November 2010 Issue
Shedding Light on Soy
By Densie Webb, PhD, RD
Vol. 12 No. 11 P. 28
Although inconclusive, studies suggest it’s modestly beneficial as part of a cholesterol-lowering diet.
Soy is good for you. Soy is bad for you. Soy can reduce your risk of breast cancer. Soy may increase your risk of breast cancer. The FDA says soy protein can lower blood cholesterol and reduce your risk of heart disease.1 The American Heart Association (AHA) says not so fast. It’s all very confusing. So what’s the bottom line? What advice should you give your patients and clients about consuming soy?
Nutrition Profile and Trends
Soybeans are high in protein (35% to 40% dry weight) and polyunsaturated fats and are the only legume that provides ample amounts of the essential omega-3 fatty acid alpha-linolenic acid.2 They are also a rich source of isoflavones (or phytoestrogens), a subclass of flavonoids that bind to estrogen receptors (though not as strongly as estrogen). Soybeans and soy foods such as tofu, soy milk, tempeh, and miso are the only significant dietary sources of these phytoestrogens. Each gram of soy protein is associated with approximately 3.5 mg of isoflavones.
Soy foods have been consumed for centuries in Asian countries. The mean daily intakes of soy protein are approximately 30 g in Japan, 20 g in Korea, 7 g in Hong Kong, and 8 g in China.3 While the average daily intake in the United States is less than 1 g3, Western vegans typically consume about 10 to 12 g of soy protein daily.4
Soy food consumption in the United States and other Western countries increased dramatically in the last decade following the 1999 FDA approval of a food-labeling health claim for soy protein for the prevention of coronary heart disease (CHD).1 Between 2000 and 2007, more than 2,700 new soy-based food products were introduced in the U.S. market, and sales of foods containing soy increased from $1 billion in 1996 to $4.5 billion in 2009.5 According to the United Soybean Board, 84% of U.S. consumers now perceive soy as healthful, and 37% purposefully consume soy products at least once per month. This increased perception of soy as a health food and its increased consumption has led to more intense evaluation of soy’s potential health effects.
In recent years, soy foods and soybean constituents, specifically isoflavones, have been the subject of nearly 2,000 papers annually.6 Several epidemiological studies suggest that soy consumption is associated with a lower incidence of chronic diseases, such as cardiovascular disease (CVD), osteoporosis, breast and prostate cancers, and type 2 diabetes, and consuming soy may help ease the symptoms associated with menopause. But while the findings are intriguing, they are far from conclusive.
Does Soy Have a Sinister Side?
Genistein, the primary soybean isoflavone, inhibits a wide range of both hormone-dependent and hormone-independent cancer cell lines in vitro.7 But it is the in vitro growth-promoting effects of genistein that have triggered safety concerns.7 The estrogenlike effects of isoflavones observed in animal studies have raised concerns about the potential harmful effects of making soy foods a regular part of the diet, including the possibility of estrogen toxicity and autoimmune thyroiditis.8
One concern has been that soy foods may be contraindicated for women with estrogen-sensitive breast cancer. However, according to Mark Messina, PhD, a soy expert and adjunct associate professor at Loma Linda University in California, the metabolism of isoflavones by mice used in studies is different from humans, meaning the results aren’t necessarily applicable to women. More importantly, he says, no human clinical studies have found that soy or isoflavones adversely affect markers of breast cancer risk, including mammographic density, estrogen levels, or breast cell proliferation.
Research from animal models also suggests that the type of soy product used in studies may be a factor. Tumor growth is positively related to the degree to which the isoflavone-containing product has been processed; the more processed the soy product, the greater the risk.7 According to Messina, whole soy flour does not stimulate tumor growth.7
No randomized controlled trials have evaluated the effects of either soy foods or isoflavone supplements on breast tumor recurrence, and definitive safety data are lacking.4 The clinical data are supportive of safety but not benefit—that is, markers of breast cancer risk are neither favorably nor adversely affected.
There have been claims in some consumer publications that soy may have feminizing effects on men. Despite these claims, a recent paper published in Fertility and Sterility concluded that soy does not have such effects. The review examined published data from more than 150 clinical studies and found that soy food consumption has no effect on circulating levels of testosterone or estrogen, sperm count, semen quality, breast size, or erectile function in men.
