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Table of Contents > Supplements > Soy
Soy
Common Forms:   Soybean, Soy Isoflavones, Soy Protein, Soy Protein Extract
 
Overview
Therapeutic Uses
Dietary Sources
Dosage and Administration
Precautions
Interactions and Depletions
Supporting Research

Overview

The soybean has been a part of the human diet for almost 5,000 years. Unlike most plant foods, the soybean is high in protein and is considered equivalent to animal foods in terms of the quality of the protein it contains. Today, researchers are interested in both the nutritional value and the potential health benefits of soy. Several studies of populations in which soy foods are a major component of the diet (particularly Asian populations) suggest that soy may be beneficial for reducing menopausal symptoms, and risk of heart disease and osteoporosis. A possible relationship between dietary soy and the prevention of hormone-related cancers (such as breast, prostate, and endometrial cancer) is also being evaluated.

Studies investigating the value of soy have focused on the protein and isoflavone content of soy foods. Isoflavones are phytoestrogens or plant compounds that weakly mimic the action of estrogen (a reproductive hormone). Soy phytoestrogens are very complex; they bind to estrogen receptors and either enhance or diminish the effects of the hormone. Soy contains other nutrients in addition to protein and isoflavones, and some researchers believe that the healthful benefits of soy foods may be due to the combined action of these nutrients. This possibility is being investigated.


Therapeutic Uses
Prevention

Heart Disease

Various studies suggest that soy protein may reduce the risk of heart disease by lowering blood levels of cholesterol. For example, in a study of nearly 5,000 men and women living in Japan, those with the highest soy intake had the lowest cholesterol levels. A recent scientific review also found that individuals with high cholesterol may reduce their total cholesterol by 9% if they replace their daily intake of animal protein with 31 to 47 grams of soy protein. This and other evidence led the United States Food and Drug Administration (FDA) in 1999 to allow for the use of a health claim regarding an association between dietary soy and reduced risk of heart disease. Health claims represent a strong statement about an association between diet and a particular disease condition. The soy protein health claim states that including 25 grams of soy protein per day in a diet low in saturated fat and cholesterol may reduce the risk of heart disease by lowering blood cholesterol levels. In order for a particular food to carry this claim, one serving of the food must contain at least 6.25 grams of soy protein per serving. In fact, it is relatively easy to add enough soy to the daily diet to meet the 25 gram recommendation: 4 ounces of firm tofu contains 13 grams of soy protein; one soy "burger" includes 10 to 12 grams of protein; and an 8-ounce glass of plain soy milk contains 10 grams of protein.

Cancer

The evidence suggesting that soy may be helpful in preventing hormone-related cancers, such as breast, prostate, and endometrial cancer, comes from studies of large populations of individuals. These types of studies suggest that populations who consume high quantities of soy have a lower incidence of hormone-related cancers.

For example, Asian-American women who eat tofu at least once a week have a lower risk of developing breast cancer than those who do not eat tofu. Yet, when Asian women migrate to the United States, their intake of tofu tends to decrease and their risk of developing breast cancer tends to increase. In comparing the soy intake of Hawaiian women with their risk of developing endometrial cancer, the women who consumed the least amount of soy had the highest risk of endometrial cancer. Incidence of prostate cancer appears similar in Asian and Western countries, but more Western men than Asian men die from prostate cancer. Other findings that suggest soy foods may have health benefits against prostate disease include the observation that prostate size in Japanese men does not increase so dramatically with age as it does in Western men.


Treatment

Heart Disease

The American Heart Association (AHA) recommends adding at least 25 grams of soy protein per day to a diet low in saturated fat and cholesterol to reduce the risk of heart disease. These recommendations are also believed to improve blood cholesterol levels in people with elevated cholesterol. The main study findings that prompted these recommendations include the following:

  • Asian populations who routinely eat soy foods (up to 55 grams of soy protein per day) have lower incidences of heart disease than populations who eat a typical Western diet (fewer than 5 grams of soy protein per day)
  • Substituting 31 to 47 grams of animal protein with soy protein in a diet low in saturated fat and cholesterol can lower blood cholesterol levels
  • Including at least 20 grams of soy protein per day in a diet low in saturated fat and cholesterol reduces blood fat levels in both men and women

Osteoporosis

Researchers have begun to investigate whether soy foods can be helpful in preventing osteoporosis by increasing bone mineral density. Most of the research has focused on the isoflavone content of soy protein. Although the studies to date are limited, they suggest that women approaching menopause who eat isoflavone-rich soy protein are significantly more likely to boost their bone mineral density than women whose diets are low in soy isoflavones. Postmenopausal Japanese women whose diets include high levels of soy isoflavones have greater bone mineral density than those whose diets include low levels of soy isoflavones.

