Soy and Health

Sales of soy foods in the U.S. have more than tripled in the past decade to about $3 billion. In a recent survey by the United Soybean Board (USB), 70% of consumers thought soy was healthy, but only 27% reported eating soy foods at least once a week.

Scientific research shows promise that soy helps prevent or reduce risk of chronic diseases. Could soy be a safe, natural alternative to Hormone Replacement Therapy (HRT) for alleviating menopausal symptoms? Recent research showed HRT may increase risk for breast and uterine cancer, heart disease and stroke. Large, long-term (three years or more) intervention trials with soy are needed.

Nutritional value: "Soybeans are a 'nutrient powerhouse' packed with high quality protein, complex carbohydrates including fiber (soluble and insoluble), B-vitamins like folate, minerals (e.g., iron, calcium, potassium) and essential polyunsaturated fats (e.g., linoleic and alpha-linolenic acids)," says USB spokesperson Eleanor Pella, MS, RD, in Gettysburg, PA.

Soy contains no cholesterol and is naturally low in sodium and saturated fat. Soy is the only bean that is a "complete" protein (contains all essential amino acids). Soybeans contain several phytochemicals (plant substances) including iso- flavones, phytate, saponins, phytosterols and protease inhibitors.

"Soybeans are the richest, natural dietary source of isoflavones (e.g., genistein, daidzein, glycitein), which along with soy protein, may help reduce risk for chronic diseases like cancer and heart disease," Pella adds.

Some researchers think iso- flavones may be responsible for about 75% of soy's health benefits. Isoflavone content varies with types of foods, brands and processing methods. Isoflavones are heat stable. Cooking won't reduce the amount. One serving of a typical soy food provides about 20-35 mg. of isoflavones (about 2-4 mg. per gram of soy protein). But, soybean oil and soy sauce contain no isoflavones.

Heart disease: USB spokesperson Jackie Newgent, RD, in New York, says: "In 1999, the Food and Drug Admin. (FDA) approved the following health claim for food labels on soy foods: 'Diets low in saturated fat and cholesterol that include 25 gm. of soy protein daily may reduce the risk of heart disease.'" That's the equivalent of about three cups soymilk or 10 oz. tofu or one cup canned soybeans or a half-cup soy nuts.

"One serving must contain at least 6.25 gm. soy protein to bear the label. FDA advises eating four servings daily to lower high blood cholesterol ('bad' LDL-cholesterol and total cholesterol). But, I think eating one or two servings daily is more practical and may also be beneficial."

Soy expert Mark Messina, PhD, an adjunct assoc. professor at Loma Linda (CA) Univ., agrees. He recommends two servings per day of soy foods (about 15 gm. soy protein and 50 mg. isoflavones) for adults. "It's easy and safe to consume this much (even for women with a history or high risk of breast cancer). Eating soy only three or four times a week is probably not enough to be beneficial," Messina says.

Cancer prevention: "The most promising research for soy's anti-cancer effects involves prostate cancer," he continues. "In one study of men with advanced prostate cancer who took isoflavone supplements, the cancer seemed to stabilize. Soy may delay or slow the progression of prostate cancer.

"Breast cancer research is speculative. The chemical structure of isoflavones is similar to estrogen, so isoflavones may bind to certain estrogen receptors blocking out estrogen (a natural hormone that may promote breast cancer). Isoflavones may act as 'selective estrogen receptor modulators' (SERMs) in some tissues, but have no effect or anti-estrogen effects in others (e.g., breast). This is one theory of how soy may help prevent breast cancer."

Soy intake during puberty (e.g., teen girls) may reduce adult breast cancer risk by changing the structure of breast cells, according to Messina. "There's also some evidence that soy (unlike HRT) has no effect or decreases breast cancer risk in women. Isoflavone supplements reduced breast tissue density (marker for breast cancer) in postmenopausal women, but had no effect in premenopausal women."

More research is needed to determine if soy can reduce or increase risk of breast cancer, especially in high-risk women and women who have had estrogen-dependent breast cancer. Meanwhile, the American Cancer Society (ACS) advises these women to limit soy foods to a few servings a week and avoid isoflavone supplements.

Bone health: "In most studies, soy had little or no impact on bone mineral density in women," Pella explains. "But, substituting soy protein for animal protein may slow or stop bone loss by reducing urinary calcium loss (from bone breakdown)."

Menopausal symptoms: According to Newgent, most clinical studies show soy and isoflavones may not be any better than a placebo (sugar pill) in relieving the severity and frequency of hot flashes. Only two studies showed a slight reduction in the severity, but not frequency, of symptoms.

Isoflavone supplements: Isoflavone supplements vary in quality (purity) and quantity, Pella notes, and safety of long-term use has not been proven in humans.

Newgent adds: "Limit isoflavone intake to 100 mg. daily. Try to eat soy foods."