April 2014 Issue

The Top 5 Soy Myths
By Judith C. Thalheimer, RD, LDN
Today’s Dietitian
Vol. 16 No. 4 P. 52

Soyfoods are the subject of several popular misconceptions. Today’s Dietitian looks at the latest research to bring the facts to light.

Soy: It’s a high-quality source of protein, containing all of the essential amino acids; it’s packed with vitamins and minerals; it has fiber (both soluble and insoluble), omega-3 and omega-6 fatty acids, no cholesterol, and very little saturated fat compared with meat.1 It’s been touted for reducing the risk of coronary heart disease, osteoporosis, and some forms of cancer, and there’s clinical evidence to back up these assertions.2 But there’s also research (and media speculation) saying that soy can be detrimental to our health.

What accounts for the conflict, what are the facts, and what advice should nutrition professionals give to clients? Here’s the latest information on five common concerns about soy:

1. All soy products are made from genetically modified organisms (GMOs). Genetic engineering is used to introduce new characteristics to crops. Some varieties of soybeans have been modified to be herbicide tolerant, allowing farmers to use large doses of herbicides to kill weeds without damaging crops. USDA data indicate that 93% of all soybeans grown in the United States in 2013 were genetically engineered.3 According to the Soyfoods Association of North America, “Many soyfoods manufacturers use soybeans and/or ingredients from soybeans that have not been genetically engineered or that are certified organic, which by USDA organic regulations exclude genetic engineering methods. These soyfoods make a statement on the label, such as ‘organic’ or ‘made from non-GMO soybeans.’” The council states that these GMO-free products include soymilk, tofu, tempeh, edamame, and some meat alternatives and nutrition bars.

2. Eating soy increases breast cancer risk. Soybeans and soy products are the richest source of isoflavones in the human diet. Isoflavones are phytoestrogens, or plant chemicals capable of exerting estrogenlike effects.

Most of the concerns surrounding soyfoods have to do with their impact on the many bodily systems influenced by estrogen. Breast cancer, especially estrogen receptor–positive breast cancer, is a primary concern.

According to Marji McCullough, ScD, RD, strategic director of nutritional epidemiology for the American Cancer Society, epidemiologic studies that followed large populations of healthy women for many years either have shown no association between soy and breast cancer or a protective association from eating soy. Even breast cancer survivors may not need to worry. Three studies looking at women’s eating habits and other lifestyle factors after breast cancer found that, in the combined total of 9,000 breast cancer survivors studied, eating soy actually lowered the risk of breast cancer recurrence, even in women with estrogen receptor–positive tumors (although less so), and regardless of whether they were taking tamoxifen.4

“I want to make sure women, including those with a history of breast cancer, know it’s OK to eat these foods,” McCullough says. “However, to find out for sure whether we should specifically recommend soyfoods to breast cancer survivors, researchers would need to replicate these findings, ideally through a controlled study. At the very least, the evidence from the studies in women reassures us that moderate consumption of soyfoods is likely to be safe.” However, information on the safety of soy supplements is inconclusive, and they shouldn’t be recommended at this time.4

McCullough offers an explanation why soy’s phytoestrogens may not be the powerful cancer causers they were once thought to be. “While isoflavones may act like estrogen, they also have antiestrogen properties,” she explains. “That is, they can block the more potent natural estrogens from binding to the estrogen receptor. In addition, they stop the formation of estrogens in fat tissue and stimulate production of a protein that binds estrogen in the blood, making it less able to bind to the receptor. They also have antioxidant and anti-inflammatory properties and may work in other ways to reduce cancer growth.”

3. Soy causes feminization in men. High doses of phytoestrogens have been shown to impair male rats’ ability to produce offspring, but the same effect hasn’t been found in male humans.5 Since many of the concerns related to soy come from studies in rats and mice, it’s important to recognize that rodents metabolize soy isoflavones differently than humans, essentially making these studies inapplicable.2

While individual cases of sex hormone changes in men eating soyfoods have been reported, these men were consuming extremely high doses of soy (3 quarts of soymilk per day, in one case), and the effects were reversed when the soy intake was discontinued.6 A 2010 review in Fertility and Sterility concluded that “neither isoflavone supplements nor isoflavone-rich soy affect total or free testosterone levels” and that there’s “essentially no evidence from nine identified clinical studies that isoflavone exposure affects circulating estrogen levels in men.”7

4. Soy phytoestrogens inhibit thyroid function. Antisoy websites claim that soy phytoestrogens are “potent anti-thyroid agents that cause hypothyroidism.”8 Indeed, in the May 2011 issue of Clinical Thyroidology, a randomized, double-blind crossover study by Sathyapalan and colleagues found that six of 60 women with subclinical hypothyroidism converted to clinical hypothyroidism after eight weeks of supplementation with soy protein containing 16 mg of phytoestrogen per day. To mimic a typical Western diet, the study authors also gave women daily soy protein supplements with 2 mg of phytoestrogen. The women in this lower-dose group showed no change in thyroid function.9

In his commentary on the published study, Jorge Mestman, MD, reviewed other research in the area and concluded that “only very high doses of soy phytoestrogen supplementation may induce clinical hypothyroidism in a minority of patients with subclinical hypothyroidism.”9

For those individuals already being treated for hypothyroidism, it’s important to note that soy consumption decreases absorption of replacement thyroid hormone, so clients should be reminded to take such medications on an empty stomach as directed. Interestingly, in the same study by Sathyapalan, the high-dose soy phytoestrogens significantly reduced insulin resistance, blood pressure, and inflammation markers.9

5. Soy-based infant formula can disrupt growth and reproductive development. Soy-based infant formulas contain significant amounts of soy isoflavones.10 “Infants absorb and metabolize these phytoestrogens,” says Victoria J. Drake, PhD, of the Linus Pauling Institute at Oregon State University, “so concern has been raised that the estrogenlike effects of soy isoflavones could potentially affect the child’s growth and development or adversely influence reproductive and immune function.”