Menopause Symptoms

Soy isoflavones may help alleviate hot flashes and night sweats that many women experience during menopause. In the studies that have been conducted to date, postmenopausal women who consume high amounts of dietary soy protein generally experience fewer and less intense hot flashes and night sweats than those with low intakes. Studies evaluating the use of soy supplements for the reduction of menopausal symptoms, however, have been more controversial.


Dietary Sources

Soy foods are available in a variety of forms. The whole soybean (called edamame) is popular in Japanese and Chinese cuisines but is not commonly consumed in the typical Western diet. Tofu, which is a curd made from cooked, pureed soybeans, is increasingly popular in Western cuisine. Tofu can be found in various textures, ranging from soft to extra firm. The softer tofus are popular for shakes, salad dressings, and creamy dishes; the firm tofus are easy to dice or crumble into mixed dishes such as stir fries or pasta. Soy milk, soy cheese, soy yogurt, and soy nuts and nut butters are also increasingly popular with Western consumers. Other soy foods available in Western markets include miso, which is a mixture of fermented soybean paste and a grain such as rice or barley; natto, which are whole soybeans that have been cooked and fermented; and tempeh, made from a mixture of whole, cooked soybeans and grains that have been cultured with an edible mold. The oil from soybeans is widely used throughout the food industry. Soy oil is a popular choice for cooking and frying oils and as a base for mayonnaise, salad dressing, and margarine.

The availability of nutrients varies with the type of soy food. In general, whole soybean-based foods such as tofu and soy milk have the highest protein and isoflavone content. The soy germ, found in whole soybeans, is particularly high in isoflavones.

In addition to soy foods, dietary supplements containing one or more components from soy are also on the market. One supplement, ipriflavone, is a synthetic derivative of a soy isoflavone and is being investigated for its benefits in improving bone health. At this time, however, most experts agree that the information on the health benefits of soy supplements is limited and prefer to recommend whole soy foods instead.


Dosage and Administration
Pediatric

Soy foods and soy-based infant formula are widely used in children, but there are no studies that have determined whether isolated soy protein or isoflavone supplements are useful or safe in this population. Therefore, isolated soy products are not recommended for use with children at this time.


Adult

Heart Disease

Most studies that have examined the effects of soy on heart disease have used between 20 and 80 grams of soy protein per day. Both the FDA and the AHA recommend 25 grams of soy protein when incorporated into a diet that is low in cholesterol and saturated fat.

No formal recommendations for isoflavone intake have been put forward but, based on the amount present in the typical Asian diet, the intake would be between 20 and 50 milligrams per day. According to the AHA report, 2 grams of soy protein contains approximately 1 milligram of soy isoflavones. As mentioned previously, 4 ounces of firm tofu contains 13 grams of soy protein, one soy "burger" includes 10 to 12 grams of protein, and an 8-ounce glass of plain soy milk contains 10 grams of protein.

Cancer

Researchers speculate that the phytoestrogens in dietary soy may be beneficial for the prevention of hormone-related cancers (such as breast, endometrial, and prostate cancer), but specifics about dosage, frequency of use, or safety are not known. The available evidence has been derived from soy foods rather than supplements or isolated components of soy. For this reason, some researchers suggest that soy foods should be recommended as part of a healthy diet for cancer prevention rather than isoflavone supplements.

Menopause

In 2000, the North American Menopause Society (NAMS), a nonprofit organization dedicated to promoting the understanding of menopause, issued a consensus statement that addressed the potential benefits of isoflavones on conditions associated with menopause. The society reported that the consumption of whole foods containing the following amounts of isoflavones may have health benefits for postmenopausal women:

  • 50 mg/day to reduce cholesterol
  • 40 to 80 mg/day to strengthen blood vessels and possibly improve blood pressure
  • 50 mg/day for bone health
  • 40 to 80 mg/day to reduce hot flashes

Other experts believe that it is still too early to recommend soy isoflavones for the treatment of menopausal symptoms. They cite the lack of specific information about effective dosage, length of administration, and long-term effects of using phytoestrogens to manage menopausal symptoms. The substitution of phytoestrogens for estrogen replacement therapy is also considered to be premature at this point in our understanding of phytoestrogens and menopause.


Precautions

Soy Allergy

Those individuals who are allergic to soy should avoid soy foods and any supplements containing soy components, such as soy isoflavones.

