Soy

Also indexed as: Daidzein, Genistein, Glycine Max, Isoflavone, Protein (Soy), Soya

What does it do? Soy, a staple food in many Asian countries, contains valuable constituents, including protein, isoflavones, saponins, and phytosterols. Soy protein provides essential amino acids. When eaten with rice, soy provides protein comparable to that found in animal products. Soy is low in fat and cholesterol-free. The isoflavones in soy, primarily genistein and daidzein, have been well researched by scientists for their antioxidant and phytoestrogenic properties.1 Saponins enhance immune function and bind to cholesterol to limit its absorption in the intestine. Phytosterols and other components of soy have been reported to lower cholesterol levels.

The soy isoflavone genistein has been reported to inhibit angiogenesis,2 the growth of new blood vessels that, when abnormal, can contribute to the development of cancer. Soy isoflavones have also been shown to inhibit 5 alpha-reductase,3 the enzyme that activates testosterone in the prostate gland and other tissues. 5 Alpha-reductase inhibition represents a potentially effective therapeutic approach to benign prostate enlargement and male pattern baldness.

Isoflavones may reduce the risk of hormone-dependent cancers, such as breast and prostate cancer, as well as other cancers. One study of soy research found that 65% of 26 animal-based cancer studies showed a protective effect of soy or soy isoflavones.4 Human research also suggests a protective role of soy against cancer,5 6 but the data are currently insufficient to form any solid conclusions.7

A review of 38 different studies revealed that soy consumption reduced cholesterol levels in 89% of the studies. A meta-analysis of these studies indicated that eating soy resulted, on average, in a cholesterol reduction of 23 mg per deciliter.8 Exactly how soy lowers cholesterol remains in debate,9 though isoflavones appear to be one key component.10

The mild estrogenic activity of soy isoflavones may ease menopause symptoms for some women, without creating estrogen-related problems. In one double-blind trial, supplementation with 60 grams of soy protein per day for 12 weeks led to a 45% decrease in the number of hot flashes, compared with a 30% reduction in the placebo group, a statistically significant difference.11 In addition, soy may help regulate hormone levels in premenopausal women.12

Soy may also be beneficial in preventing osteoporosis. Isoflavones from soy have protected against bone loss in animal studies.13 In a double-blind study of postmenopausal women, supplementation with 40 grams of soy protein powder per day (containing 90 mg of isoflavones per day) protected against bone mineral loss in the spine.14 Although the use of soy in the prevention of osteoporosis looks hopeful, no long-term human studies have examined the effects of soy or soy-derived isoflavones on bone density or fracture risk.

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

Soy has been used in connection with the following conditions (refer to the individual health concern for complete information):

Rating Health Concerns
3Stars High cholesterol
Menopause
2Stars Osteoporosis
1Star Vaginitis
3Stars Reliable and relatively consistent scientific data showing a substantial health benefit.
2Stars Contradictory, insufficient, or preliminary studies suggesting a health benefit or minimal health benefit.
1Star An herb is primarily supported by traditional use, or the herb or supplement has little scientific support and/or minimal health benefit.

Who is likely to be deficient? Although deficiencies do not occur, people who do not consume soy foods will not gain the benefits of soy.

How much is usually taken? 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.15 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.16

Are there any side effects or interactions? 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.17 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.18 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 function19 or produce no change in thyroid function.20 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.21

Most research, including animal studies, report anticancer effects of soy extracts,22 though occasional animal studies have reported cancer-enhancing effects.23 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.24 When pregnant rats were given genistein injections, their female offspring were reported to be at greater risk of breast cancer.25 Although premenopausal women have shown decreases in estrogen levels in response to soy,26 27 pro-estrogenic effects have also been reported.28 When pre-menopausal women were given soy isoflavones, an increase in breast secretions resulted—an effect thought to elevate the risk of breast cancer.29 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.30

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

Are there any drug interactions? Certain medications may interact with soy. Refer to the drug interactions safety check for a list of those medications.

References:

1. Wei H, Bowen R, Cai Q, et al. Antioxidant and antipromotional effects of the soybean isoflavone genistein. Proc Soc Exp Biol Med 1995;208:124–9.

2. Fotsis T, Pepper M, Adlercreutz H, et al. Genistein, a dietary-derived inhibitor of in vitro angiogenesis. Proc Natl Acad Sci 1993;90:2690–4.

3. Evans BA, Griffiths K, Morton MS. Inhibition of 5 alpha-reductase in genital skin fibroblasts and prostate tissue by dietary lignans and isoflavonoids. J Endocrinol 1995;147:295–302.

4. 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.

5. Adlercreutz H, Markkanen H, Watanabe S. Plasma concentrations of phyto-oestrogens in Japanese men. Lancet 1993;342:1209–10.

6. Lee HP, Gourley L, Duffy SW, et al. Dietary effects on breast-cancer risk in Singapore. Lancet 1991;337:1197–200.

7. Messina MJ. Legumes and soybeans: overview of their nutritional profiles and health effects. Am J Clin Nutr 1999;70:439S–50S.

8. Anderson JW, Johnstone BM, Cook-Newell ME. Meta-analysis of the effects of soy protein intake on serum lipids. N Engl J Med 1995;333:276–82.

9. Potter SM. Overview of proposed mechanisms for the hypocholesterolemic effect of soy. J Nutr 1995;125:6065–115.

10. Crouse JR 3rd, Morgan T, Terry JG, et al. 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;159:2070–6.

11. Albertazzi P, Pansini F, Bonaccorsi G, et al. The effect of dietary soy supplementation on hot flushes. Obstet Gynecol 1998;91:6–11.

12. Cassidy A, Bingham S, Setchell KDR. Biological effects of a diet of soy protein rich isoflavones on the menstrual cycle of premenopausal women. Am J Clin Nutr 1994;60:333–40.

13. Anderson JJB, Ambrose WW, Garner SC. Biphasic effects of genistein on bone tissue in the ovariectomized, lactating rat model (44243). Proc Soc Exp Biol Med 1998;217:345–50.

14. Potter SM, Baum JA, Teng H, et al. Soy protein and isoflavones: Their effects on blood lipids and bone density in postmenopausal women. Am J Clin Nutr 1998;68(suppl):1375S–79S.

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

16. 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.

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

18. 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].

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

20. 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.

21. 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.

22. 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.

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

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

25. 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.

26. 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.

27. 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.

28. 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.

29. 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.

30. 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.