Is Soy Safe?
by Jack Norris, RD
Dear Jack,
I read on the Internet that soy is bad for you. Is this true?
I am also aware of many articles circulating on the Internet and in health magazines which say that soy is bad for you. First of all, let me say that 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. (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. 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. Future studies will be looking at higher intakes of soy.
Dementia
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. So, there doesn’t seem to be much to fear here.
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.
Footnotes
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.
Other References
"Is It True What They Say About Soy?" UC Berkeley Wellness Letter, November 2002.