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.
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):
Reliable
and relatively consistent scientific data showing a substantial health benefit. Contradictory, insufficient, or preliminary studies
suggesting a health benefit or minimal health benefit. For an herb, supported by traditional use but minimal
or no scientific evidence. For a supplement, 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.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
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.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 anticancer 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 premenopausal
women have shown decreases in estrogen levels
in response to soy,1213 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.
Are there any drug
interactions?
Certain medicines may interact with soy. Refer to
drug interactions for a list of those medicines.
References (To view, roll mouse over the "References" heading; to hide, click on the heading)
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].
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.
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 June 2009.
