Tofu. The word strikes fear into the hearts of livestock producers across America. Humans nearly doubled their soybean consumption in the decade between 1985 and 1996. [1] Tofu has become a household name and a feature on many restaurant menus. [2] Why has this ancient bean, eaten and revered for centuries by certain Asian cultures, caught America’s attention so suddenly? Three factors explain this phenomenon. First, research indicates that soybeans contain many substances that prevent or fight cancer, heart disease, osteoporosis, menopausal symptoms, diabetes, kidney problems, gallstones, and high blood pressure. [3] This is exciting news to a country plagued by such diseases! [4] Second, American consumers are discovering that soybeans are a nutritional powerhouse! [5] Soybeans are high in protein, containing all nine essential amino acids. [6] Soyfoods vary in their nutrient composition but are generally rich in calcium, iron, zinc, B vitamins, and fiber. [7] Soyfoods are fairly low in total fat, low in saturated fat, and contain no cholesterol. [8] Third, the prevalence of soyfoods has helped to eliminate Americans’ ingrained resistance to an unfamiliar food, especially a food with the unusual appearance and texture of tofu, tempeh, textured vegetable protein, or miso. [9] As more Americans sample the endless variety of soyfoods in different recipes, they realize that soyfoods not only possess many healthful qualities, they also taste delicious! [10]

Let’s first examine the heart health benefits of soy consumption. Soyfoods decrease the risk of heart disease by lowering LDL-cholesterol levels, preventing the oxidation of LDL-cholesterol molecules ("bad cholesterol"), and generally promoting the health of the artery walls. [11] The components of soyfoods that seem to lower the risk of heart disease include soy protein, phytoestrogens (specifically isoflavones), linolenic acid, fiber, saponins, and phytosterols.

An analysis of thirty-eight soy protein studies, published in 1995 in the New England Journal of Medicine, demonstrates that consuming soy protein daily (rather than animal protein) decreases significantly blood cholesterol levels in persons of all ages. [12] Specifically, the report showed that soy protein significantly reduces LDL-cholesterol while raising, albeit insignificantly, HDL-cholesterol ("good cholesterol"). [13] This is important because "most dietary interventions such as oat bran intake or decreased saturated fat intake significantly decrease HDL-cholesterol levels." [14] The individuals with the highest initial blood cholesterol concentrations exhibit the greatest reduction in LDL-cholesterol. [15]

The research subjects averaged 47 grams of soy protein per day, obtained from isolated soy protein, textured soy protein, or a combination of both. [16] When a similar study required that the subjects consume only 25 grams of soy protein, only those individuals with initial concentrations above 220 mg per deciliter reduced their cholesterol levels. [17] These studies demonstrate that soy protein can effectively lower blood cholesterol levels, but the threshold amount of soy protein necessary to produce beneficial effects remains unclear. However, these individuals didn’t have to consume outrageous amounts of soy, when you realize that 4 oz. of tofu contains 11 grams of protein and 4 oz. of tempeh contains 17 grams. [18] Furthermore, researchers have shown that merely substituting soy milk for cow’s milk can reduce cholesterol levels. [19]

Soybeans also contain isoflavones, which play a crucial role in soy’s disease-fighting properties. Isofavones reduce the risk of heart disease by adding to the cholesterol-lowering properties of soy protein, preventing the oxidation of LDL-cholesterol, and promoting the health of the artery walls. [20] Soybeans are the only commonly consumed food that contains significant amounts of isoflavones. [21] LDL-cholesterol does not harm the arteries unless oxidized and the isoflavones inhibit that process, according to Mark Messina, Ph.D., co-author of "The Simple Soybean and Your Health." Isoflavones also protect against heart disease by helping to relax the artery walls, which promotes flexibility, thereby ameliorating the harmful effects of cholesterol buildup, says Dr. Messina. Additionally, Dr. Messina reports that isoflavones are heat-stable and therefore unaffected by cooking temperatures; the fermentation involved in making soy items such as tempeh or miso has no effect on isoflavones. One caveat, however: according to Dr. Messina, soy oil and the soy protein concentrates used in many soy burgers and other soy products have lower concentrations of isoflavones than other soyfoods and may not lower cholesterol levels.

