Soy has long been recognized as a nutrient-dense food and as an excellent source of protein by respected dietitians and clinical nutritionists. (1) The soybean contains all of the essential amino acids, as well as an impressive list of micronutrients (vitamins and minerals). Micronutrients in rich supply in soy include: calcium, iron, magnesium, manganese, phosphorus, potassium, vitamins B1, B2, B3, B5, B6, B9, C and zinc. Fiber and omega-3 and 6 fatty acids are also present in soy. The composition of these nutrients varies among preparations, but is in the highest quantity in whole soy foods such as edamame (whole soy beans), soy milk, tofu and tempeh.

Yet despite the powerful health benefits of whole soy foods, myths and misinformation regarding the ‘dangers’ of soy consumption are being widely circulated and presented as fact. I will address a few of these myths by taking a closer look at some of the sources of confusion and controversy.

“All soy is GMO!”

I would like to begin by explaining that the largest consumer of commercially grown GMO soybeans, both in the US and globally, is farmed animals. GMOs are genetically modified organisms, and their safety for human consumption is a hot topic of debate; many European countries have banned GMOs. While long term studies and conclusive data on the health effects of GMOs are lacking, GMOs are ubiquitous in our food supply. Soybeans are one of several major food staples now dominated by genetic modification. Currently, 81% of the global soybean crop is genetically modified, and approximately 85% of all GMO soybeans end up in farmed animal feed. The GMO soy consumed by farmed animals is utilized as a source of protein by them, and does not just magically evaporate in the slaughterhouse or the milk processing plant. It ends up on your plate.

But while an alarming percentage of soybeans are genetically modified, the claim that “all soy is GMO” is one of the great soy myths. Of the soy directly consumed by humans, non-GMO soy foods such as tofu, tempeh and soy milk are widely available in stores which offer soy products, and they are clearly labeled non-GMO.

“But I heard soy causes cancer!”

Misinformation regarding soy’s relationship to cancer largely stems from confusion around the presence of phytoestrogens in soy. Phytoestrogen is not estrogen. Estrogen and testosterone are steroid hormones, and occur naturally in both sexes of humans, as well as in animals used for food. They help regulate sexual function and secondary sexual characteristics, in addition to nonsexual cellular functions. While estrogen plays many important beneficial roles in humans, it also naturally promotes proliferation of cells, and, at high levels, can increase risk of some cancers by encouraging cells to multiply more than they usually would. Hormone replacement therapy in postmenopausal women (specifically, taking only estrogen) has also been implicated in cancer growth. (2)

While soy does not contain estrogen, animal foods do. Many consumers are aware that animals used for meat and dairy are commonly supplemented with synthetic growth hormones, but what they don’t consider is that animal flesh and cow milk also contain their own naturally occurring estrogen— and this is true even of “grass-fed” and “organic” animals. Furthermore, meat, dairy and eggs all contain phytoestrogens; they are pervasive in our food, both plant and animal-derived, and you are not avoiding them entirely by avoiding soy.

Phytoestrogen is just a catchall term for numerous naturally occurring plant compounds which are structurally similar to mammalian estrogen, and functionally are weakly estrogenic (weakly mimicking estrogen) or antiestrogenic (blocking estrogen’s effects). The metabolism and functionality of phytoestrogens are incredibly complex, and vary between individuals. The concern over soy and cancer stems from the fact that soy-based foods contain phytoestrogens (specifically, isoflavones) in varying amounts (depending on the preparation), and these react with the estrogen receptor. There are two types of estrogen receptors in humans: alpha and beta. Alpha are distributed widely throughout the body, whereas beta are localized in the ovary, prostate, lung, and epididymis (testicle). While isoflavones, like estrogen, bind to both alpha and beta receptors (preferentially to beta), isoflavones do not have the estrogenic effect of inducing tumor growth. In fact, isoflavones have demonstrated a protective benefit against hormone-dependent cancers.

The inverse relationship between soy consumption and risk of developing premenopausal breast cancer has been clearly established. In other words, higher rates of soy intake are associated with lower rates of breast cancer. (3) However, large clinical trials have not yet been conducted regarding the effect of phytoestrogen consumption on tumor growth in established cancer patients. (4) To date, such studies have utilized small sample sizes, and the methods for obtaining data were highly variable. There have been ‘promising’ results from multiple animal models, demonstrating a reduction in tumor size with consumption of soy protein. Yet, aside from the immorality of artificially inducing cancer in unwilling participants, it is dangerous to compare laboratory animals to humans. We are not mice. Even within our species, there is tremendous variability in the metabolic processing of phytoestrogens and pharmacological agents, thus establishing the difficulty and complexity of this area of research. Instead, attention should be placed on the already available mass of epidemiological data which compares Asian cultures to those consuming a Western diet.