Breast Cancer

Findings from laboratory studies have raised concerns that soy consumption may stimulate the growth of breast cancer cells. Although the issue is not completely resolved, the bulk of the evidence presently available suggests that women who can safely take hormone replacement therapy may also use soy phytoestrogens without increasing their risk of breast cancer. Phytoestrogens may, however, reduce the effectiveness of the breast cancer medication tamoxifen and should not be used by women with breast cancer who are also taking tamoxifen.

Kidney Disease

Although soy protein is considered nutritionally equivalent to animal-based protein, the phosphorus and potassium content of soy is greater than that found in comparable amounts of meat, poultry, or fish. Individuals with kidney disease should consult with their nephrologist or renal dietitian before including soy foods in their diet.

Hypothyroidism

Isoflavones may reduce the iodine content in the body, which can lead to underfunctioning of the thyroid (hypothyroidism). A few cases of hypothyroidism have been reported in infants fed soy formula. This effect, however, is not widespread in the United States where most people use iodized salt.


Side Effects

For individuals who are not allergic to soy, no serious short-term or long-term side effects have been reported from eating soy foods. Common mild side effects include stomach upset and digestive problems including constipation and diarrhea.


Pregnancy and Breastfeeding

The consumption of soy foods during pregnancy and while breastfeeding is considered to be safe, but further studies are needed to demonstrate beyond any reasonable doubt that soy is safe during these life stages.


Pediatric Use

Because phytoestrogens influence the activity of certain hormones in the body, there has been some concern that infants fed soy formula may experience negative effects on general health or reproduction later in life. A recent study of over 800 young adults who (as infants) were fed either soy formula or cow's milk formula, however, found that there were no differences between the two groups in over 30 different parameters measured, including fertility and general health.


Interactions and Depletions

The phytoestrogen components of soy may interfere with the drug tamoxifen, used in breast cancer treatment for men and women, and the drug raloxifene, used to prevent and treat osteoporosis. The minerals zinc and iron may also interfere with the absorption of soy and could theoretically lower soy levels in the body. Further research is needed.


Supporting Research

Adlercreutz H, Mazur W. Phyto-estrogens and western diseases. Ann Med. 1997;29(2):95-120.

Albertazzi P, Pansini F, Bonaccorsi G, Zanotti L, Forini E, De Aloysio D. The effect of dietary soy supplementation on hot flushes. Obstet Gynecol. 1998;91(1):6-11.

Alekel DL, St. Germain A, Peterson CT, Hanson KB, Stewart JW, Toda T. Isoflavone-rich soy protein isolate attenuates bone loss in the lumbar spine of perimenopausal women. Am J Clin Nutr. 2000;72(3):844-852.

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.

Anderson JW, Smith BW, Washnock CS. Cardiovascular and renal benefits of dry bean and soybean intake. Am J Clin Nutr. 1999;70(suppl 3):464S-474S.

Barnes S. Phytoestrogens and breast cancer. Baillieres Clin Endocrinol Metab. 1998;12(4):559-579.

Chorazy PA, Himelhoch S, Hopwood NJ, Greger NG, Postellon DC. Persistent hypothyroidism in an infant receiving a soy formula: case report and review of the literature. Pediatrics. 1995;96:148-150.

Constantinou AI, Xu H, Cunningham E, Lantvit DD, Pezzuto JM. Consumption of soy products may enhance the breast cancer-preventive effects of tamoxifen. Paper presented at: American Association of Cancer Research 92nd Annual Meeting; March 24-28, 2001; New Orleans, La.

Crouse JR, 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;159:2070-2076.

Erdman JW Jr. AHA Science Advisory: Soy protein and cardiovascular disease: a statement for healthcare professionals from the Nutrition Committee of the AHA.Circulation. 2000;102(20):2555-2559.

Food and Drug Administration. Dietary Supplement Health and Education Act of 1994. Center for Food Safety and Applied Nutrition. December 1, 1995. Accessed at http://vm.cfsan.fda.gov/~dms/dietsupp.html on September 21, 2001.

Food and Drug Administration. Food labeling: health claims; soy protein and coronary heart disease. Federal Register, Department of Health and Human Services; October 26, 1999. 64(206):57699-57733.

Fournier DB, Erdman JW Jr, Gordon GB. Soy, its components and cancer prevention: a review of the in vitro, animal and human data. Cancer Epidemiol Biomarkers Prev. 1998;7(11):1055-1065.

Glazier MG, Bowman MA. A review of the evidence for the use of phytoestrogens as a replacement for traditional estrogen replacement therapy. Arch Intern Med. 2001;161(9):1161-1172.