Linolenic acid (an omega-3 fatty acid), fiber, saponins, and phytosterols found in soybeans may also play a role in heart disease prevention. [22] Beware the partially hydrogenated soybean oil used in many processed foods, however: not only does it contain 75% less omega-3 fatty acids, it contains trans fatty acids, which may increase cholesterol levels in the same way that saturated fats do. [23] Some soyfoods (generally those made from whole soybeans) are high in soluble fiber and therefore contribute to the amount of total soluble fiber in the diet. [24] A diet high in soluble fiber reduces the risk of heart disease, although the effects are more modest than the effects of soy protein alone. [25] Authorities dispute the role of saponins (molecules similar to cholesterol in their chemical makeup) in reducing blood cholesterol levels, but they may either block cholesterol absorption or cause the body to excrete more cholesterol. [26] Phytosterols, plant compounds that also resemble cholesterol, compete with dietary cholesterol for absorption by the intestines, thus reducing blood cholesterol levels. [27]

Isoflavones constitute the most exciting area of research into the role of soy in cancer prevention and treatment. [28] Genistein is one of the primary isoflavones in soybeans, and it has received much attention from researchers because of its potential to prevent and treat cancer. [29] Genistein has the potential to influence many different types of cancers, such as breast, colon, lung, prostate, skin, uterine, kidney, mouth, esophagus, and leukemia, because it affects very fundamental aspects of the cancer process. [30] Genistein inhibits enzymes intimately involved in the growth and regulation of cancer cells, as well as other key enzymes involved in the cancer process. [31] Genistein also reverses the cancer process of cell de-differentiation, causing cells to differentiate from cancer cells to normal cells. [32] Furthermore, genistein inhibits angiogenesis, the growth of blood vessels needed for tumors to enlarge. [33] Finally, genistein, as well as the other isoflavones, functions as a powerful antioxidant. [34] Antioxidants prevent the cell damage that can lead to cancer. [35]

Researchers dispute the role of genistein in preventing and treating specific types of cancer. According to Dr. Mark Messina, the studies conducted on the relationship between soyfoods consumption and prostate cancer have yielded encouraging results. Unfortunately, Dr. Messina reports that the relationship between soyfoods consumption and breast cancer has become less clear as the research has progressed. This had been a promising avenue of research, because of the low rates of breast cancer in Asian countries, where individuals consume a lot of soyfoods. [36] There is some evidence, however, that consumption of soyfoods by young girls may decrease the risk of developing breast cancer as an adult. [37] Researchers who have conducted encouraging initial studies believe that "the effectiveness of genistein as an anti-cancer agent in humans awaits further preclinical, clinical, and epidemiologic testing." [38]

In addition to genistein, protease inhibitors may contribute to the cancer-fighting properties of soyfoods. [39] Protease inhibitors prevent the activation of specific cancer-causing genes; they also protect the body from radiation and free radicals, which can attack DNA. [40] Unfortunately, protease inhibitors also interfere with the digestion of dietary protein. [41] In animals, however, the amount of protease inhibitor needed to prevent cancer is insufficient to cause adverse affects, thus suggesting that humans could consume enough protease inhibitors in soybeans to lower the risk of cancer while avoiding their adverse effects. [42] This is an exciting area of soybean research that the public should watch closely. [43]

Phytate, phytosterols, and saponins all seem to contribute to soyfoods’ ability to prevent the growth of cancerous cells. Phytate acts as an anti-oxidant by binding iron in the intestines. [45] It also may help to prevent cancer by bolstering the immune system and directly controlling cell growth. [46] Phytosterols protect the colon from the harmful effects of bile acids, and researchers have demonstrated that one type of phytosterol can reduce the development of colon tumors by as much as 50 percent. [47] Saponins act to decrease cancer risk through their antioxidant properties. [48]

Researchers have linked soy consumption to good bone health . The role of soy in preventing osteoporosis is currently a hot research area. [49] According to Dr. Mark Messina, the ease of studying soy’s effects on bone mass makes this area attractive for soy researchers; researchers can easily measure increases and decreases in bone mass. Three substances in soyfoods contribute to bone health: calcium, protein, and isoflavones. [50]

The calcium content of soyfoods contributes to bone health. The amount of calcium in various soyfoods varies widely, but the calcium bioavailability in soybeans is comparable to cow’s milk. [51] This is surprising, because soybeans contain oxalic acid and phytic acid, compounds that inhibit calcium absorption from other plant foods, says Connie Weaver, Ph.D., Head of the Foods and Nutrition Department at Purdue University and a calcium bioavailability researcher. Calcium-set tofu and calcium-fortified soy milk are good sources of calcium. [52] Dr. Weaver reports that a 4 oz. serving of calcium-set tofu provides as much calcium as an 8 oz. glass of milk. According to Dr. Weaver, "There is no food outside of dairy products and other calcium-fortified foods that can accomplish that feat."

