, to discover that soy isoflavones can bind to estrogen receptors and exert hormonal effects on mammals. Of particular interest is the observation that the binding of isoflavones may be tissue-selective, which mean isoflavones mimic the effects of estrogen in some tissues and blocks the effects of estrogen in others. The extent to which soy isoflavones exert estrogenic and anti-estrogenic effects in animals and humans is currently the focus of considerable scientific research.
State of the current research
In animal and laboratory studies, isoflavones appear to have both hormonal and nonhormonal properties. For example, genistein is known to inhibit the activity of many enzymes and cellular factors that control the growth of cells. It also has antioxidant properties. Some studies suggest that these substances may help maintain bone strength and inhibit certain cancers. Other studies suggest that it is not the genistein and daidzein—perhaps not any of the isoflavones—but something else in soy that may have these biological effects. Tthere have been other studies into various proposed health benefits of soy or soy compounds that have not found a positive effect.
Some researchers are concerned about the biological activity of plant-derived estrogens such as isoflavones and consider their long-term effect on humans an unknown. The prime concern is the potential for chronic changes to the endocrine (hormonal) system that could affect reproductive health. Another concern is possible alterations to the immune system.
As the role of soyfoods and soy isoflavones in human health have been studied more intensively in clinical trials, the evidence for health benefits has become a more contradictory. In January 2006, a review by the American Heart Association (AHA) of 22 studies on the effects of soy protein with isoflavones demonstrated minimal or no benefit on cholesterol, which led the AHA to state that they could not recommend the use of isoflavone supplements in pills or food for the prevention of heart disease. This was a reversal of their previous position stated in the late 1990's (Soy Protein, Isoflavones, and Cardiovascular Health, AHA, 2006).
The bottom line is that although diets rich in soy or soy-containing products appear safe and potentially beneficial, the long-term safety of high doses of soy isoflavone supplements is not yet known.
Isoflavone content of foods
Isoflavones are found in small amounts in a number of legumes, grains, and vegetables, but soybean are by far the most concentrated source of isoflavones in the human diet. Whole (dry) soy contains about 200 milligrams of isoflavones per 100 gram serving. A good source of information on the isoflavone content of specific foods can be found in the USDA-Iowa State University Database on the Isoflavone Content of Foods.
Soyfoods differ somewhat in their concentration of isoflavones, but all of the traditional soyfoods, such as tofu, soymilk, soynuts, tempeh, miso, and edamame soybean (varieties of soybean that are harvested and eaten in their green phase) are rich sources of isoflavones.
Soy flour and textured soy protein also contain high amounts of isoflavones. Soy protein concentrates, a widely used ingredient in soyfoods such as soy burgers, generally do not contain nutritionally-significant amounts of isoflavones, but some do—depending on how the product was processed. “Second-generation products", such as soy hot dogs and soy-based ice cream, also have much lower amounts of isoflavones because they usually contain considerable amounts of non-soy ingredients. Soy oil and soy sauce do not contain isoflavones.
What about soy isoflavone supplements?
Because of the health claims of soy isoflavones, there are now many isoflavone supplements on the market. Most of these supplements contain isoflavones which are extracted from soybean or red clover. However, the effect of concentrated isoflavones on human health remains unclear.
One problem with soy isoflavone supplements is the variable amounts of isoflavones in the supplements, and how these differ from the levels in soy foods. Pills may contain much more, or much less, than the label states, since there is no regulation. In addition, we do not know yet how much you would need to get a benefit, if there is one, and how much would be too much.
The best way to consume isoflavones is in the form of soy or whole soyfoods, so you can benefit from the high protein content and other healthy components of soy. It would be difficult to consume too much isoflavone from natural soy products, but there might be a risk associated with the consumption of dietary estrogens in the form of concentrated isoflavone pills or supplements. Most nutritionists agree that one to two servings a day of whole soy foods is enough to give you the benefits of soy without the theoretical risks.
More on soy isoflavones
Soy and Isoflavones. United Soybean Board (pdf)
Soy Isoflavones, Linus Pauling Institute, Micronutrient Research for Optimum Health, Micronutrient Information Center.
National Cancer Institute, Division of Cancer Prevention, Soy Isoflavones.
U.S. Department of Agriculture, Agricultural Research Service. 2002. USDA-Iowa State University Database on the Isoflavone Content of Foods, Release 1.3 - 2002. Nutrient Data Laboratory Website.
Soy Protein, Isoflavones, and Cardiovascular Health, An American Heart Association Science Advisory for Professionals from the Nutrition Committee, 2006.