Soy Safety
Health experts, including the Food and Drug Administration (FDA) (1) and the American Cancer Society (2) have all weighed in on the safety of soyfoods, such as tofu and soymilk, as well as meat and dairy free products such as veggie burgers and soy cheeses. The conclusion: soyfoods are low in fat, high in protein, fiber and iron, they contain essential omega-3 fatty acids and are absolutely safe to eat. Here are the facts you need to know:
FACT: The U.S. Food and Drug Administration approved a health claim for foods that contain at least 6.25 grams of soy protein which states that, “25 grams of soy protein a day, as part of a diet low in saturated fat and cholesterol, may reduce the risk of heart disease.” FDA extensively reviewed research to support this health claim and found that soy protein, when included in a low-fat and low-cholesterol diet, could lower blood total cholesterol and low-density lipoprotein (LDL) or “bad” cholesterol levels. To get 25 grams daily, enjoy three or four servings of soyfoods, such as 1 cup of soymilk, 3 to 4 ounces of tofu, one 3 ounce soy burger, 3 ounces of meatless chicken strips or nuggets, 1 cup of soy pasta, 1 cup of cultured soy, 2 tablespoons soy nut butter, a frozen soy bar, a ¼ cup of soy nuts or edamame.
FACT: The isoflavones naturally found in soy benefit human health. Isoflavones are bioactive compounds, and recent studies find that the isoflavones contained in soyfoods are associated with such benefits as improving the health of arteries, (3) preventing certain cancers(4, 5) and reducing bone loss. (6) Unfortunately, there has been a lot of confusion resulting from some studies in animals that were given very high concentrations of isoflavones compared to the bioactive compounds that one typically eats in soyfoods. Some animal studies that are looking for effects inject or feed a very concentrated source of one specific isoflavone, such as genistein, which is very different from the way isoflavones are consumed in food. (7) Applying these study findings is like mixing apples and oranges. Other problems seen in some animals but not humans are believed to be due to differences between humans and animals in the way soy isoflavones are metabolized.(8)
FACT: Isoflavones in soyfoods have no significant effects on hormone levels in men or women. Although isoflavones are called “phytoestrogens” because they look similar to the chemical structure of estrogens, they can act differently in the human body and shouldn’t be confused with the estrogen hormone. As a result, there is no validity to the claim that consuming soy leads to testosterone levels that are lower than normal. In fact, a recent study involving active young men found that soy protein did not lower testosterone concentrations in comparison to control groups, but did produce a beneficial antioxidant effect.(9) Other studies in people found that men consuming 40-70 mg/day of soy isoflavones from soyfoods or soy supplements had no significant changes in testosterone levels compared to control groups.(10,11,12)
FACT: Soyfoods are safe for the developing fetus.
According to a scientific panel convened in March 2006 by the National Institute of Environmental Health Sciences (NIEHS), even pregnant women who eat soy regularly consume such low amounts of genistein, the most heavily concentrated isoflavone in soy, that the likelihood of reproductive or developmental effects are of “negligible concern."(13) Such problems have only been observed in targeted studies on animals, not humans, using very large amounts of genistein.
FACT: Women who eat soyfoods have a lower risk of developing breast cancer.
Eating soyfoods appears to have a protective effect against developing breast cancer at any age, which may be strongest when soy is consumed during childhood and adolescence. (14,15, 4) This may be a major reason why Asian diets are linked to lower breast cancer death rates. Observations have found that Asian women who follow a traditional Asian diet have a much lower breast cancer rate: in one estimate, one-fifth that of Western women.(16)
FACT: No studies demonstrate a link between eating soy and breast cancer recurrence or tumor growth in humans.
Because some studies in mice observed that very high levels of soy protein or isoflavones led to faster tumor growth or recurrence, this has been an area of study in women, but to date there have been no studies in humans that demonstrate a link between eating soy and tumor recurrence or growth. Nor has soy or its bioactive compounds been observed to cause changes associated with breast cancer, such as thickening breast tissue,(17,18,19) increasing the number of cells in breast tissue,(20) or increasing estrogen circulating in the blood.(21) Researchers also make it clear that it is difficult to apply findings from animal studies using excessive doses of isoflavones to humans because mice and rats produce lower levels of hormonal estrogen than do women,(22) and the digestion of isoflavones produces different end products.(23)
FACT: Major national institutes for cancer research and treatment advise that breast cancer survivors can safely consume moderate amounts of soy.
