by Jack Norris, RD • Last updated April 2011
Soy contains isoflavones, which have the ability to bind to estrogen receptors. There are about 25 mg of isoflavones in one serving of soy.
Case-control studies on soy and breast cancer have been generally encouraging to those with soy in their diets, with about half associating soy with a lower risk for breast cancer and the other half showing no effects.
Prospective studies, which are generally a higher level of evidence than case-control studies, have also been very positive. Of the six studies done on populations with higher soy intakes (about one to two servings per day is the typical upper intake amount), the Singapore Chinese Health Study (1), the Shanghai Women’s Study (2, 3), and the Japan Public Health Center study (4) all found that higher intakes of soy were associated with a reduced risk. The Japan Collaborative Cohort Study (5) and the Japan Life Span Study (6) found no association.
The European Prospective Investigation into Cancer-Oxford (7), which contained a large number of vegetarians, also found no association. Regarding the lack of association in the European study, some have speculated that in order to receive benefits from soy, exposure must occur during adolescence when breasts are developing, while Western vegetarians often add soy to their diets as adults.
As for women with breast cancer, including those with tumors that grow in response to contact with estrogen (known as estrogen receptor positive), the authors of the recently published Women’s Healthy Eating and Living Study (8) write:
Our study is the third epidemiological study to report no adverse effects of soy foods on breast cancer prognosis. These studies, taken together, which vary in ethnic composition (two from the US and one from China) and by level and type of soy consumption, provide the necessary epidemiological evidence that clinicians no longer need to advise against soy consumption for women diagnosed with breast cancer.
Also see The Bottom Line on Soy and Breast Cancer Risk by Marji McCullough, ScD, RD of the American Cancer Society.
The 2000 Honolulu-Asia Aging Study linked soy with lower cognitive function and brain shrinkage. Here’s a more thorough review of the research on soy and mental cognition.
Epidemiological studies (unlike clinical trials) examine patterns of soy consumption and cognition in specific populations. One such study found tempeh (a fermented soy food) to be associated with improved cognition (21). Three reports from epidemiological studies have associated tofu with reduced cognition in some groups (21, 22, 23), but increased cognition in another group (24), and neutral in others (23, 24). The harmful findings for tofu in the epidemiological studies are likely due to confounding caused by the fact that people of lower economic status have traditionally eaten more tofu in Asian cultures as well as the fact that some Indonesian tofu has been prepared using formaldehyde.
The research as a whole provides little cause for concern.
The concern about infant formula is understandable giving that infants will be eating a large portion of their diet as soy.
The American Academy of Pediatrics and the National Toxicology Program considers soy formula safe.
The most important study to date, tracking adults who were fed soy formula as infants, provides assurance that there’s no reason to be concerned about thyroid or reproductive function (25).
The Beginnings Study is an ongoing study examining the effects of formula on child development (26). It’s in its early stages with findings from children only a year old, but to date, no negative effects of soy have been found on growth, sex organs, or neurological development compared to children on cow’s milk formula.
Some research shows that it’s best to choose a soy formula with DHA, and it’s important to note that soy formula isn’t intended for pre-term infants.
Regarding the concern that soy could cause feminizing characteristics in men, there have been two case studies. In one, a man eating twelve servings per day of soymilk developed enlarged, sensitive breast tissue (27). In another, a man with type 1 diabetes was eating 14 servings per day of mostly processed soy foods for one year and developed erectile dysfunction (28), which normalized after ceasing the soy. While I would not recommend eating this much soy, one study used even much higher amounts of isoflavones and found no problems for most men (29).
Isoflavones can affect thyroid hormone levels—especially if someone has iodine deficiency. A 2011 clinical trial found that 16 mg/day of isoflavones in people with mild hypothyroidism appeared to cause an increased rate of advancing to overt hypothyroidism (33).
Nine other clinical trials showed no effect of soy on the thyroid compared to placebo in people with presumably healthy thyroids (34, 35, 36, 37, 38, 39, 40, 41, 42), while the remaining five studies found small changes, all without physiological significance (43, 44, 45, 46, 47).
People without hypothyroidism should have no problems with soy as long as they get enough iodine, but until we know more, people with mild hypothyroidism might want to avoid soy just to be safe.
