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By Jack Norris, RD,

Dear Jack,

I read on the Internet that soy is bad for you. Is this true?

I'm kidding about the above intro question because it implies that not everyone has seen articles on the Internet about how bad soy is for you. If I had a nickel for every time someone said something like that to me...

There are at least 30 - 40 scientific papers on soy published each month. If you do a search on PubMed, you will find almost 7,000 papers with “soy” in the title and over 19,000 with soy in the abstract. So, it would be quite easy to build a false case against soy by citing a handful of these thousands of studies.

To sum up the research on humans, the bulk of the evidence indicates that 2 to 3 servings of soy is perfectly safe, possibly even protective against disease. A serving of soy is 1/2 cup of tofu, tempeh, soybeans or textured vegetable protein, or 1 cup of soymilk.

The following is a list of problems said to be caused by soy. I will quickly cover each one.

Thyroid Problems

Isoflavones in soy have a similar structure to thyroid hormone and can inhibit thyroid peroxidase (TPO),1 an enzyme involved in the synthesis of thyroid hormones. Numerous compounds of plant origin, known as goitrogens, can inhibit TPO activity. As their name implies, goitrogens can cause an enlarged thyroid, known as a goiter by increasing the amount of thyroid stimulating hormone. People can get goiters from both an iodine deficiency and an iodine excess, which can cause metabolic problems since the thyroid regulates metabolism.

Most people who have a reliable source of iodine can safely eat soy without it causing a problem for their thyroid. Some vegans might not get enough iodine because it is inconsistently found in foods. I recommend that vegans (who do not each much seaweed) get 75-150 mcg of iodine every few days from a supplement to ensure they are getting enough. Seaweed can also be a good source of iodine (sometimes too much, so be careful).

Breast Cancer

There is a debate about whether soy prevents or causes breast cancer. Some women have estrogen-positive breast cancer, meaning their tumors have estrogen receptors and are thought to grow from contact with estrogen. Soy contains isoflavones which are weak estrogens. It is not known if this is good or bad. The isoflavones could stimulate estrogen positive breast tumors, or they could dull the effect of real estrogen on the tumor. There has been little research performed on humans. If I was a woman who had breast cancer or who was at high risk for breast cancer, I might limit soy until more is known. But, that might be cutting out a food that is protective -- we just don’t know at this time.

In April 2006, the results of a meta-analysis of soy intake and breast cancer risk showed a slightly decreased risk among women who ate more soy.5 It was the combined results of 17 previous studies (supposedly all that existed at the time the study was written in 2004) where soy intake and breast cancer rates were measured. When the results were combined, it showed that the women who had the highest intake of soy had a slightly smaller risk of breast cancer than those in the lowest intake categories. However, the intake categories were rather small. In the individual studies showing a protective effect of soy, the highest intake category was an average of 8 grams of soy protein per day, or about one serving per day. The highest intake category for soy in any of the 17 studies was 19 grams per day, or about 2 servings. That study showed no effect of soy. This meta-analysis indicates that one serving of soy per day is not going to raise the risk of breast cancer and might even lower it a bit.

In April of 2009, a paper was published in the American Journal of Clinical Nutrition, written by Mark Messina and Anna Wu.6 Here are highlights from this paper:

  • Daily per capita isoflavone intake in the United States and in Europe is about 3 mg per day. Daily per capita isoflavone intake among older adults in Japan and in Chinese cities such as Shanghai is about 25-50 mg per day.
  • They noted that the study by Trock et al (mentioned above) found no statistially significant protective effect of higher intake on breast cancer rates in Asian countries. But they reported the results of a 2008 meta-analysis (Wu et al.), with the following results:

Isoflavone intake (mg/day)        Risk   
~5                                                1.00
~10                                              .88, .78-.98
~20                                               .71, .60-.85

Reduced risks were found for both pre- and postmenopausal breast cancer.

  • When you pick apart these various studies, there appears to be evidence that eating soy early in life is important for gaining a protection against breast cancer.
  • Four studies have taken breast tissue biopsies before and after exposure to isoflavones and none found any increased breast cell proliferation.
  • Two epidemiologic studies looking at survival after breast cancer diagnosis found no reduced survival among women eating more soy.
  • The authors conclude, "The concern that products containing isoflavone might be contraindicated for patients with breast cancer and women at increased risk of breast cancer is based almost exclusively on results from rodent studies. In contrast, however, the clinical and epidemiologic data suggest that isoflavones pose no risk to such women. This suggestion is consistent with the relatively unimpressive data showing that postmenopausal therapy with oral estrogen increases breast cancer risk. Nevertheless, it must be acknowledged that, because there is an imprecise understanding of the effect of soy and isoflavones on breast tissue, women at high risk of developing breast cancer and patients with breast cancer should first discuss any dietary changes with their primary health care provider."

