Soybeans have been accused of everything from causing breast cancer to thyroid damage to flooding men and women with estrogen. But is soy really that unhealthy to eat or is its rap rooted in myth? We break down the potential health risks and benefits of eating soy below.
Read more: The Benefits of Soymilk — and Some Drawbacks
"When I talk about soy in class, I talk about it like beer. If you're an adult and you go out and have one or two beers, you can expect not to have any serious adverse health outcomes. If you drink 10, then you're going to have a problem."
Heather B. Patisaul, associate professor at North Carolina State University
Doesn't Soy Contain Phytoestrogens?
One of the knocks against soy is that it contains phytoestrogens, a group of natural compounds that resemble estrogen organically.
According to a March 2010 review by Heather B. Patisaul of North Carolina State University and Wendy Jefferson of the National Institute of Environmental Health Sciences, published in Frontiers in Neuroendocrinology, the jury is still out on phytoestrogens.
Patisaul and Jefferson wrote, "A litany of health benefits including a lowered risk of osteoporosis, heart disease, breast cancer and menopausal symptoms are frequently attributed to phytoestrogens, but many are also considered endocrine disruptors, indicating that they have the potential to cause adverse health effects as well. Consequently, the question of whether or not phytoestrogens are beneficial or harmful to human health remains unresolved." (Endocrine disruptors are chemicals that may interfere with the body's endocrine system and produce adverse effects.)
The researchers say that soy's effects are most likely complex and could depend on someone's age, health status and even the presence or absence of specific gut bacteria in the individual. So what's a phytoestrogen-wary individual to do?
"It's all things in moderation," says Patisaul. "When I talk about soy in class, I talk about it like beer. If you're an adult and you go out and have one or two beers, you can expect not to have any serious adverse health outcomes. If you drink 10, then you're going to have a problem. It sort of depends on your life stage, how much you're consuming and what other types of health issues you have."
Many non-meat-eaters prefer soy because it's a complete protein. But Patisaul cautions that soy should not be the only protein a person eats — it should be one of many.
Read more: Soy Protein Vs. Meat Protein
Can Soy Affect Thyroid Function?
The thyroid is an endocrine gland that produces hormones, which control the rate of many metabolic activities in the body.
A March 2006 review of 14 trials by researchers from Loma Linda University published in the journal Thyroid looked at the effects of soy food on at least one measure of thyroid function in healthy humans. The review revealed that soy and its isoflavones, a class of phytoestrogens, present little evidence of adversely affecting thyroid function. However, the study was conducted by Mark Messina, who, in addition to being a Loma Linda University professor and internationally recognized expert on the health effects of soy, is also a consultant for companies that manufacture and/or sell soy-based foods.
An August 2018 review in Archives of Toxicology investigated clinical studies and found that isoflavones didn't have an effect on breast cancer risk or the thyroid hormone system in healthy women. However, the study cautions that women with breast cancer or a history of breast cancer as well as people with an iodine deficiency (especially during pregnancy), hypothyroidism, and/or thyroid dysfunction should limit their intake of soy.
But researchers are still searching for connections.
For example, a February 2011 in the Journal of Clinical Endocrinology and Metabolism connected phytoestrogen with thyroid function. Sixty people with subclinical hypothyroidism (mild thyroid failure) were randomly assigned either a typical Western dose of phytoestrogen (consisting of 2 milligrams) or a dose that coincides with a vegetarian diet (16 milligrams).
Six participants, or 10 percent, progressed from subclinical to overt hypothyroidism after consuming the higher dose; no one on the lower dose had further development of thyroid issues. The study also found a significant reduction in systolic and diastolic blood pressure, insulin resistance and inflammation with the higher dosage. This study adds weight to Patisaul's recommendation that moderation is important for people who choose to consume soy.
Is Soy Connected With Breast Cancer?
Soy is not a known cause of breast cancer. In fact, the American Cancer Society calls soy "an excellent source of protein and a good alternative to meat." But estrogen plays a major role in breast cancer, which is why researchers are interested in revealing how soy and its phytoestrogens affect breast cancer survivors.
A September 2001 study published in the Annals of Pharmacotherapy found that low concentrations of genistein and daidzein, the major phytoestrogens in soy, stimulated breast tumor growth in animals. Genistein and daidzein also antagonized the antitumor effect of the breast cancer drug tamoxifen in the lab. The researchers concluded that genistein and daidzein may stimulate existing breast tumor growth and that women with current or past breast cancer "should be aware of the risks of potential tumor growth when taking soy products."
Some people might read that and say, "OK, well, soy must be bad for you." But contrary to that animal study, a body of research on humans says soy might help prevent breast cancer and reduce its recurrence, or it might have no effect at all.
A September 2002 study in Carcinogenesis suggests that eating soy foods during adolescence and adult life is associated with a significantly reduced risk of breast cancer in Asian-American women.
The American Cancer Society says: "There is growing evidence that eating traditional soy foods such as tofu may lower the risk of cancers of the breast, prostate, or endometrium, and there is some evidence it may lower the risk of certain other cancers. Whether this applies to foods that contain soy protein isolates or textured vegetable protein derived from soy is not known. There is little data to support the use of supplements of isolated soy phytochemicals for reducing cancer risk."
