Close to 40 million American women will reach menopause in the next 20 years. Many will seek alternatives to relieve menopausal symptoms as researchers and healthcare practitioners continue to question the safety of hormone therapy. Among these alternatives are soyfoods. Prized for their versatility, soyfoods have been dietary staples in many Asian countries for centuries.
Nutritionally, soybeans are higher in protein than other legumes, and the quality of their protein compares to that of animal protein. Traditional soyfoods, such as tofu, miso, tempe, and soymilk, provide about 7 to 15 g protein per serving. Although soybeans have more fat than other beans, their fatty acid profile is heart-healthy—high in polyunsaturated fat and low in saturated fat. Also, the soybean is one of the few good plant sources of omega-3 fatty acids.
Soyfoods are unique dietary sources of isoflavones—naturally occurring compounds whose estrogen-like properties may relieve menopausal symptoms and reduce the risk of osteoporosis and heart disease. Part of a group of chemicals called phytoestrogens (plant estrogens), isoflavones have a chemical structure similar, but not identical, to that of estrogen. They bind to the same receptors in cells as estrogen, and sometimes exert estrogen-like effects. But because isoflavones prefer a particular type of estrogen receptor, they don’t always exert estrogenic effects and sometimes have antiestrogenic effects on tissues or biological processes typically affected by estrogen. Because of these variable actions, isoflavones are often referred to as SERMS—selective estrogen receptor modulators. One of the best known SERMS in medical practice is tamoxifen, a drug widely used to treat breast cancer.
Older adults in Japan consume about 30 to 50 mg isoflavones daily. Westerners, in contrast, typically have low soyfood intakes and consume less than 3 mg isoflavones daily, on average. One serving of a traditional soyfood provides about 25 mg isoflavones. The U.S. Department of Agriculture maintains an online database of the isoflavone content of a large variety of soyfoods (www.nal.usda.gov/fnic/foodcomp/Data/isoflav/isoflav.html).
The low incidence of hot flashes among native Japanese women first prompted speculation that soyfoods may relieve hot flashes because they contain isoflavones. About 10% of menopausal women in Japan report having hot flashes, compared to roughly 50% in the United States. (But recent research raised the possibility that Japanese women are reluctant to report having hot flashes, so their incidence may be higher than 10%.) The estrogen decrease that occurs around menopause is certainly one trigger for the onset of hot flashes, providing ample basis for investigating isoflavones’ effects.
A recent comprehensive literature review found isoflavone supplements reduced both the frequency and severity of hot flashes by approximately 50%. About half of this decrease resulted from isoflavones and half from the placebo effect. Isoflavones have clinically meaningful effects; a survey found that among women seeking nonhormonal alternatives to estrogen for hot-flash relief, 70% would be pleased by a 50% improvement. To promote compliance, most recent trials have used supplements rather than soyfoods, but approximately two servings of traditional soyfoods provide the amount of isoflavones found to be effective (50 mg).
Bone loss increases dramatically around the time of menopause, largely due to the estrogen decline. Several clinical trials, including the Women’s Health Initiative, show estrogen therapy reduces bone loss and fracture risk.
The potential skeletal benefits of isoflavones have piqued researchers’ interest. Two prospective epidemiologic studies have evaluated the relationship between soy intake and fracture risk. Comparing women in the highest soy-intake category to those in the lowest, both studies found high intake reduced the fracture risk by about one-third. In one study, 1,770 fractures of all types occurred among more than 24,000 Shanghai women followed for 4.5 years. In the other study, 692 hip fractures occurred in more than 34,000 Singaporean women followed for 7 years.
Since 1998, more than 25 clinical trials have evaluated the effects of isoflavone-containing products on bone mineral density (BMD) in peri- and postmenopausal women. How-ever, most of these trials had a short duration. Trials that include BMD as the primary endpoint should be conducted for at least 2 years, or ideally, 3 years.