Also in April of 2009, a prospective study on soy intake and breast cancer risk in the Shanghai Women's Health Study was published. They found that among premenopausal women, higher soy intake was protective, but not among postmenopausal women. The results for premenopausal women were:

      Isoflavone intake    Risk              Soy protein intake     Risk
      (mg/day)                                                     (g/day)

              11                      1.00                       3.5                        1.00
              20                    .66, .44-.98              6.0                        .79, .54-1.15
              28                    .80, .55-1.18            8.2                        .59, .39-.91
              38                    .48, .30-.77              11                         .62, .40-.95
              55                     .44, .26-.73             16                        .41, .25-.70

A 2008 case control study from Japan found that soy consumption was linked with a significantly reduced risk of estrogen positive breast cancer.8 The risk for the top one-third was .74 (.58 - 0.94).

In conclusion, at the very least it appears safe for women without breast cancer to eat moderate amounts of soy, and it is also probably safe for women with breast cancer.


One study from Hawaii linked tofu consumption with lower cognitive function.2 Other studies have linked soy to better cognitive function.3 A study of Seventh-day Adventists, many of whom have consumed soyfoods all of their lives, showed lower dementia in old age than the general population.

A 2008 study from Indonesia found that among people aged 52 to 98, increased tofu consumption was linked to slightly worse memory scores (-0.18, p = .05).9 Tempeh, on the other hand, was linked to slightly better memory scores.

The authors state that

"According to the Departments of Public Health at the Universities of Jakarta and Yogyakarta, formaldehyde is often added to tofu (but not tempeh) to preserve its freshness. Formaldehyde can induce oxidative damage to the frontal cortex and hippocampal tissue...."

As of September 2008, the government was trying to end the practice (Formaldehyde-laced tofu still selling despite raids, The Jakarta Post, 09/11/2008). In light of the formaldehyde problem and how slight the differences were in memory scores, there is probably little to worry about.

The authors surmised that tempeh might be good for memory because the bacteria used in the tempeh starter, Rhizopus oligosporus produce folate, which is thought to protect memory.

Kidney Stones

People prone to oxalate kidney stones might want to limit their intake of soy as many soyfoods are high in oxalates.

On a Brighter Note

There is evidence that soy intake may be protective against heart disease, prostate cancer, osteoporosis4, and menopausal symptoms. Tempeh is also a good source of absorbable zinc.

Of course, some people are allergic to soy and should avoid it. Other people say they feel better when not eating soy. And other people feel better when they do eat soy. But the research indicates it’s “not unsafe” for most people at 2 to 3 servings a day.


1. Duncan AM, Dillingham BL. Soy & Thyroid Function: Safety Issues Examined (PDF). The Soy Connection. 2006 (Summer);14(3):1-3.

2. White LR, Petrovitch H, Ross GW, Masaki K, Hardman J, Nelson J, Davis D, Markesbery W. Brain aging and midlife tofu consumption. J Am Coll Nutr. 2000 Apr;19(2):242-55.

3. Messina V, Messina M. "Is It Safe to Eat Soy?"

4. Ma DF, Qin LQ, Wang PY, Katoh R. Soy isoflavone intake increases bone mineral density in the spine of menopausal women: meta-analysis of randomized controlled trials. Clin Nutr. 2008 Feb;27(1):57-64. Epub 2007 Dec 11.

5. Trock BJ, Hilakivi-Clarke L, Clarke R. Meta-analysis of soy intake and breast cancer risk. J Natl Cancer Inst. 2006 Apr 5;98(7):430-1.

6. Messina M, Wu AH. Perspectives on the soy-breast cancer relation. Am J Clin Nutr. 2009 May;89(5):1673S-1679S. Epub 2009 Apr 1.

7. 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. Epub 2009 Apr 29.

8. Suzuki T, Matsuo K, Tsunoda N, Hirose K, Hiraki A, Kawase T, Yamashita T, Iwata H, Tanaka H, Tajima K. Effect of soybean on breast cancer according to receptor status: a case-control study in Japan. Int J Cancer. 2008 Oct 1;123(7):1674-80. Abstract.

9. 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.

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We began this archive as a means of assisting our visitors in answering many of their health and diet questions, and in encouraging them to take a pro-active part in their own health. We believe the articles and information contained herein are true, but are not presenting them as advice. We, personally, have found that a whole food vegan diet has helped our own health, and simply wish to share with others the things we have found. Each of us must make our own decisions, for it's our own body. If you have a health problem, see your own physician.