In addition to reducing the risk for breast cancer as mentioned above, soy also provides benefits for preventing prostate cancer, lowering LDL cholesterol, and improving menopausal symptoms. Please see Soy Part 2—Research for more details.
When you add up all the research on soy, there’s no reason to think that two servings per day are harmful to most people, and good reason to think soy provides some health benefits.
1. Butler LM, Wu AH, Wang R, Koh WP, Yuan JM, Yu MC. A vegetable-fruit-soy dietary pattern protects against breast cancer among postmenopausal Singapore Chinese women. Am J Clin Nutr. 2010 Apr;91(4):1013-9.
3. Lee SA, Shu XO, Li H, Yang G, Cai H, Wen W, Ji BT, Gao J, Gao YT, Zheng W. Adolescent and adult soy food intake and breast cancer risk: results from the Shanghai Women’s Health Study. Am J Clin Nutr. 2009 Jun;89(6):1920-6.
4. Yamamoto S, Sobue T, Kobayashi M, Sasaki S, Tsugane S; Japan Public Health Center-Based Prospective Study on Cancer Cardiovascular Diseases Group. Soy, isoflavones, and breast cancer risk in Japan. J Natl Cancer Inst. 2003 Jun 18;95(12):906-13.
5. Nishio K, Niwa Y, Toyoshima H, Tamakoshi K, Kondo T, Yatsuya H, Yamamoto A, Suzuki S, Tokudome S, Lin Y, Wakai K, Hamajima N, Tamakoshi A. Consumption of soy foods and the risk of breast cancer: findings from the Japan Collaborative Cohort (JACC) Study. Cancer Causes Control. 2007 Oct;18(8):801-8.
7. Travis RC, Allen NE, Appleby PN, Spencer EA, Roddam AW, Key TJ. A prospective study of vegetarianism and isoflavone intake in relation to breast cancer risk in British women. Int J Cancer. 2008 Feb 1;122(3):705-10.
8. Caan BJ, Natarajan L, Parker BA, Gold EB, Thomson CA, Newman VA, Rock CL, Pu M, Al-Delaimy WK, Pierce JP. Soy Food Consumption and Breast Cancer Prognosis. Cancer Epidemiol Biomarkers Prev. 2011 Feb 25.
10. Duffy R, Wiseman H, File SE. Improved cognitive function in postmenopausal women after 12 weeks of consumption of a soya extract containing isoflavones. Pharmacol Biochem Behav. 2003 Jun;75(3):721-9.
12. Kritz-Silverstein D, Von Mühlen D, Barrett-Connor E, Bressel MA. Isoflavones and cognitive function in older women: the Soy and Postmenopausal Health In Aging (SOPHIA) Study. Menopause. 2003 May-Jun;10(3):196-202.
13. Fournier LR, Ryan Borchers TA, Robison LM, Wiediger M, Park JS, Chew BP, McGuire MK, Sclar DA, Skaer TL, Beerman KA. The effects of soy milk and isoflavone supplements on cognitive performance in healthy, postmenopausal women. J Nutr Health Aging. 2007 Mar-Apr;11(2):155-64.
14. Islam F, Sparkes C, Roodenrys S, Astheimer L. Short-term changes in endogenous estrogen levels and consumption of soy isoflavones affect working and verbal memory in young adult females. Nutr Neurosci. 2008 Dec;11(6):251-62.
17. Celec P, Ostatníková D, Cagánová M, Zuchová S, Hodosy J, Putz Z, Bernadic M, Kúdela M. Endocrine and cognitive effects of short-time soybean consumption in women. Gynecol Obstet Invest. 2005;59(2):62-6.
18. Kreijkamp-Kaspers, S., Kok, L., Grobbee, D.E., de Haan, E.H.F., Aleman, A., Lampe, J.W., van der Schouw, Y.T. Effect of soy protein containing isoflavones on cognitive function, bone mineral density, and plasma lipids in postmenopausal women: A randomized controlled trial, JAMA. 2004;292:65-74.
19. Ho SC, Chan AS, Ho YP, So EK, Sham A, Zee B, Woo JL. Effects of soy isoflavone supplementation on cognitive function in Chinese postmenopausal women: a double-blind, randomized, controlled trial. Menopause. 2007 May-Jun;14(3 Pt 1):489-99.
