Soybeans are native to East Asia, where they have been an important food staple for centuries. There is documentation of soy being cultivated as early as 1100 BC and of the use of soybean oil around 1061 AD.1, 2 Within the last two decades, soy products are increasingly being incorporated into the American diet. Soybeans are legumes (as are beans and peas). Some foods made from soybeans include tofu, tempeh, miso, soybean oil, and, of course, soy sauce. However, with increasing popularity of soy-based foods, a multitude of other soy products—so called “second generation” soy products—and soy ingredients have become available, such as soy milk, soy flour, soy protein concentrate, and soy protein isolate. Soy protein isolates and concentrates have been used to develop a range of food products including beverages and meat alternatives such as “veggie burgers” as they can be processed to function similarly to the traditional sources of protein in meat and dairy products.
Among plant-based foods, soy is unique because the protein in soy products—processed in ways that retain protein integrity—can, like the protein found in meat, milk, and eggs, be considered a complete protein (all essential amino acids are present in sufficient quantity to support normal growth and development after correcting for protein digestibility). The soybean itself is composed of approximately 40 percent protein, 20 percent oil, 35 percent carbohydrates, and 5 percent trace minerals and other compounds. Among these key compounds, isoflavones have been extensively researched for potential health effects. Depending on the method and/or degree of extraction from the bean, soy protein sources may differ in the amount of isoflavones they contain. 3
A lower incidence of cardiovascular disease, certain types of cancer, bone fractures, and menopausal symptoms is observed in many Asian countries compared to that found in the United States and other Western cultures. Differences in diet and, more specifically, soy consumption are presently being studied in an attempt to explain the variance in incidences of these chronic diseases and other health concerns. Soy protein has many clinically significant health benefits in humans. These benefits include reduced total and LDL (“harmful”) cholesterol in the blood while maintaining HDL (“beneficial”) cholesterol; some studies show an increase in HDL cholesterol.4-7 Emerging evidence suggests that soy may reduce the risk of certain cancers.8-12 Soy protein with isoflavones may also help maintain bone mass, and some evidence suggests a reduction in the number and severity of “hot flashes” in post-menopausal women.6, 13
In October 1999, the U.S. Food and Drug Administration (FDA) authorized a health claim for conventional foods, associating soy protein consumption with reduced risk of coronary heart disease (CHD).14, 15 Product labels bearing the claim would read, “25 grams of soy protein a day, as part of a diet low in saturated fat and cholesterol, may reduce the risk of heart disease.”14 The FDA further clarifies, “In order to qualify for this health claim, a food must contain at least 6.25 grams of soy protein per serving, the amount that is one-fourth of the effective level of 25 grams per day. Because soy protein can be added to a variety of foods, it is possible for consumers to eat foods containing soy protein at all three meals and for snacks.”15
Research indicates that soy foods assist with decreasing LDL oxidation, and improve arterial compliance (“stiffness”). Numerous studies since 1967 demonstrate the effect of soy protein on lowering total and LDL cholesterol levels in the blood.4, 5, 7, 16-21 Epidemiological research suggests that soy can help lower LDL cholesterol, which lowers the risk for coronary heart disease.22 In populations with elevated levels of cholesterol, the integration of soy into the diet has a more profound effect.23 Experts believe consumption of intact soy protein has cholesterol-lowering advantages over extracted isoflavones.16 Soy protein, as part of a dietary portfolio (which also includes almonds, plant sterols, and viscous fiber), can bring about similar LDL cholesterol-lowering effects to statin medication treatment; the effects of both the statin medication and dietary portfolio treatments are greater than a low-saturated fat diet alone.24, 25 Studies demonstrate that soy protein with isoflavones may increase HDL cholesterol levels in the blood although the level of significance varies based on sex, amount of soy consumed, initial lipid profile, and duration of time soy is incorporated into the diet.5-7 Soy foods also appear to protect against oxidation of the LDL particle; this effect may be attributed to soy’s isoflavone content.26 This is important since oxidation of LDL particles is widely believed to have a role in the formation of atherosclerosis (“hardening of the arteries”), a causative first step toward heart disease. Furthermore, arterial compliance is also linked to atherosclerosis.27 Results from another study involving healthy perimenopausal and menopausal women indicate that consumption of purified soy isoflavones results in a 26 percent improvement in arterial compliance, a similar extent to that achieved with conventional hormone replacement therapy. This study, however, did not find cholesterol-lowering benefits.28 Soy protein with isoflavones also significantly improves vascular function through observed vasodilatory response (“widening of the arteries”).29