But not everyone is as confident about the safety of isoflavones. “While the potentially beneficial effects of phytoestrogen consumption have been eagerly pursued and frequently overstated, the potentially adverse effects of these compounds are likely underappreciated,” says Heather Patisaul, PhD, an associate professor of biology at North Carolina State University and coauthor of a recent review article on the pros and cons of phytoestrogens. While she believes soy foods are safe for most people, she recommends that women struggling with infertility or those who are pregnant moderate their intake. Also, Patisaul notes that most concerns about high doses of isoflavones from soy have been the result of studies using pills or powders in quantities much larger than those usually found in the diet.
Soy and CVD
Soy and CVD prevention is the most studied of all soy/health links. The latest statistics from the AHA, published in Circulation, indicate that one in every three deaths in the United States is the result of CVD. Anything that promises to reduce the risk of CVD garners attention. In a research review, the Agency for Healthcare Research and Quality found that most studies on soy and CVD have been designed to look at cardiovascular surrogate end points such as blood pressure, total cholesterol, LDL cholesterol, HDL cholesterol, triglycerides, and C-reactive protein. No study has evaluated the effect of soy consumption on clinical cardiovascular events.
According to Messina, the cholesterol-lowering effects of soy protein came to the forefront in 1995 with the publication of a meta-analysis that found soy protein lowered LDL cholesterol by 12.9%.9 When the FDA approved a food-labeling health claim for soy protein and reducing the risk of CHD, it set 25 g/day as the threshold soy protein intake. The health claim stated that “25 grams of soy protein per day, as part of a diet low in saturated fat and cholesterol, may reduce the risk of heart disease.” However, several epidemiologic studies have found an inverse relationship between soy protein and blood cholesterol levels at intakes less than 25 g/day. The 25 g/day threshold was set not because any data showed lower amounts were ineffective but because few intervention studies used less than this amount.5
While the FDA approved a food-labeling health claim for soy, it clearly indicated in its opinion that “the evidence did not support a significant role for soy isoflavones in cholesterol-lowering effects of soy protein.” The emphasis has always been on soy protein, leaving little or no role for isolated isoflavone supplements. In fact, according to Patisaul, isoflavone supplements are a waste of money. “The data do not convincingly establish that they confer any health benefits,” she says.
A Change of Heart?
Health claims for soy’s ability to lower cholesterol are also approved in Japan, the United Kingdom, South Africa, the Philippines, Brazil, Indonesia, Korea, Malaysia, and Turkey. However, earlier this year, the European Food Safety Authority issued an opinion that there is insufficient evidence to prove soy protein can lower cholesterol.
Within the past five years, several analyses have indicated that the cholesterol-lowering effects of soy protein are much more modest than originally thought, with most studies showing LDL cholesterol reductions of only 3% to 5%. The FDA is currently reevaluating the evidence in support of the health claim. The agency has stated that it is conducting this reexamination due to the large number of clinical trials published since the claim was first approved, not because the data no longer support the approved health claim.
Since the FDA’s health claim for soy was approved and since the AHA published its statement on soy’s role in reducing CVD risk in Circulation in 2000, the AHA has reevaluated the evidence. In 2006, the AHA’s Nutrition Committee assessed 22 randomized trials and found that isolated soy protein with isoflavones slightly decreased LDL cholesterol but had no effect on HDL cholesterol, triglycerides, lipoprotein(a), or blood pressure. No other effects of soy consumption were evident. The AHA then revised its statement on soy protein by stating in Circulation that “the direct cardiovascular health benefit of soy protein or isoflavone supplements is minimal at best.” In 2008, the AHA wrote a letter to the FDA recommending that the agency revoke the soy protein and CHD health claim.
Earlier this year, a study published in Menopause found no favorable changes in the lipid profiles of overweight, hypercholesterolemic, postmenopausal women as a result of the incorporation of 25 g of soy protein and 60 mg of isoflavones per day in the diet.
