Also Listed As:  Daidzein, Genistein, Glycine Max, Isoflavone, Protein (Soy), Soya
See Also:  Soy Protein (Sports and Fitness), Soy Protein (Weight Management), Drug Interactions

Introduction to Soy

Soy, a stable food in many Asian countries, contains valuable constiuents, including protein, isoflavones, saponins, and phytosterols.  Soy protein provides essential amino acids.  When eaten with rice, soy provides protein comparable with that found in animal products.  Soy is low in fat and cholesterol-free. 

What are the Sources of Soy?

Relatively large amounts of isoflavones are present in whole soybeans, roasted soy nuts, tofu, tempeh, soy milk, meat substitues, soy flour, and some soy protein isolates.  In addition, the isoflavones present in soy are available as supplements, in capsules or tablets. 

What are the Benefits of Soy?

Soy has benefited people with the following health concerns.  For more informatin, click on an individual health concern.

Health Concerns	Supplement Effectiveness
High Blood Pressure    High Cholesterol    Menopause	Reliable and relatively consistent data has shown    a substantial health benefit
Osteoporosis	Contradictory, insufficient, or preliminary studies    suggest a minimal to moderate health benefit
Premenstual Syndrome (PMS)    Vaginitis	Little scientific support and/or minimal health benefit

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Who is Commonly Defecient?

Although deficiencies do not occur, people who do not consume soy foods will not gain the benefits of soy.

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What is a good Amount of Soy to Take?

The ideal intake of soy is not known. Researchers suggest the equivalent of one serving of soy foods per day supports good health, and the benefits increase as soy intake increases.¹ Societies in which large amounts of soy are consumed ingest between 50 and 100 mg per day of soy isoflavones. The cholesterol lowering effects of soy have been observed at amounts as low as 20 grams of soy protein per day, if it replaces animal protein in the diet.²

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What are the Possible Side Effects of Taking Soy?

Soy products and cooked soybeans are safe at a wide range of intakes. However, a small percentage of people have allergies to soybeans and thus should avoid soy products.

Soy isoflavones have been reported to reduce thyroid function in some people.³ A preliminary trial of soy supplementation among healthy Japanese, found that 30 grams (about one ounce) per day of soybeans for three months, led to a slight reduction in the hormone that stimulates the thyroid gland.⁴ Some participants complained of malaise, constipation sleepiness, and even goiter.  These symptoms resolved within a month of discontinuing soy supplements. However, a variety of soy products have been shown to either cause an increase in thyroid function⁵ or produce no change in thyroid function.⁶ The clinical importance of interactions between soy and thyroid function remains unclear. However, in infants with congenital hypothyroidism soy formula must not be added, nor removed from the diet, without consultation with a physician, because ingestion of soy may interfere with the absorption of thyroid medication.⁷

Most research, including animal studies, report anti-cancer effects of soy extracts,⁸ though occasional animal studies have reported cancer-enhancing effects.⁹ The findings of several recent studies suggest that consuming soy might, under some circumstances, increase the risk of breast cancer.  When ovaries have been removed from animals—a situation related to the condition of women who have had a total hysterectomy—dietary genistein has been reported to increase the proliferation of breast cancer cells.¹⁰ When pregnant rats were given genistein injections, their female offspring were reported to be at greater risk of breast cancer.¹¹ Although pre-menopausal women have shown decreases in estrogen levels in response to soy,^{12 13} pro-estrogenic effects have also been reported.¹⁴ When pre-menopausal women were given soy isoflavones, an increase in breast secretions resulted—an effect thought to elevate the risk of breast cancer.¹⁵ In yet another trial, healthy breast cells from women previously given soy supplements containing isoflavones showed an increase in proliferation rates—an effect that might also increase the risk of breast cancer.¹⁶

Of 154 healthy postmenopausal women who received 150 mg of soy isoflavones per day for five years, 3.9% developed an abnormal proliferation of the tissue that lines the uterus (endometrial hyperplasia). In contrast, none of 144 women who received a placebo developed uterine hyperplasia.¹⁷ Although no case of uterine cancer was diagnosed during the study, endometrial hyperplasia is a potential forerunner of uterine cancer. The amount of isoflavones used in this study is two to three times as much as that used in many other studies. Nevertheless, the possibility exists that long-term use of isoflavones could cause uterine hyperplasia, and women taking isoflavones should be monitored appropriately by their doctor.

