Soy

A member of the legume family, the soybean is rich in omega-3 fatty acids , fiber , folic acid, calcium , magnesium, potassium, and the B vitamins and is also cholesterol free and low in saturated fat . The protein in soybeans is complete, containing all the essential amino acids found in animal sources (4 ounces of tofu [soybean curd] contain the same amount and quality of protein as a similar-size hamburger). For individuals who want to include more plant-based protein in their diet and particularly for those on a vegetarian diet, soy products provide a way to add nonmeat protein to the diet.

Soy and Your Heart

Among the many benefits of soy are the potential for lowering one's risk of heart disease , menopausal bone loss, breast and prostate cancer, and osteoporosis . The results of a 1995 meta-analysis (combining results from separate but related studies) published in the New England Journal of Medicine found that consuming an average of 47 grams of soy protein a day, rather than animal protein, significantly decreased LDL cholesterol in people with moderately elevated or elevated cholesterol levels (low-density lipoproteins (LPLs) are the "bad" type of cholesterol and have been associated with clogged arteries and heart attacks). The study also found that high-density lipoprotein HDL cholesterol (the beneficial, or "good," cholesterol) was not affected by the consumption of soy protein.

Scientists suggest that soy protein and isoflavones are the active substances helping to keep blood vessels flexible and preventing deadly blood clots. Isoflavones are phytochemicals (naturally occurring compounds) found in plants, and they have potentially strong biological activity, meaning they exert a physiological effect, in the body. Phytochemicals give plants their color, flavor, and odor, and they have benefits to the body beyond basic nutrition . The U.S. Food and Drug Administration (FDA) has approved a health claim stating that consuming 25 grams of soy protein per day, along with a diet low in saturated fat and cholesterol, may reduce the risk for heart disease. The FDA also suggests that four servings of soy foods per day can lower LDL cholesterol by 10 percent. To use this claim, a food product must contain 6.25 grams of soy protein per serving. The claim does not include a recommendation for isoflavone level, which remains an issue of debate. The 25 gram recommendation applies to all ages.

Many of today's common diseases are not diseases of aging, but of lifestyle , and they can take twenty to thirty years to develop. Typically, when soy protein replaces animal protein, the consumption of saturated fat and cholesterol goes down. About 40 percent of the soybean's calories come from fat , with the majority (54 %) being unsaturated. The polyunsaturated fat in the soybean includes omega-3 fatty acids, which are not frequently found in plants.

Soy and Cancer

Soybeans and soy foods in the diet may provide strong anticancer activity because they are natural sources of isoflavones. A specific isoflavone called genistein, which is found is soy, appears to help block tumor-cell growth. Current studies indicate that consuming soy may reduce the risk of developing prostate cancer, while isoflavone supplements may help physicians stabilize prostate cancer by decreasing the prostate-specific antigen (PSA) level used to measure how well the cancer is being controlled.

There has been much debate and disagreement about soy consumption and its role in breast cancer. Similar in chemical structure to estrogen , isoflavones are in fact, weak estrogens, and they may act as such in the body. Hormone replacement therapy (HRT) has been shown to increase breast density, a factor in breast-cancer risk (as breast density increases, so does the risk for breast cancer), while recent soy studies have found that soy use in both premenopausal and postmenopausal women did not affect breast density.

Soy and Menopause

The popularity of soy-based foods is also due to their potential for reducing the symptoms of menopause. In a study published in Obstetrics and Gynecology, researchers found that women who consumed 60 grams of isolated soy protein daily reported a reduction in moderate to severe hot flashes. Other studies have contradicted this finding, however, Japanese women, who typically have a soy-rich diet, do experience a lower incidence of most post-menopausal symptoms than women in Western countries, including hot flashes, hormone-related cancers, and osteoporosis.

Soy and Osteoporosis

Another area with conflicting studies is the link between soy and osteoporosis. One method for determining your bone health and changes in bone density over time is calcium metabolism (a process where a substance, necessary for life, is synthesized or broken down). For bone density to increase, more calcium must be kept in the bones. This retention is measured by tests that look at calcium absorption versus calcium loss as measured in the urine. In studies that have compared a soy diet to a calcium/whey diet, calcium loss through the urine was much lower on the soy diet. Some researchers suggest that the amino-acid content of soy protein, as compared to that of animal protein, is the reason for less calcium loss in the urine.

The versatile soybean remains a popular food choice, and adding soy to the diet is one component of a healthful eating program. Even the soybean

Soybeans are soaked, mashed, mixed with water, and then pressed to extract soy milk, which is curdled to make tofu. Combining 25 grams of soy protein per day with a low-fat diet can have significant health benefits.

[Hans & Cassidy. The Gale Group. Reproduced by permission.]

SEE ALSO Functional Foods ; Legumes ; Meat Analogs ; Plant-Based Diets ; Vegetarianism .

Susan Mitchell

Bibliography

Anderson, James W., et al. (1995). "Meta-Analysis of the Effects of Soy Protein Intake on Serum Lipids." New England Journal of Medicine 333:276–82.

Albertazzi P., et al. (1998). "The Effect of Dietary Soy Supplementation on Hot Flashes." Obstetrics and Gynecology 91:6–11.

Internet Resources

Messina, Mark (2002). "Symposium Highlights Significant Research On Soy and Human Health." The Soy Connection, Winter. Chesterfield, MO: United Soybean Board. Also available from <http://www.talksoy.com>

Stevens and Associates (2002). "The U.S. Soyfoods Directory." Available from <http://www.soyfoods.com>

United States Department of Agriculture, Agricultural Research Service (2002). "USDA-Iowa State University Database on the Isoflavone Content of Foods." Available from <http://www.nal.usda.gov/fnic/foodcomp/Data/isoflav/isoflav.html>