Lessons from Asia

Soy has been a major staple in Asian cultures for centuries, and their incidence of coronary artery disease, hypertension, ischemic stroke, hormone-dependent cancers, osteoporosis, postmenopausal hip fracture, diabetes, and obesity are all markedly lower than what is seen here in the US. However, when sectors of these populations begin to consume foods based more on the Western diet, not surprisingly, their patterns of disease begin to mimic ours as well. I recall this point being made in medical school. The terms ‘vegan’ and ‘plant-based’ were not yet widely in use, yet there was at least recognition of the association between geographic variability in diet, and health conditions. During this time, the human genome project was completed. I had made the connection that independent of the mass of genetic information all humans share, how we live and what we eat will have a huge and direct impact on our health.

To cite just one example, the Okinawa Centenarian Study analyzed the health and dietary patterns of over 900 centenarians (individuals of 100 years of age or older) living on the Japanese island of Okinawa. Individuals in their 70s, 80s and 90s were evaluated as well. The Japanese Ministry of Health has been maintaining a family register for the entire country since the 1870s, and it is updated every 5 years. The world’s highest known concentration of centenarians live in Okinawa. Regular physical activity, lean BMIs (body mass index), and high consumption of fruits, vegetables and soy are all a part of traditional Okinawan lifestyle. Their aging population enjoy healthier lives and much lower rates of cancer (breast, ovarian, prostate and colon) as compared to those in the U.S., and even to those on the Japanese mainland. Their rates of dementia, osteoporosis, and coronary artery disease are also impressively low. By comparison, based on my 10 years of clinical experience, I can attest to the fact that our aging population suffers a high incidence of debilitating disease, dependency on pharmaceuticals, depression, and difficulty with mobility and physical activity.

The myth of ‘moobs’ (man boobs)

“Moobs” is another of the heavily circulated soy myths with no actual basis in scientific fact. I am reminded of the fear I had of swallowing bubblegum as a child — “because it will take 7 years to digest!” Gynecomastia is the medical term for developed breasts in men. Fetal sexual organ formation and secondary sexual characteristic development are essentially hormone driven. The ‘soy causes man boobs’ urban legend is thus likely rooted in the confusion between estrogen and phytoestrogen, but, as previously explained, phytoestrogen is not estrogen. If indeed this were the case, there would be a lot of men in need of bras.

In reality, clinical studies in men show that isoflavones do not affect testosterone levels or circulating estrogen levels. Even at levels of isoflavone exposure significantly higher than those of a typical Asian male consuming a soy rich diet, isoflavones have not been found to have feminizing effects. (6)

Soy myths and hysteria from the Weston A. Price Foundation

Soy myths and hysteria and the Weston A. Price Foundation (WAPF) are inseparable. Much of the fear-mongering around soy is a direct result of misinformation disseminated by the WAPF’s relentless anti-soy campaigns. The WAPF, registered as a nonprofit organization, is a multimillion dollar operation that lobbies for raw milk and grass-fed beef. Its members (often farmers) make financial contributions and in turn benefit from WAPF promotion. One of the WAPF’s ongoing strategies for promoting animal farming interests is a concerted effort to discredit veganism in general, and soy in particular. Soyfoods sales have climbed from $500 million in 1992 to $5.2 billion in 2011. The soy industry is expanding exponentially, thus posing a potential threat to the products the WAPF are trying to peddle. In response, the Weston Price Foundation actively publishes articles which propagate the supposed dangers of soy consumption, citing clinical and medical journals in an attempt to appear credible.

To give one example, there is a recent blog entry on the WAPF website from board member Kaayla Daniel, who attempted to interpret an article from the Journal of American Medical Association (JAMA) in a way that would promote the WAPF’s anti-soy agenda. On July 10 2013, JAMA published ‘Effect of Soy Protein Isolate Supplementation on Biochemical Recurrence of Prostate Cancer After Radical Prostatectomy’. (5) The study was well conducted (randomized and double-blind), and aimed to analyze whether or not soy intake would have any effect on patients being treated for advanced prostate cancer. Oncologists measured PSA (prostate specific antigen) in the blood at specific time intervals to assess response. It is not surprising that in patients with a lifetime consumption of foods high in animal fat and protein, pesticides, preservatives and antibiotics, and after a diagnosis of advanced cancer — that the biochemical markers were not affected by soy ingestion. The study could yield no solid conclusions about soy, except that with advanced prostate cancer, the consumption of soy will not reduce biochemical markers. Yet, the WAPF skewed data and selectively interpreted the study to support claims that further their anti-soy agenda.