The National Cancer Institute(24) states that for breast cancer survivors “soyfoods, as part of a healthy diet and in moderate amounts, are safe to consume.” The American Cancer Society suggests up to 3 servings a day is safe, while the American Institute for Cancer Research(25) advises women who have had hormone-dependent breast cancer to limit themselves to a few servings of soyfoods per week. All agencies specify that these recommendations apply to whole soyfoods, and do not advise consumption of supplements of soy protein or isoflavones. Additionally, those who are taking tamoxifen should consult their oncologists for advice on soy and other matters concerning nutrition, since animal studies have shown that isoflavones can alter the actions of tamoxifen.(26)
FACT: Soyfoods protect against thyroid cancer and have no effect on thyroid function in healthy people.
A recent study indicates that eating soyfoods may actually protect against the development of thyroid cancer in women.(27) More research needs to be conducted to determine if these benefits are consistent in men and women. Moreover, in a recent review of clinical trials involving healthy men and women, the weight of the evidence found that consuming soyfoods or isoflavones had either no effects or only very modest changes (but within normal range) on thyroid function.(28) It is important to note that the adults studied in these trials got enough iodine in their diet. In other studies or observations that linked soyfood consumption to goiter, iodine deficiencies, rather than soy, were to blame.(29)
FACT: Soyfoods are safe for people with hypothyroidism (under-active thyroid) who are under medical care.
Although people who are being treated for hypothyroidism need to discuss their diet with their physician so that an appropriate level of medication is prescribed, soyfoods can be consumed safely. Soyfoods are among many commonly eaten foods, including high-fiber foods as well as iron and calcium supplements (30) that have chemical components which can affect to some extent medication for hypothyroidism. Some physicians suggest taking the medication between meals, so that the chemical components in food do not interfere with the actions of the medicine.(31)
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References:
1. U.S. Department of Health and Human Services, Food and Drug Administration. FDA approves new health claim for soy protein and coronary heart disease. October 20, 1999. Accessed at http://www.fda.gov/ on January 8, 2007.
2. American Cancer Society. Nutrition and Physical Activity During and After Cancer Treatment: Answers To Common Questions. Accessed at http://www.cancer.org/ Dec. 28, 2006.
3.Colacurci N, Chiantera A, Fornaro F, de Novellis V, Manzella D, Arciello A, Chiantera V, Improta L, Paolisso G. Effects of soy isoflavones on endothelial function in healthy postmenopausal women. Menopause 2005;12:299-307.
4. Messina M. Emerging evidence on the role of soy in reducing prostate cancer risk. Nutr Rev 2003;61:117-131.
5. Korde L FT, Wu A, et al. Adolescent and childhood soy intake and breast cancer risk in Asian-American women. Breast Cancer Res Treat 2005;88:S149.
6. Messina M, Ho S, Alekel DL. Skeletal benefits of soy isoflavones: a review of the clinical trial and epidemiologic data. Curr Opin Clin Nutr Metab Care 2004;7:649-658.
7. NTP-CERHR Expert Panel on the Reproductive and Developmental Toxicity of Soy Formula. April 2006 http://cerhr.niehs.nih.gov/.
8. Gu L, House SE, et al. Metabolic phenotype of isoflavones differ among female rats, pigs, monkeys, and women. J Nutr. 2006; 136:1215-21.
9. DiSilvestro RA, Mattern C, Wood N, Devor ST. Soy protein intake by active young adult men raises plasma antioxidant capacity without altering plasma testosterone. Nutrition Research 2006; 26: 92-95.
10. Kurzner MS. Hormonal effects of soy in premenopausal women and men. J Nutr 2002;132(3):570S-573S.
11. Rubin S, Kalman D, Martinez M, Krieger DR, A Randomized Double-Blind Clinical Pilot Trial Evaluating the Effect of Protein Source when Combined with Resistance Training on Body Composition and Sex Hormones in Adult Males, FASEB, 2005.