A recently published meta-analysis of several epidemiological studies shows that the consumption of soy foods decreases the risk of developing breast cancer in both pre- and postmenopausal women. Findings from the studies also suggest that consuming soy foods in adolescence reduces the risk of developing breast cancer later in life with the highest risk reduction seen in those who consume soy foods throughout adolescence and adulthood.30 However, based on data from animal research, questions remain about soy consumption and whether it should be consumed by women with estrogen-sensitive breast cancer, or by women with high risk of developing breast cancer. A long-term study found that isoflavones have no significant impact on breast cancer risk.31 In a review by Messina and Loprinzi, animal models are discussed regarding the study of soy and breast cancer. Some suggest that in ovariectomized rodents, genistein, an isoflavone found naturally in soy, stimulates tumor growth, whereas when using intact rodents, genistein plays a role in the inhibition of tumor growth.32-34, 9-12 Clinical studies currently are underway to continue examining how soy components, including isoflavones, may affect cancer risk.
Research suggests improvements in bone mass in postmenopausal women and slowing of bone loss at the lumbar spine in perimenopausal women in response to feeding soy protein with higher levels of isoflavones.6, 35 The effects are modest, and only the lumbar spinal region appears to benefit as no effects are observed at the hip. The short duration of these studies (approximately 24 weeks) demonstrate a compelling need for longer-term studies, which are currently underway. 36
Other potential health effects of soy foods are being investigated. A statement from The North American Menopause Society suggests women transitioning through menopause may benefit from incorporating soy protein with isoflavones and isoflavone concentrates into their diets.37 Studies show that “hot flashes” are modestly reduced in menopausal women consuming soy protein with higher levels of isoflavones, equivalent to 60 grams of isolated soy protein with 76 mg isoflavones per day during the study trial period.13, 38 However, experts note that the health effects of soy foods and soy isoflavones should not be considered equal to known estrogen actions.39 A recent government review of studies indicates investigators employ a variety of symptom scores and frequency indexes, which makes comparisons across studies difficult. However, evidence of benefit is stronger among the randomized trials of isoflavone supplements, which mostly show positive results.40
Soy continues to be one of the most intriguing food components studied today. Emerging areas of research indicate that soy may have a role in weight management and reducing risk of prostate cancer. One study demonstrates that soy protein provides satiety equal to other proteins.41 Current research suggests that utilizing soy protein as part of a low-calorie diet for weight management provides the additional benefit of cholesterol reduction and improved lipid profiles not evidenced in low-calorie diets alone.42 In relation to prostate cancer, studies show that dietary components, including soy isoflavones, reduce prostate cancer risks; a large epidemiological study suggests that prostate cancer mortality is inversely related to estimated soy intake.43 Further clinical studies will continue to increase understanding of the role of soy in maintaining and improving health.
Legume crop originating in East Asia and now also grown in the United States and South America
Tofu, miso, tempeh, soy milk, soy flour, soy protein concentrate, soy protein isolate, texturized soy protein, meat alternatives, okara, yuba, dry blended beverages, food bars, in addition to many other foods
*NOTE: Soy isoflavones and other bioactives may not be found in all of these products.
The Bottom Line
Incorporating soy protein-rich foods into the diet is beneficial in helping to reduce LDL- cholesterol and thus the risk of cardiovascular disease. The FDA has approved the following health claim for soy protein and reduced risk of heart disease: “Diets low in saturated fat and cholesterol that include 25 grams of soy protein a day may reduce the risk of heart disease.” Soy protein also may help maintain bone mass in women. A modest reduction in the severity of “hot flashes” commonly associated with menopause may be attributed to soy protein with higher levels of soy isoflavones. Emerging science appears to show that soy protein may also play a role in weight management and soy components, including isoflavones, may have additional health effects with improved arterial health and a reduction in oxidative stress, factors believed to be associated with heart disease and cancer risk reduction, respectively. Further clinical studies will continue to increase understanding of the role of soy in maintaining and improving health.