To date, this hasn’t been found to be the case. A 2012 study in Pediatrics compared breast-fed infants to those fed soy- and cow’s milk–based formula. Although breast-fed babies scored higher on tests of cognitive development, all of the formula-fed babies showed normal growth and development in the first year of life.11

Moreover, a retrospective study of 811 men and women aged 20 to 34 found no difference in height, weight, time of puberty, general health, or pregnancy outcomes between those fed soy-based formula as infants and those fed cow’s milk–based formula.10

“If you look at the latest research,” Drake says, “there is no convincing evidence that healthy infants fed soy-based formula are at greater risk for adverse effects than those fed cow’s milk-based formula.”

Moving Forward
Our understanding of the effects of soy isoflavones on the human body is evolving with ongoing research. The majority of current studies indicate that the typical amount of soy consumed in the Western diet is safe, and even beneficial, for most people. Excessive consumption of soy, or soy supplementation, however, isn’t recommended. Since myths and misconceptions about soy are common, it’s essential for dietitians to keep up with emerging research and information from reliable sources to adequately advise clients and answer their questions.

— Judith C. Thalheimer, RD, LDN, is a freelance nutrition writer and community educator living outside Philadelphia.


Sources of Soy Isoflavones12
Soybeans are by far the most concentrated source of isoflavones in the human diet.

• The highest concentrations of isoflavones are found in soyfoods that aren’t highly processed, such as tofu, soymilk, soynuts, tempeh, miso, and edamame. Soy flour and textured soy protein also contain significant amounts of isoflavones.

• Soy protein concentrates vary dramatically in their isoflavone content depending on how the protein was extracted.

• Processed soyfoods that contain considerable amounts of non-soy ingredients, such as soy hot dogs, ice cream, or snack bars, have much lower amounts of isoflavones than whole soyfoods.

• Soy oil and soy sauce contain no isoflavones.



1. Wilson H. A vegan doctor addresses soy myths and misinformation. Free From Harm website. http://freefromharm.org/health-nutrition/vegan-doctor-addresses-soy-myths-and-misinformation/ - sthash.HC9A9xF1.dpuf. Updated January 14, 2014. Accessed January 22, 2014.

2. Soy myths & facts. United Soybean Board website. http://www.soyconnection.com/sites/default/files/pictures/soy-myths-facts.pdf. Accessed January 22, 2014.

3. Genetically engineered varieties of corn, upland cotton, and soybeans by state and for the United States, 2000-13. US Department of Agriculture Economic Research Service website. http://www.ers.usda.gov/data-products/adoption-of-genetically-engineered-crops-in-the-us.aspx - .Uvofu_3bQds. Updated July 8, 2013. Accessed January 23, 2014.

4. McCullough M. The bottom line on soy and breast cancer risk. The American Cancer Society website. http://www.cancer.org/cancer/news/expertvoices/post/2012/08/02/the-bottom-line-on-soy-and-breast-cancer-risk.aspx. August 2, 2012.

5. Glover A, Assinder SJ. Acute exposure of adult male rats to dietary phytoestrogens reduces fecundity and alters epididymal steroid hormone receptor expression. J Endocrinol. 2006;189(3):565-573.

6. Martinez J, Lewi JE. An unusual case of gynecomastia associated with soy product consumption. Endocr Pract. 2008;14(4):415-418.

7. Messina M. Soybean isoflavone exposure does not have feminizing effects on men: a critical examination of the clinical evidence. Fertil Steril. 2010;93(7):2095-2104.

8. Soy alert! Weston A. Price Foundation website. http://www.westonaprice.org/soy-alert/soy-alert-brochure. Updated March 9, 2009. Accessed February 3, 2014.

9. Sathyapalan T, Manuchehri AM, Thatcher NJ, et al. 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;96(5):1442-1449.

10. Higdon J, Drake VJ. Soy isoflavones. Oregon State University Linus Pauling Institute website. http://lpi.oregonstate.edu/infocenter/phytochemicals/soyiso/. Updated December 2009. Accessed January 24, 2014.

11. Andres A, Cleves MA, Bellando JB, Pivik RT, Casey PH, Badger TM. Developmental status of 1-year-old infants fed breast milk, cow’s milk formula, or soy formula. Pediatrics. 2012;129(6):1134-1140.

12. Soy isoflavones. Iowa State University Soybean Extension and Research Program website. http://extension.agron.iastate.edu/soybean/uses_isoflavones.htm. Updated July 2007. Accessed February 9, 2014.