Soyfoods may be better sources of dietary calcium than dairy products because soy protein causes the body to excrete much less calcium in the urine than animal protein. [53] In a study in which people consumed the same amount of calcium and protein, those who consumed animal protein lost 50% more calcium from their bodies than those who consumed soy protein. [54] Not surprisingly, countries with high animal protein intake tend to have higher rates of hip fracture, a common result of osteoporosis. [55] This is true even though those same countries tend to exhibit a correspondingly high calcium intake. [56]

Isoflavones also seem to play a role in the prevention of osteoporosis. Dr. Mark Messina reports a study in which researchers fed post-menopausal women either high-isoflavone soy protein, low-isoflavone soy protein, or milk protein instead of soy protein. The women who ate the high-isoflavone soy protein gained significantly more bone mass over 6 months than either of the other two groups. Researchers are unsure exactly how isoflavones influence bone mass. [57] They may stimulate bone formation and inhibit bone resorption (disintegration and absorption by the body). [58] The research into the relationship between soy consumption and bone health is both provocative and promising, says Dr. Weaver, who plans to study the role of soybean phytoestrogens on calcium metabolism in post-menopausal women.

Soyfoods seem to assist in the prevention of kidney disease largely because they replace animal protein, which stresses the kidneys, with healthful plant protein. [59] Kidney stones are composed mainly of calcium, and the excess calcium excretion caused by animal protein consumption may cause their formation. [60] Scientists have demonstrated that the soluble fiber in soy plays a role in regulating glucose levels in diabetics. [61] Soy helps to dissolve gallstones after they have formed, and may even prevent their formation. [62] Researchers speculate that soy lowers blood pressure by allowing the body to excrete more sodium than the body excretes after animal protein consumption; miso and natto also may affect blood pressure because they contain antihypertensive peptides. [63] Soyfoods have a modest effect on symptoms of menopause, [64] such as hot flashes, reports Mark Messina.

The research on the relationships between soy consumption and heart disease, prostate cancer, and osteoporosis demonstrates a link between soy consumption and a decreased risk of succumbing to those diseases. Researchers are less sure of the connection between soyfoods and other cancers, menopausal symptoms, diabetes, kidney problems, gallstones, and high blood pressure, but the research appears promising. It may be that scientists merely have not figured out the amounts or forms of soy necessary to impact those diseases. Belinda Smith, M.S., R.D., Nutrition Research Coordinator for the Metabolic Research Group at the University of Kentucky, suggests eating whole soybean foods such as tempeh, miso, natto, and soynuts because the health benefits of soy may derive from the synergistic effects of all the healthful substances in soybeans. Ms. Smith also advises increasing soyfood consumption gradually while increasing fluid intake, as some individuals experience gastrointestinal discomfort due to a sudden increase in fiber intake. In general, researchers recommend 1 serving of soy daily to maintain general good health, more if you have a higher risk of certain diseases or are using soy to fight a specific disease. The bottom line is this: there are no risks involved in soy consumption and the potential benefits are enormous. [65] So get out there and enjoy soy!

1 Soyatech, Inc., "Soyfoods Production in the United States – Retail Value and Soybean Usage" (chart on file with author).

² Nancy Ross Ryan, "Oh, Boy! Soy! Top Chefs Celebrate the Diversity of Soy Foods with 8 Show-Stopping Dishes," Vegetarian Times, April 1, 1998, p36.

³ Mark Messina, et al., The Simple Soybean and Your Health, p35 (Avery Publishing Group 1994).

⁴ See note 3, p77-121.

⁵ See note 3, p46, 47, 49, 89. I group menopausal symptoms with diseases for organizational purposes only. Menopause is a natural stage in a woman’s life cycle, not a disease.

⁶ See note 3, p19.

⁷ See note 3, p27-29.

⁸ See note 3, p21, 27-29.

⁹ See note 3, p30.

¹⁰ See note 2.

¹¹ Hsiao-Ching Chou, "Soy’s Tasty Side Comes to Light," Denver Post, April 1, 1998, pE2.