1. Translations courtesy of Soyfoods Center. Shijing \[Book of Odes\] (compilation of poems and songs) by various authors, and Bencao Tujing \[Illustrated Pharmacopoeia\] by Su Song. Lafayette, CA: Soyfoods Center; 2005.
2. Shurtleff WR, Hymowitz T. Debunking soybean myths and legends in the historical and popular literature. Crop Sci. 2005;45:473-476.
3. USDA-Iowa State University Database on the Isoflavone Content of Foods. Available at: http://www.nal.usda.gov/fnic/foodcomp/Data/isoflav/isfl_tbl.pdf. Accessed: March 29, 2005.
4. Tonstad S, Smerud K, Hoie L. A comparison of the effects of 2 doses of soy protein or casein on serum lipids, serum lipoproteins, and plasma total homocysteine in hypercholesterolemic subjects. Am J Clin Nutr. 2002;76:78-84.
5. 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.
6. Potter SM, Baum HA, Teng H, Stillman RJ, Shay NF, Erdman JW. Soy protein and isoflavones; their effects on blood lipids and bone density in postmenopausal women. Am J Clin Nutr. 1998;68(6 Suppl):1375S-1379S.
7. Zhan, S, Ho, S. Meta-analysis of the effects of soy protein containing isoflavones on the lipid profile. Am J Clin Nutr. 2005; 81:397-408.
8. Greenwald P. Clinical trials in cancer prevention: Current results and perspectives for the future. J Nutr. 2004;134:3507S-3512S.
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14. FDA. Health claims: Soy protein and risk of coronary heart disease (CHD). Fed Reg. October 26, 1999;64(206):57699-57733.
15. FDA talk paper: FDA approves new health claim for soy protein and coronary heart disease. Available at: http://www.cfsan.fda.gov/~lrd/tpsoypr2.html. Accessed March 9, 2005.
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18. van Raaij JM, Katan MB, West CE, Hautvast JG. Influence of diets containing casein, soy isolate, and soy concentrate on serum cholesterol and lipoproteins in middle-aged volunteers. Am J Clin Nutr. 1982;35(5):925-934.
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23. Erdman JW. Soy protein and cardiovascular disease. A statement for healthcare professionals from the Nutrition Committee of the AHA. Circulation. 2000;102:2555-2559.
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26. Tikkanen MJ, Wahala K, Ojala S, Vihma V, Adlercreutz H. Effect of soybean phytoestrogen intake on low density lipoprotein oxidation resistance. Proc Natl Acad Sci USA. 1998;95:3106-3110.
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30. Yan L, Spitznagel E. A Meta-Analysis of Soyfoods and Risk of Breast Cancer in Women. International Journal of Cancer Prevention. 2004; 1 (4):281-293.
31. Maskarinec, G, Takata, Y, Franke, AA, Williams, AE, Murphy SP. A 2-year soy intervention in premenopausal women does not change mammographic densities. J Nutr. 2004:134(11): 3089-3094.
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33. Allred CD, Allred KF, Ju YH, Virant SM, Helferich WG. Soy diets containing varying amounts of genistein stimulate growth of estrogen-dependent (MCF-7) tumors in a dose-dependent manner. Cancer Research. 2001;61:5045-5050.
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35. Alekel DL, St Germain A, Peterson CR, Hanson KB, Stewart JW, Toda T. Isoflavone-rich soy protein isolate attenuates bone loss in the lumbar spine of premenopausal women. Am J Clin Nutr. 2000;72(3):844-852.
36. Erdman JW, Stillman RJ, Boileau RA. Provocative relation between soy and bone maintenance. Am J Clin Nutr. 2000;72(3):679-680 (Editorial).
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38. Messina M, Hughes C. 2003. Efficacy of soyfoods and soybean isoflavone supplements for alleviating menopausal symptoms. J Med Food. 2003;6(1):1-11.
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