Goodman MT, Wilkens LR, Hankin JH, Lyu LC, Wu AH, Kolonel LN. Association of soy and fiber consumption with the risk of endometrial cancer. Am J Epidemiol. 1997;146(4):294-306.

Hargreaves DF, Potten CS, Harding C, et al. Two-week dietary soy supplementation has an estrogenic effect on normal premenopausal breast. J Clin Endocrinol Metab. 1999;84(11):4017-4024.

Heaney RP, Weaver, CM, Fitzimmons ML. Soybean phytate content: effect on calcium absorption. Am J Clin Nutr. 1991;53(3):745-747.

Hermansen K, Sondergaard M, Hoie L, Carstensen M, Brock B. Beneficial effects of a soy-based dietary supplement on lipid levels and cardiovascular risk markers in type 2 diabetic subjects. Diabetes Care. 2001;24(2):228-233.

Hsieh CY, Santell RC, Haslam SZ, Helferich WG. Estrogenic effects of genistein on the growth of estrogen receptor-positive human breast cancer (MCF-7) cells in vitro and in vivo. Cancer Res. 1998;58(17):3833-3838.

Kim H, Peterson TG, Barnes S. Mechanisms of action of the soy isoflavone genistin: emerging role for its effects via transforming growth factor beta signaling pathways. Am J Clin Nutr. 1998;68(suppl 6):1418S-1425S.

Kurzer MS, Xu X. Dietary phytoestrogens. Annu Rev Nutr. 1997;17:353-381.

Lee HP, Gourley L, Duffy SW, Esteve J, Lee J, Day NE. Dietary effects on breast-cancer risk in Singapore. Lancet. 1991;337(8751):1197-1200.

Lichtenstein AH. Soy protein, isoflavones and cardiovascular disease risk. J Nutr. 1998;128:1589-1592.

Liu K. Soybeans: Chemistry, Technology and Utilization. New York, NY: Chapman & Hall; 1997:1-4, 48-49, 64-69, 83-92, 446-447.

Messina, MJ. Legumes and soybeans: overview of their nutritional profiles and health effects. Am J Clin Nutr. 1999;70(3Suppl):439S-450S.

Messina M, Bennink M. Soyfoods, isoflavones and risk of colonic cancer: a review of the in vitro and in vivo data. Baillieres Clin Endocrinol Metab. 1998;12(4):707-728.

Messina M, Messina V. Soyfoods, soybean isoflavones and bone health: a brief overview. J Ren Nutr. 2000;10(2):63-68.

Messina MJ, Persky V, Setchell KDR, Barnes S. Soy intake and cancer risk: a review of the in vitro and in vivo data. Nutr Cancer. 1994;21(2):113-131.

McMichael-Phillips DF, Harding C, Morton M, et al. Effects of soy-protein supplementation on epithelial proliferation in the histologically normal human breast. Am J Clin Nutr. 1998;68(suppl 6):1431S-1435S

Moyad MA. Soy, disease prevention and prostate cancer. Semin Urol Oncol. 1999;17(2):97-102.

Nagata C. Ecological study of the association between soy product intake and mortality from cancer and heart disease in Japan. Int J Epidemiol. 2000;29(5):832-836.

Nagata C, Takatsuka N, Kawakami N, Shimizu H. Soy product intake and hot flashes in Japanese women: results from a community-based prospective study. Am J Epidemiol. 2001;153(8):790-793.

Nagata C, Takatsuka N, Kurisu Y, Shimizu H. Decreased serum total cholesterol concentrations is associated with high intake of soy products in Japanese men and women. J Nutr. 1998;128(2):209-213.

North American Menopause Society. The role of isoflavones in menopausal health: consensus opinion of the North American Menopause Society. Menopause. 2000;7(4):215-229.

Nestel PJ, Yamashita T, Sasahara T, et al. Soy isoflavones improve systemic arterial compliance but not plasma lipids in menopausal and perimenopausal women. ArteriosclerThromb Vasc Biol. 1997;17(12):3392-3398.

SM, Baum JA, Teng H, Stillman RJ, Shay NF, Erdman JW Jr. Soy protein and isoflavones: their effects on blood lipids and bone density in postmenopausal women. Am J Clin Nutr. 1998;68(suppl 6):1375S-1379S.

Quella SK, Loprinzi CL, Barton DL, et al. Evaluation of soy phytoestrogens for the treatment of hot flashes in breast cancer survivors: a North Central Cancer Treatment Group trial. J Clin Oncol. 2000;18(5):1068-1074.

Setchell KD. Phytoestrogens: the biochemistry, physiology, and implications for human health of soy isoflavones. Am J Clin Nutr. 1998;68(suppl 6):1333S-1346S.