According to Messina, many clinical studies have demonstrated that soy foods, soy protein, or isoflavones favorably affect a variety of CHD markers, but because of limited or inconsistent data, no definitive conclusions can be reached about their effects on health. Although the reasons behind the discrepancies in study findings are not fully understood, the source of the soybeans used and different processing methods may affect bioactive compounds and ultimately the products’ ability to lower cholesterol.3
Reports from the Eighth International Symposium on the Role of Soy in Health Promotion and Chronic Disease Prevention and Treatment concluded that even if less than originally believed and modest in comparison with statins and phytosterols, the cholesterol-lowering effects of soy protein are still similar to those of soluble fiber and could be relevant from a public health perspective. Incorporating soy foods into diets that include multiple cholesterol-lowering dietary factors can provide impressive benefits, such as those seen with the Portfolio diet. The Portfolio dietary approach, in which soy foods provide most of the protein, has been shown to result in decreases in LDL cholesterol ranging from 20% to 30%.10
Further, a body of epidemiologic evidence associates soy intake with a reduction in the risk of CVD events beyond that which could be attributed to the benefits of cholesterol reduction.11 Studies have found blood pressure-lowering effects of soy foods as well as improvements in endothelial function, reduced LDL cholesterol oxidation, and larger LDL particle size associated with reduced CHD risk.12,13 Some of these effects could be due to soy’s isoflavone content. However, the studies are few and the findings inconsistent.
In future studies, careful characterization and identification of soy proteins will be necessary to accurately compare findings from different studies.3 Making soy one part of a cholesterol-lowering diet is likely to confer modest cardiovascular benefits with no identifiable risk. However, the use of phytoestrogen supplements, rather than substituting soy protein for some animal protein as part of a balanced diet, is unlikely to confer meaningful cardiovascular benefits. Messina believes an optimal soy intake for adults is between 2 and 4 servings of soy foods per day.
— Densie Webb, PhD, RD, is a freelance writer, editor, and industry consultant based in Austin, Tex.
Easy Soy Food Additions
• Add roasted soy nuts to cookie or brownie recipes.
• In recipes calling for cream cheese, replace half with puréed tofu.
• Add firm tofu to meatless chili, soups, and stews.
• Cook extra-firm tofu on the grill.
• Skewer extra-firm tofu chunks for shish kebobs.
• Use tofu sausage links and soy burgers.
• Use soy nut butter in place of peanut butter.
• Make smoothies using soy milk and silken tofu.
• Make muffins with soy flour.
• Keep soy protein bars on hand for snacks.
• Try jalapeño-flavored soy cheese on sandwiches and soy Parmesan on spaghetti sauce.
• Look for soy yogurt that contains healthful bacterial cultures and is fortified with calcium and vitamin D.
• Substitute edamame for chickpeas to make hummus.
• Try soy pasta.
— Author adapted tips from multiple sourcesReferences
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3. Xiao CW. Health effects of soy protein and isoflavones in humans. J Nutr. 2008;138(6):1244S-1249S.
4. Messina M, Nagata C, Wu AH. Estimated Asian adult soy protein and isoflavone intakes. Nutr Cancer. 2006;55(1):1-12.
5. Soyfoods Association of North America. Sales and trends. Available at: http://www.soyfoods.org/products/sales-and-trends
6. Messina M, Messina V. The role of soy in vegetarian diets. Nutrients. 2010;2(8):855-888.
7. Messina M, Abrams DI, Hardy M. Can clinicians now assure their breast cancer patients that soyfoods are safe? Women’s Health. 2010;6(3):335-338.
8. Doerge DR, Sheehan DM. Goitrogenic and estrogenic activity of soy isoflavones. Environ Health Perspect. 2002;110(Suppl 3):349-353.
9. Anderson JW, Johnstone BM, Cook-Newell ME. Meta-analysis of the effects of soy protein intake on serum lipids. N Engl J Med. 1995;333(5):276-282.
10. Jenkins DJ, Kendall CW, Faulkner D, et al. A dietary portfolio approach to cholesterol reduction: Combined effects of plant sterols, vegetable proteins, and viscous fibers in hypercholesterolemia. Metabolism. 2002;51(12):1596-1604.
11. Zhang X, Shu XO, Gao YT, et al. Soy food consumption is associated with lower risk of coronary heart disease in Chinese women. J Nutr. 2003;133(9):2874-2878.
12. Hooper L, Kroon PA, Rimm EB, et al. Flavonoids, flavonoid-rich foods, and cardiovascular risk: A meta-analysis of randomized controlled trials. Am J Clin Nutr. 2008;88(1):38-50.
13. Messina M, Lane B. Soy protein, soybean isoflavones and coronary heart disease risk: Where do we stand? Future Lipidology. 2007;2(1):55-74.