The two isoflavone trials conducted for 3 years produced conflicting results. In an Italian study, postmenopausal women who consumed 54 mg per day of genistein (the primary isoflavone in soyfoods) showed dramatic increases in spinal and hip BMD, compared to marked BMD decreases in the placebo group. But in a U.S. study, isoflavones didn’t substantially affect BMD.
Despite some impressive epidemiologic data, we can’t draw conclusions about the direct skeletal effects of isoflavones. But because of their protein and calcium content, many soyfoods are good choices for protecting bone health. (See Soyfoods glossary by clicking the PDF icon above.)
Coronary heart disease
Among postmenopausal women, heart disease is the leading cause of death. Soyfoods may play a protective role due to their isoflavone and nutrient content. Based on the cholesterol-lowering properties of soy protein, the Food and Drug Administration (FDA) allows foods containing at least 6.25 g soy protein per serving to carry a health claim on their labels about the role of soy protein in reducing heart-disease risk.
Soy protein reduces low-density lipoprotein (LDL) levels by about 4%. Over time, even this modest reduction may reduce heart disease risk by 4% to 8%. Soyfoods also are low in saturated fat and high in polyunsaturated fat (a mixture of omega-6 and omega-3 fatty acids). Because of the improved fatty acid content, some experts believe that replacing more traditional protein sources in the U.S. diet with soyfoods would lower LDL levels by 4%. The direct effect of soy protein combined with the indirect benefit of displacing less healthy foods in the diet can be expected to substantially reduce heart disease risk.
What’s more, soyfoods may reduce risk independent of their cholesterol effects. Two recent meta-analyses found soy lowers blood pressure, and compelling evidence exists that isoflavones improve endothelial function. Clearly, for multiple reasons, soyfoods can make important contributions to heart-healthy diets.
Much of the initial research interest in soy was based on evidence suggesting soyfoods reduce breast cancer risk. Research conducted over the past 20 years supports this hypothesis—but with a twist. Most evidence now indicates that to derive protection from breast cancer, females must consume soyfoods early in life—during childhood, adolescence, or both. Perhaps as little as one serving per day may reduce risk.
Yet concern has arisen that soyfoods may be harmful to breast cancer patients and women at high risk for developing this disease. Greater lifelong exposure to estrogen is thought to increase breast cancer risk, so some experts suspect isoflavones’ estrogen-like effects also may increase risk. Nonetheless, recently published clinical and epidemiologic data are reassuring. Studies show that isoflavones, whether from soyfoods or supplements, don’t affect markers of breast cancer risk, including breast-
tissue density and breast-cell proliferation.
Furthermore, two Chinese and two U.S. prospective epidemiologic studies show soy consumption improves prognosis in breast cancer patients. So, while it may be premature to recommend soyfoods specifically to improve prognosis, the evidence no longer justifies advising breast cancer patients not to consume soyfoods.
Although soy protein is among the top eight most common allergens, soy allergies are rare. An FDA survey found that one in 2,500 adults is allergic to soy, compared to 1 in 90 adults who are allergic to dairy. Nonetheless, those with soy allergies need to pay close attention to food choices, as many foods contain small amounts of soy protein as additives.
Contributing to a healthy diet
According to the FDA, people who want to reduce their cholesterol level using soyfoods should consume at least 25 g soy protein daily—although many experts believe smaller amounts of soy can still be effective. To relieve menopausal symptoms and improve endothelial function, two to three servings daily providing a total of 50 to 75 mg isoflavones appear to be beneficial. A serving is 1 cup of soymilk, ½ cup of tofu or edamame, or ¼ cup of soynuts. Based on several lines of evidence, soyfoods clearly can make important contributions to a healthy diet.
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Virginia L. Messina is Vice-President and Mark Messina is President of Nutrition Matters, Inc. in Port Townsend, Washington, a consulting company that focuses on providing nutritional information about plant-based diets and soyfoods to healthcare professionals and consumers.