Some postmenopausal women taking the soy isoflavone genistein have experienced gastrointestinal side effects (abdominal pain, epigastric pain, dyspepsia, vomiting, or constipation).¹⁸

Soy contains a compound called phytic acid which can interfere with mineral absorption.

Certain medicines may interact with Soy.  Refer to drug interactions for a list of those medicines.

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References

1. Messina M. To recommend or not to recommend soy foods. J Am Diet Assoc 1994;94:1253–4.

2. Teixeira SR, Potter SM, Weigel R, et al. 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;71:1077–84.

3. Divi RL, Chang HC, Doerge DR. Antithyroid isoflavones from soybean. Biochem Pharmacol 1997;54:1087–96.

4. Ishizuki Y, Hirooka Y, Murata Y, Togashi K. [The effects on the thyroid gland of soybeans administered experimentally in healthy subjects.] Nippon Naibunpi Gakkai Zasshi 1991;67:622–9. [in Japanese].

5. Forsythe WA. Soy Protein, thyroid regulation and cholesterol metabolism. Forsythe WA. Soy protein, thyroid regulation and cholesterol metabolism. J Nutr 1995;125:619S–23S.

6. Bennink MR, Mayle JE, Bourquin LD, Thiagarajan D. Evaluation of soy protein in risk reduction for colon cancer and cardiovascular disease: Preliminary results. Second International Symposium on the Role of Soy in Preventing and Treating Chronic Disease. September 15–18, 1996. Brussels, Belgium.

7. Jabbar MA, Larrea J, Shaw RA. Abnormal thyroid function tests in infants with congenital hypothyroidism: the influence of soy-based formula. J Am Coll Nutr 1997;16:280–2.

8. Messina MJ, Persky V, Setchell KD, Barnes S. Soy intake and cancer risk: a review of the in vitro and in vivo data. Nutr Cancer 1994;21:113–31.

9. Rao CV, Wang C-X, Simi B, et al. Enhancement of experimental colon cancer by genistein. Cancer Res 1997;57:3717–22.

10. Barnes S. The chemopreventive properties of soy isoflavonoids in animal models of breast cancer. Breast Cancer Res Treat 1997;46:169–79 [review].

11. Hilakivi-Clarke L, Cho E, Onojafe I, et al. Maternal exposure to genistein during pregnancy increases carcinogen-induced mammary tumorigenesis in female rat offspring. Oncol Rep 1999;6:1089–95.

12. Lu L-JW, Anderson KE, Grady JJ, Nagamani M. Effects of soya consumption for one month on steroid hormones in premenopausal women: implications for breast cancer risk reduction. Cancer Epidemiol Biomarkers Prev 1996;5:63–70.

13. Nagata C, Takatsuka N, Inaba S, et al. Effect of soymilk consumption on serum estrogen concentrations in premenopausal Japanese women. J Natl Cancer Inst 1998;90:1830–5.

14. Hargreaves DNF, Potten CS, Harding C, et al. Two-week dietary soy supplementation has an estrogenic effect on normal premenopausal breast. J Clin Endocrinol Metab 1999;84:4017–24.

15. Petrakis NL, Barnes S, King EB, et al. Stimulatory influence of soy protein isolate on breast secretion in pre- and postmenopausal women. Cancer Epidemiol Biomarkers Prev 1996;5:785–94.

16. 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):1431S–6S.

17. Unfer V, Casini ML, Costabile L, et al. Endometrial effects of long-term treatment with phytoestrogens: a randomized, double-blind, placebo-controlled study. Fertil Steril 2004;82:145–8.

18. Marini H, Minutoli L, Polito F, et al. Effects of the phytoestrogen genistein on bone metabolism in osteopenic postmenopausal women: a randomized trial. Ann Intern Med 2007;146:839–47.

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