12. Dillingham BL, McVeigh BL, Lampe JW, Duncan AM. Soy protein isolates of varying isoflavone content exert minor effects on serum reproductive hormones in healthy young men. Journal of Nutrition 2005; 135:584-591.
13. NTP-CERHR Brief on the potential human reproductive and developmental toxicity of genistein. Draft October, 2006. Accessed at http://cerhr.niehs.nih.gov/chemicals/genistein-soy/genistein/Genistein_Brief_10-12-06.pdf on January 11, 2007.
14. Trock BJ, Hilakivi-Clarke L, Clarke R. Meta-analysis of soy intake and breast cancer risk. J Nat Cancer Inst. 2006;98:459 – 71.
15. Yan L, Spitznagel E. A meta-analysis of soyfoods and risk of breast cancer in women. Int J Cancer Preventio 2005;1:281-293.
16. American Cancer Society. Soy and Breast Cancer. Accessed at http://www.cancer.org/ on January 3, 2007.
17. Maskarinec G, Takata Y, Franke AA, Williams AE, Murphy SP. A 2-year soy intervention in premenopausal women does not change mammograhpic densities. J Nutr 2004;134:3089-94.
18. Maskarinec G, Williams AE, Inouye JS, Stanczyk FZ, Frankie AA. A randomized isoflavone intervention among premenopausal women. Cancer Epidemiol Biomarkers Prev 2002;11:195-201.
19. Maskarinec G, Williams AE, Carlin L. Mammorgraphic densities in a one-year isoflavone intervention. Eur J Cancer Prev 2003;12:165-9.
20. Palomares MR, Hopper L, Goldstein L, Lehman CD, Storer BE, Gralow JR. Effect of soy isoflavones on breast proliferation in postmenopausal breast cancer survivors. Breast Cancer Res Treatment 2004;88 (Suppl 1):4002.
21. Kurzer MS. Hormonal effects of soy in premenopausal women and men. J Nutr 2002;132:570S-573S.
22. Clarkson TB, Appt SE, Wood CE, Cline JM. Lessons to be learned from animal studies on hormones and the breast. Maturitas 2004 ; 49 : 79 – 89.
23. Setchell KD, Brown NM, Lydeking-Olsen E. The clinical importance of the metabolite equol — a clue to the effectiveness of soy and its isofl avones. J Nutr 2002 ; 132 : 3577 – 84.
24. National Cancer Institute. Nutrition in Cancer Care (PDQ®). Accessed at http://www.cancer.gov/ on December 28, 2006.
25. American Institute of Cancer Research. New Soy-Breast Cancer Study Finds Small but Significant Protective Effect. Accessed at http://www.aicr.org/ on December 28, 2006.
26. Ju YH, Doerge DR, Allred KF, et al.: Dietary genistein negates the inhibitory effect of tamoxifen on growth of estrogen-dependent human breast cancer (MCF-7) cells implanted in athymic mice. Cancer Res 2002;62: 2474-7.
27. Horn-Ross PL, Hoggatt KJ, Lee MM. Phytoestrogens and thyroid cancer risk: the San Francisco Bay Area thyroid cancer study, Cancer Epidemiol Biomarkers Prev, 2002; 11:43-9.
28. Messina M and Redmond G. Effects of soy protein and soybean isoflavones on thyroid function in healthy adults and hypothyroid patients: a review of the relevant literature. Thyroid. 2006; 16:249-259.
29. Food Standards Agency, United Kingdom. Phytoestrogens and Health. London: Committee on Toxicity of Chemicals in Food, Consumer Products and the Environment, 2003:1-444.
30. Nippoldt T. Soy: Does it worsen hypothyroidism? Accessed at http://www.mayoclinic.com/ on Jan. 2, 2007.
31. Bell DS, Ovalle F. Use of soy protein supplement and resultant need for increased dose of levothyroxine. Endocr Prac, 2001;7:193-4
