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 with that found in animal products. Soy is low in fat and cholesterol-free.
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.
Some dieters say that soy protein
Protein is necessary for rebuilding tissue, which is especially important for bodybuilders. All types of protein provide the body with amino acids.
From an athletic point of view, soy is just another type of protein. Choosing soy protein over other types of protein adds variety to the protein choices available. However, because soy lowers cholesterol, it is an excellent choice for those also trying to lower their cholesterol levels.
Soy is a common ingredient in many meal replacements that are designed to provide a large amount of nutrients in a minimal amount of calories.
Animal and human studies have suggested that when soy is used as a source of dietary protein, it may have several biological effects on the body that might help with weight loss.1 A preliminary study found that people trying to lose weight using a meal-replacement formula containing soy protein lost more weight than a group not using any formula.2 However, controlled studies comparing soy protein with other protein sources in weight loss diets have not found any advantage of soy.3 4 5 When soy protein is used for other health benefits, typical daily intake is 20 grams per day or more.
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.6 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.7 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 function8 or produce no change in thyroid function.9 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.10
Most research, including animal studies, report anticancer effects of soy extracts,11 though occasional animal studies have reported cancer-enhancing effects.12 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.13 When pregnant rats were given genistein injections, their female offspring were reported to be at greater risk of breast cancer.14 Although premenopausal women have shown decreases in estrogen levels in response to soy,15 16 pro-estrogenic effects have also been reported.17 When pre-menopausal women were given soy isoflavones, an increase in breast secretions resulted—an effect thought to elevate the risk of breast cancer.18 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.19
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.20 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.
Soy contains a compound called phytic acid, which can interfere with mineral absorption.
Are there any drug
interactions?
Certain medicines may interact with soy. Refer to drug
interactions for a list of those medicines.
*Dieters and weight-management advocates may claim benefits for soy protein based on their personal or professional experience. These are individual opinions and testimonials that may or may not be supported by controlled clinical studies or published scientific articles on soy protein. For more complete and detailed information, including references and safety information, see Soy as a nutritional supplement.
1. Bhathena SJ, Velasquez MT. Beneficial role of dietary phytoestrogens in obesity and diabetes. Am J Clin Nutr 2002;76:1191–201 [review].
2. Allison DB, Gadbury G, Schwartz LG, et al. A novel soy-based meal replacement formula for weight loss among obese individuals: a randomized controlled clinical trial. Eur J Clin Nutr 2003;57:514–22.
3. Bosello O, Cominancini L, Zocca I, et al. Short- and long-term effects of hypocaloric diets containing proteins of different sources on plasma lipids and apoproteins of obese subjects. Ann Nutr Metab 1988; 32:206–14.
4. Yamashita T, Sasahara T, Pomeroy SE, et al. Arterial compliance, blood pressure, plasma leptin, and plasma lipids in women are improved with weight reduction equally with a meat-based diet and a plant-based diet. Metabolism 1998;47:1308–14.
5. Jenkins DJ, Wolever TM, Spiller G, et al. Hypocholesterolemic effect of vegetable protein in a hypocaloric diet. Atherosclerosis 1989;78:99–107.
6. Divi RL, Chang HC, Doerge DR. Antithyroid isoflavones from soybean. Biochem Pharmacol 1997;54:1087–96.
7. 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].
8. Forsythe WA. Soy Protein, thyroid regulation and cholesterol metabolism. Forsythe WA. Soy protein, thyroid regulation and cholesterol metabolism. J Nutr 1995;125:619S–23S.
9. 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.
10. 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.
11. 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.
12. Rao CV, Wang C-X, Simi B, et al. Enhancement of experimental colon cancer by genistein. Cancer Res 1997;57:3717–22.
13. Barnes S. The chemopreventive properties of soy isoflavonoids in animal models of breast cancer. Breast Cancer Res Treat 1997;46:169–79 [review].
14. 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.
15. 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.
16. 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.
17. 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.
18. 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.
19. 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.
20. 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.
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The information presented in Healthnotes is for informational purposes only. It is based on scientific studies (human, animal, or in vitro), clinical experience, or traditional usage as cited in each article. The results reported may not necessarily occur in all individuals. For many of the conditions discussed, treatment with prescription or over the counter medication is also available. Consult your doctor, practitioner, and/or pharmacist for any health problem and before using any supplements or before making any changes in prescribed medications. Information expires August 2007.