It’s also worth noting: Dr. Weston A Price (1870-1948) was a dentist, not a physician or a dietician. He studied teeth from primitive cultures and formulated dietary recommendations for modern society based on dental decay observations.

Soy and thyroid function

The thyroid gland has the essential function of controlling metabolism. There are multiple dietary nutrients required for the production of thyroid hormone, iodine being the most widely recognized. The relationship between soy consumption, iodine deficiency and goiter (enlarged thyroid) was first described in 1960 in The New England Journal of Medicine. Infants consuming nonfortified soy-based formula developed goiter, yet the exact nature of the relationship was unclear. Since then, there have been numerous studies which have disproven the causal relationship between soy and thyroid toxicity. (7)

TPO (thyroid peroxidase) is an enzyme located in the thyroid gland that catalyzes the necessary reactions to formulate thyroid hormone. Studies involving rats, pigs and humans demonstrated decreased TPO activity when fed isolated genistein and daidzein (the isoflavones in soy that react most strongly with TPO). Although some TPO activity was lost, there was no overall negative effect on thyroid function. Thyroid hormone levels measured in the blood of both experimental and control groups were the same. Furthermore, humans (as well as rats) only demonstrated hypothyroidism if their soy diets were iodine-depleted. Please see table for National Institute of Health recommendations.

What types of soy foods are healthiest?

In discussions of soy foods, the assertion is sometimes made that only fermented soy foods are safe and healthful to consume, with the eating habits of traditional Asian cultures cited as support for this claim. In fact, contrary to this common misconception, the soy products regularly consumed in Asian countries are not all, or even primarily, fermented. According to research from Ginny Messina, R.D., “In Japan, about half of soy consumption comes from the fermented foods miso and natto, and half comes from tofu and dried soybeans. In Shanghai, most of the soy foods consumed are unfermented, with tofu and soymilk making the biggest contributions. In fact, even in Indonesia, where tempeh is a revered national food, unfermented soy products like tofu account for around half of soy intake.”

Personally, I enjoy a wide variety of healthful foods. I frequent my local farmers market, and buy locally grown, organic produce that is in season. When it comes to soy foods, I am always careful to buy non-GMO products, which are easy to find and are clearly labeled. Thai-style stir fry and tofu scramble are among my favorite tofu dishes; they are easy to prepare and can be made in one pan. I also season and bake firm tofu, then place it in a wrap for a quick lunch. In the winter months, I make creamy vegetable soups with a soy milk base, and grilled tempeh is great on sandwiches and crumbled over salads. There are numerous websites and cookbooks which offer delicious soy recipes, ranging from super-easy to gourmet.

Whole soy foods are safe and nutritious. I recommend incorporating them into a diet which contains a good variety of fresh fruits and vegetables, grains and legumes. For more on plant sources of protein, please see my article, “A Vegan Doctor Addresses the Protein Question.”

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(1) Tucker, Katherine L. et al. “Simulation with Soy Replacement Showed That Increased Soy Intake Could Contribute to Improved Nutrient Intake Profiles in the U.S. Population.” The Journal of Nutrition, doi: 10.3945/jn.110.123901; 27 October 2010

(2) Morito, Keiko. et al. “Interaction of Phytoestrogens with Estrogen Receptors Alpha and Beta.” Biological and Pharmaceutical Bulletin, Vol. 24, Issue 4, pp. 351-356, April 2001

(3) Lee, Sang-Ah. et al. “Adolescent and Adult Soy Intake and Breast Cancer Risk: Results from the Shanghai Women’s Health Study.” The American Journal of Clinical Nutrition, Vol 89, Issue 6, pp. 1920-1926, June 2009

(4) Wu, A H. et al. “Epidemiology of Soy Exposures and Breast Cancer Risk.” British Journal of Cancer, Vol 98, Issue 1, pp. 9-14, 15 January 2008

(5) Bosland, Maarten C. et al. “Effect of Soy Protein Isolate Supplementation on Biochemical Recurrence of Prostate Cancer After Radical Prostatectomy: A Randomized Trial.” The Journal of the American Medical Association, Vol. 310, Issue 2, pp. 170-178, 10 July, 2013

(6) Messina, Mark. “Soybean isoflavone exposure does not have feminizing effects on men: a critical examination of the clinical evidence.” Fertility and Sterility, Vol. 93, Issue 7 , pp. 2095-2104, 1 May 2010

(7) Chang, Hebron C. et al. “Dietary Genistein Inactivates Rat Thyroid Peroxidase in Vivo without an Apparent Hypothyroid Effect.” Toxicology and Applied Pharmacology, Vol 198, Issue 3, pp. 244-252, 1 November 2000