20. Pilsáková L, Riecanský I, Ostatníková D, Jagla F. Missing evidence for the effect one-week phytoestrogen-rich diet on mental rotation in two dimensions. Neuro Endocrinol Lett. 2009 Mar;30(1):125-30.
21. Hogervorst E, Sadjimim T, Yesufu A, Kreager P, Rahardjo TB. High tofu intake is associated with worse memory in elderly Indonesian men and women. Dement Geriatr Cogn Disord. 2008;26(1):50-7. (Epub 2008 Jun 27)
24. Hogervorst E, Mursjid F, Priandini D, Setyawan H, Ismael RI, Bandelow S, Rahardjo TB. Borobudur revisited: Soy consumption may be associated with better recall in younger, but not in older, rural Indonesian elderly. Brain Res. 2010 Oct 28. (Epub ahead of print)
25. Strom BL, Schinnar R, Ziegler EE, Barnhart KT, Sammel MD, Macones GA, Stallings VA, Drulis JM, Nelson SE, Hanson SA. Exposure to soy-based formula in infancy and endocrinological and reproductive outcomes in young adulthood. JAMA. 2001 Aug 15;286(7):807-14.
29. Fischer L, Mahoney C, Jeffcoat AR, Koch MA, Thomas BE, Valentine JL, et al. Clinical characteristics and pharmacokinetics of purified soy isoflavones: multiple-dose administration to men with prostate neoplasia. Nutr Cancer 2004;48(2):160–70.
31. Beaton LK, McVeigh BL, Dillingham BL, Lampe JW, Duncan AM. Soy protein isolates of varying isoflavone content do not adversely affect semen quality in healthy young men. Fertil Steril. 2010 Oct;94(5):1717-22.
32. Messina M, Watanabe S, Setchell KD. Report on the 8th International Symposium on the Role of Soy in Health Promotion and Chronic Disease Prevention and Treatment. J Nutr. 2009 Apr;139(4):796S-802S.
33. Sathyapalan T, Manuchehri AM, Thatcher NJ, Rigby AS, Chapman T, Kilpatrick ES, Atkin SL. The Effect of Soy Phytoestrogen Supplementation on Thyroid Status and Cardiovascular Risk Markers in Patients with Subclinical Hypothyroidism: A Randomized, Double-Blind, Crossover Study. J Clin Endocrinol Metab. 2011 Feb 16. (Epub ahead of print)
36. Tousen Y, Ezaki J, Fujii Y, Ueno T, Nishimuta M, Ishimi Y. Natural S-equol decreases bone resorption in postmenopausal, non-equol-producing Japanese women: a pilot randomized, placebo-controlled trial. Menopause. 2011 Jan 19. (Epub ahead of print)
37. Ham JO, Chapman KM, Essex-Sorlie D, Bakhit RM, Prabhudesai M, Winter L, Erdman JW, Potter S. Endocrinological response to soy protein and fiber in mildly hypercholesterolemic men. Nutr Res 1993; 13:873-884.
41. Roughead ZK, Hunt JR, Johnson LK, Badger TM, Lykken GI. Controlled substitution of soy protein for meat protein: effects on calcium retention, bone, and cardiovascular health indices in postmenopausal women. J Clin Endocrinol Metab. 2005 Jan;90(1):181-9.
44. Hampl R, Ostatnikova D, Celec P, Putz Z, Lapcík O, Matucha P. Short-term effect of soy consumption on thyroid hormone levels and correlation with phytoestrogen level in healthy subjects. Endocr Regul. 2008 Jun;42(2-3):53-61.
45. Jayagopal V, Albertazzi P, Kilpatrick ES, Howarth EM, Jennings PE, Hepburn DA, Atkin SL. Beneficial effects of soy phytoestrogen intake in postmenopausal women with type 2 diabetes. Diabetes Care. 2002 Oct;25(10):1709-14.
46. Persky VW, Turyk ME, Wang L, Freels S, Chatterton R Jr, Barnes S, Erdman J Jr, Sepkovic DW, Bradlow HL, Potter S. Effect of soy protein on endogenous hormones in postmenopausal women. Am J Clin Nutr. 2002 Jan;75(1):145-53.