There is a lot of controversy surrounding Soy and it's benefits or even risks. Here is one positive synopsis of some of the gathered evidence.
" Believe nothing, no matter where you read it, or who said it, no matter if I have said it, unless it agrees with your own reason and your own common sense."
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% protein, 20% oil, 35% carbohydrates, and 5% 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
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
The Bottom Line
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 BencaoTujing [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-IowaState UniversityDatabase 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.
9. Chan HY& Leung LK. A potential protective mechanism of soy isoflavones against 7,12-dimethybenz[a]anthracene tumour initiation. Br J Nutr. 2003;90:457-465.
10. Gallo D, Giacomelli S, Cantelmo F, Zannoni GF, Ferrandina G, Fruscella E, Riva A, Morazzoni P, Bombardelli E, Mancuso S, Scambia G. Chemoprevention of DMBA-induced mammary cancer in rats by dietary soy. Breast Cancer Res Treat. 2001;69:153-154.
11. Shao ZM, Wu J, Shen ZZ, Barsky SH. Genistein exerts multiple suppressive effects on human breast carcinoma cells. Cancer Res. 1998;58(21):4851-4857.
12. Zhou JR, Yu L, Mai Z, Blackburn GL. Combined inhibition of estrogen-dependent human breast carcinoma by soy and tea bioactive components in mice. Int J Cancer. 2003;108:8-14.
13. Albertazzi P. Pansini F, Bonaccorsi G, Zanotti L, Forini E, De Aloyiso D. The effect of dietary soy supplementation on hot flushes. ObstetGynocol. 1998;91(1):6-11.
14. FDA. Health claims: Soy protein and risk of coronary heart disease (CHD). Fed Reg. October 26, 1999;64(206):57699-57733.
16. Crouse JR 3rd, Morgan T, Terry JG, Ellis J, Vitolins M, Burke GL. A randomized trial comparing the effect of casein with that of soy protein containing varying amounts of isoflavones on plasma concentrations of lipids and lipoproteins. Arch Intern Med. 1999;159(17):2070-2076.
17. Potter SM, Bakhit, RM, Exxes-Sorlie EL, Weingartner KE, Chapman KM, Nelson RA, Prabhudasai M, Savage WD, Nelson AI, Winter LW. Depression of plasma cholesterol in men by consumption of baked products containing soy protein. Am J Clin Nutr. 1993;58(4):501-506.
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.
19. van Raaij JM, Katan MB, Hautvast JG, Hermus RJ. Blood pressure and diet in normotensive volunteers: Absence of an effect of dietary fiber, protein, or fat. Am J Clin Nutr. 1981;34(7):1261-1271.
20. Merritt JC. Metabolic syndrome: Soybean foods and serum lipids. J Natl Med Assoc. 2004;96:1032-1041.
21. Clarkson TB. Soy, soy phytoestrogens and cardiovascular disease. J Nutr. 2002;132(3):566S-569S.
22. Zhang X, Shu XO, Gao YT, Yang G, Li Q, Li H, Jin F, Zheng W. Soy food consumption is associated with lower risk of coronary heart disease in Chinese women. J Nutr. 2003:2874-2878.
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.
24. Jenkins DJ, Kendall CW, Marchie A, Faulkner DA, Wong JM, de Souza R, Emam A, Parker TL, Vigden E, Lapsley KG, Trautwein EA, Josse RG, Leiter LA, Connelly PW. Effects of a dietary portfolio of cholesterol-lowering foods vs. lovastatin on serum lipids and C-reactive protein. JAMA. 2003;290:502-510.
25. Kendall CW, Jenkins DJA. A dietary portfolio: Maximal reduction of low-density lipoprotein cholesterol with diet. CurrAtherosclerRep. 2004;6:492-498.
26. Tikkanen MJ, Wahala K, Ojala S, Vihma V, Adlercreutz H. Effect of soybean phytoestrogen intake on low density lipoprotein oxidation resistance. ProcNatl Acad SciUSA. 1998;95:3106-3110.
27. Van Popele NM, Grobee DE, Bots ML, Asmar R, Ropouchian J, Reneman RS, Hoeks APG, van der Kuip DAM, Hofman A, Witteman JCM. Association between arterial stiffness and atherosclerosis: The Rotterdam study. Stroke. 2001;32:454-460.
28. Nestel PJ, Yamashita T, Sasahara T, Pomeroy, S, Dart A, Komesaroff P, Owen A, Abbey M. Soy isoflavones improve systemic arterial compliance but not plasma lipids in menopausal and premenopausal women. ArteriosclerThromb VascBiol. 1997;17(12):3392-3398.
29. Steinberg FM, Guthrie NL, Villablanca AC, Kumar K, Murray MJ. Soy protein with isoflavones has favorable effects on endothelial function that are independent of lipid and antioxidant effects in healthy postmenopausal women. Am J Clin Nutr. 2003;78:123-130.
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.
32. Allred CD, Ju YH, Allred KF, Chang J, Helferich WG. Dietary genistein stimulates growth of estrogen-dependent breast cancer tumors similar to that observed with genistein. Carcinogenesis. 2001;22(10):1667-1673.
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.
34. Messina MJ, Loprinzi CL. Soy for breast cancer survivors: A critical review of the literature. J Nutr. 2001;131:3095S-3108S.
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).
37. NAMS Consensus. The role of isoflavones in menopausal health: Consensus opinion of The North American Menopause Society. Menopause. 2000;7(4):215-229.
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.
39. Messina M, Erdman J, Setchell KDR. Introduction to and perspectives from the fifth international symposium on the role of soy in preventing and treating chronic disease. J.Nutr. 2004;134:1205S-1206S.
40. Balk E, Chung M, Chew P, Ip S, Raman G, Kupelnick B, Tatsioni A, Sun Y, Wolk B, DeVine D, Lau J. Effects of Soy on Health Outcomes. Evidence Report/Technology Assessment No. 126. AHRQ Publication No. 05-E024-2. Rockville, MD: Agency for Healthcare Research and Quality. August 2005.
41. Lang, V, Bellisle, F, Oppert, JM, Craplet, C, Bornet, FR, Slama, G, Guy-Grand, B. Satiating effect of proteins in healthy subjects: a comparison of egg albumin, casein, gelatin, soy protein, pea protein, and wheat gluten. Am J Clin Nutr. 1998;67:1197-204.
42. Anderson, JW, Hoie, LH. Weight loss and lipid changes with low-energy diets: comparator study of milk-based versus soy-based liquid meal replacement interventions. J Am Coll Nutr. 2005 Jun; 24(3):210-216.
43. Djavan, B, Zlotta, A, Schulman, C, Teillac, P, Iversen, P, Boccon Gibod, L, Bartsch, G, Marberger, M. Chemotherapeutic Prevention Studies of Prostate Cancer. J Urology. 2004: 171:S10-S14.