Setchell KD, Cassidy A. Dietary isoflavones: biological effects and relevance to human health. J Nutr. 1999;129(3):758S-767S.

Setchell KD, Zimmer-Nechemias L, Cai J, Heubi JE. Isoflavone content of infant formulas and the metabolic fate of these phytoestrogens in early life. Am J Clin Nutr. 1998;68(suppl 6):1453S-1461S.

Slavin JL, Karr SC, Hutchins AM, Lampe JW. Influence of soybean processing, habitual diet and soy dose on urinary isoflavonoid excretion. Am J Clin Nutr. 1998;68(suppl 6):1492S-1495S.

Somekawa Y, Chiguchi M, Ishibashi T, Aso T. Soy intake related to menopausal symptoms, serum lipids and bone mineral density in postmenopausal Japanese women. Obstet Gynecol. 2001;97(1):109-115.

St Germain A, Peterson CT, Robinson JG, Alekel DL. Isoflavone-rich or isoflavone-poor soy protein does not reduce menopausal symptoms during 24 weeks of treatment. Menopause. 2001;8(1):17-26.

Strom BL, Schinnar R, Ziegler EE,et al. Exposure to soy-based formula in infancy and endocrinological and reproductive outcomes in young adulthood. JAMA. 2001;286(7):807-814.

Teixeira SR, Potter SM, Weigel R, Hannum S, Erdman JW Jr, Hasler CM. Effects of feeding 4 levels of soy protein for 3 and 6 wk on blood lipids and apolipoproteins in moderately hypercholesterolemic men.Am J Clin Nutr. 2000;71(5):1077-1084.

Tham DM, Gardner CD, Haskell WL. Clinical review 97: potential health benefits of dietary phytoestrogens: a review of the clinical, epidemiological and mechanistic evidence. J Clin Endocrinol Metab. 1998 Jul;83(7):2223-2235.

This P, De La Rochefordiere A, Clough K, Fourquet A, Magdelenat H; The Breast Cancer Group of the Institut Curie. Phytoestrogens after breast cancer. Endocr Relat Cancer. 2001;8(2):129-134.

United Soybean Board. Soy and health: the healthful, balanced nutrient. Accessed at http://www.talksoy.com on August 22, 2001.

Upmalis DH, Lobo R, Bradley L, Warren M, Cone FL, Lamia CA. Vasomotor symptom relief by soy isoflavone extract tablets in postmenopausal women: a multicenter, double-blind, randomized, placebo-controlled study. Menopause. 2000;7(4):236-242.

Vincent A, Fitzpatrick LA. Soy isoflavones: are they useful in menopause? Mayo Clin Proc. 2000;75(11):1174-1184.

Washburn S, Burke GL, Morgan T, Anthony M. Effect of soy protein supplementation on serum lipoproteins, blood pressure and menopausal symptoms in perimenopausal women. Menopause. 1999;6(1):7-13.

Wong WW, Smith EO, Stuff JE, Hachey DL, Heird WC, Pownell HJ. Cholesterol-lowering effect of soy protein in normocholesterolemic and hypercholesterolemic men. Am J Clin Nutr. 1998;68(suppl 6):1385S-1389S.

Wu AH, Ziegler RG, Horn-Ross PL, et al.Tofu and risk of breast cancer in Asian-Americans. Cancer Epidemiol Biomarkers Prev. 1996;5(11):901-906.


Review Date: October 2001
Reviewed By: Participants in the review process include: Jacqueline A. Hart, MD, Department of Internal Medicine, Newton-Wellesley Hospital, Harvard University and Senior Medical Editor Integrative Medicine, Boston, MA; Steven Ottariano, RPh, Veteran's Administrative Hospital, Londonderry, NH; Pamela Williams, MPH, RD, Nutrition Technical Writer, Nutrilite, A Division of Access Business Group, Buena Park, CA.

Copyright © 2004 A.D.A.M., Inc

The publisher does not accept any responsibility for the accuracy of the information or the consequences arising from the application, use, or misuse of any of the information contained herein, including any injury and/or damage to any person or property as a matter of product liability, negligence, or otherwise. No warranty, expressed or implied, is made in regard to the contents of this material. No claims or endorsements are made for any drugs or compounds currently marketed or in investigative use. This material is not intended as a guide to self-medication. The reader is advised to discuss the information provided here with a doctor, pharmacist, nurse, or other authorized healthcare practitioner and to check product information (including package inserts) regarding dosage, precautions, warnings, interactions, and contraindications before administering any drug, herb, or supplement discussed herein.

 
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