¹² Belinda Smith, "Soyfoods Protect from Heart Disease." (http://www.soyfoods.com/symposium97/heart_disease.html)

¹³ James W. Anderson, et al., "Meta-Analysis of the Effects of Soy Protein Intake on Serum Lipids," N Engl J Med 333:276-282, 1995.

¹⁴See note 13.

¹⁵See note 13.

¹⁶ Raga M. Bakhit, et al., "Intake of 25 g of Soybean Protein With or Without Soybean Fibers Alters Plasma Lipids in Men With Elevated Cholesterol Concentrations," J Nutr 124: 213-222, 1994.

¹⁷ "Good News About Soyfoods," brochure published by the United Soybean Board.

¹⁸ See note 3, p70.

¹⁹ See note 3, p103-105.

²⁰Peter L. Zock & Martijn B. Katan, "Hydrogenation Alternatives: Effects of Trans Fatty Acids and Stearic Acid Versus Linolenic Acid on Serum Lipids and Lipoproteins in Humans," J Lipid Res 33: 399-410, 1992.

²¹Joanne Slavin, "Nutritional Benefits of Soy Protein and Soy Fiber," JADA 91: 816-819, 1991.

²²See note 3, p103-104.

²³ See note 3, p70. However, other substances in soybeans, although not unique to soybeans, may play a significant role in the relationship between soy and cancer prevention. These substances are protease inhibitors, phytate, phytosterols, and saponins. Furthermore, although researchers have not delved into this complex area, all these compounds might interact synergistically to protect a body from cancer. See note 3, p65-69.

²⁴ Mark Messina, "Soy as a Possible Alternative to Hormone Replacement Therapy." (http://www.soyfoods.com/symposium97/hormone.html)

²⁵ See note 3, p75.

²⁶ Agostino Molteni, et al., "In Vitro Hormonal Effects of Soybean Isoflavones," J Nutr 125: 751S-755S, 1995.

²⁷ C. Herman, et al., "Soybean Phytoestrogen Intake and Cancer Risk," J Nutr 125: 757S-767S, 1995.

²⁸ Huachen Wei, et al., "Antioxidant and Antipromotional Effects of the Soybean Isoflavone Genistein," Proc. Soc. Exp. Biol. And Med. 124-129, 1993.

²⁹ "California Study Finds Evidence that Soy May Protect Against Cancer: Genistein Cited as Anti-Cancer Agent," PR Newswire, April 24, 1998.

³⁰ See note 3, p65-69.

³¹ See note 3, p65.

³²Dr. Ann Kennedy, a soy researcher, is currently focusing on a specific protease inhibitor, known as the Bowman-Birk-Inhibitor (BBI). Her research has proceeded to the human trial stage, focusing on BBI’s affect on oral, prostate, and lung cancers. She uses BBI extracted in such a manner as to eliminate its interference with protein digestion while retaining the beneficial aspects of protease inhibitor activity. "ProtoMed Announces Protease Inhibitor in Soybeans Capable of Preventing Cancer," PR Newswire, May 6, 1998.

³³ See note 24.

³⁴See note 3, p113-119.

³⁵Robert P. Heaney, et al., "Soybean Phytate Content: Effect on Calcium Absorption," Am. J. Clin. Nutr. 53: 745-747, 1991.

³⁶Connie Weaver, "Meeting Calcium Needs for Optimal Bone Health." (http://www.soyfoods.com/symposium97/calcium.html)

³⁷ See note 3, p118.

³⁸N.A. Breslau, et al., "Relationship of Animal Protein-Rich Diet to Kidney Stone Formation and Calcium Metabolism," J Clin Endocrinol Metabol 66: 140-146 (1988).

³⁹ See note 24.

⁴⁰ See note 24.

⁴¹ See note 3, p119-120.

⁴² See note 3, p119-120.

⁴³Alan C. Tsai, et al., "Effects of Soy Polysaccharide on Postprandial Plasma Glucose, Insulin, Glucagon, Pancreatic Polypeptide, Somatostatin, and Triglyceride in Obese Diabetic Patients," Am J Clin Nutr 45: 596-601, 1987.

⁴⁴ David Kritchevsky & David M. Klurfeld, "Influence of Vegetable Protein on Gallstone Formation in Hamsters," Am J Clin Nutr 32:2174-2176, 1979.

⁴⁵ Felice J. Freyer, "The Humble Soybean May Hold the Key to Healthier, Happier Lives," Providence Sunday Journal, March 29, 1998, pE1.