Vanderbilt University professor of cancer research Xiao Ou Shu has studied the link between soy and breast cancer prognosis. Shu co-authored a December 2009 study in JAMA that found that soy food does not harm breast cancer survivors and may be associated with a reduced risk of recurrence and mortality.
What About Soy and Men?
An August 2010 meta-analysis published in Fertility and Sterility found that neither soy foods nor isoflavone supplements alter measures of testosterone concentrations. A July 2013 study published in PLoS One concluded that short-term intake of soy isoflavones does not affect serum hormone levels, total cholesterol or prostate-specific antigen in men with prostate cancer.
And What About Soy Allergies?
Soy induces an allergic reaction in a small portion of the population. Some babies, for example, are allergic to soy-based formula. Most children lose their allergy as they age, but soy allergies also can spring up during adulthood.
There is good news for soy eaters with soy allergies: Fermented soy may cause drastically fewer allergic reactions. In 2008, University of Illinois researchers revealed that fermented soy reduces potential allergenicity and increases essential amino acids.
Read more: The Dangers of Expired Soy Products
The Trouble With Highly Processed Soy, aka Soy Protein Isolate
Sales of soy foods in the United States crossed the $36 billion mark in 2017 and is predicted to climb to nearly $57 billion by 2025. Today, processed soy protein (in the form of soy protein isolate) is used in protein bars, breakfast cereals, salad dressings, soups, cheeses, nondairy creamers, whipped toppings and infant formulas.
Some doctors and nutritionists — among them Caldwell Esselstyn Jr., MD, of the Cleveland Clinic; Dean Ornish, MD; Andrew Weil, MD; Mark Hyman, MD; and Ashley Koff, a registered dietitian and co-author of "Mom Energy: A Simple Plan To Live Fully Charged" — recommend avoiding foods containing soy protein isolate, including fake meats, soy cheeses and protein bars. Dr. Weil, Dr. Hyman and Koff have spoken out to say that these highly processed soy products do not have the health benefits of whole natural soy. Koff has expressed concern that soy protein isolate may disrupt hormones.
Dr. Hyman urges his website readers to "say no to processed soy products" because "they don't have the thousands of years of traditional use that whole soy foods do, they're processed, and they contain unhealthy fats and other compounds."
Almost All Soy Is Genetically Modified (GMO)
You've probably heard that most soy in the United States is genetically modified, and it's true. Soybeans were among the first FDA-approved genetically modified organisms during the mid-1990s. By 2012, 93 percent of soy grown in the United States was genetically modified, according to the USDA.
Some people worry that genetically modified soy is unhealthy, citing Russian research that claims it causes infertility in hamsters. That particular piece of research was unpublished, meaning it was not exposed to a standard peer-review process. The organizations that conducted the research are not well-known. Articles and blog posts that cited the study failed to provide a link to the study itself. In conclusion: That research failed to meet the minimum standards of accountability and should not be taken seriously.
There might not be hard proof that genetically modified soy is bad, but are all GMOs unhealthy? The answer is unclear at this point. What we do know with certainty is that GMOs are highly controversial, with some polls finding that more than half of the American public believes GMOs are unsafe and many countries banning them outright. A nationwide June 2013 ABC News phone poll of 1,024 American adults found that 57 percent said they are less likely to buy GMO foods while only five percent said they would be more likely to buy a food labeled as genetically modified.
At least 44 countries — including the U.S., Australia, Italy, France, Germany, Mexico, Russia and Switzerland — had total or partial bans on GMOs.
However, major scientific bodies, such as the World Health Organization, believe that GMOs currently on the market are not likely to present any risk to humans, but also states that it is not possible to make general statements on the safety of all GM foods.
Are There Any Benefits to Eating Soy?
Whole soy (such as steamed edamame beans) and fermented soy (in the form of miso, tempeh and some types of tofu) have long been staples of Asian diets. It is hailed as a complete plant protein that contains all the essential amino acids, which are necessary for carrying out life functions and building and maintaining lean muscle.
Soy has also been linked to major heart health benefits. A June 2019 meta-analysis of 46 trials led by University of Toronto researchers, published in the Journal of the American Heart Association, found that soy reduces total cholesterol and LDL cholesterol (the dangerous kind), supporting previous FDA statements that soy is heart healthy.
So, Is Soy Bad for You?
Are there problems with soy? Yes, and the ones we know about are associated with the processed versions. Soybeans are massively subsidized, which means it's cheaper for farmers to produce soy compared to other foods. So that's one problem: Potentially healthier foods are being crowded out of the market by Big Soybean.
The other problem is that when food is cheap, food manufacturers stick it in everything. Highly processed soy is stripped of its nutritional value, and it winds up in manufactured foods that often are less healthy than whole foods.
If you decide to consume soy after weighing the pros and cons, choose whole soy (such as steamed edamame) or fermented soy (such as miso or tempeh), and minimize your intake of highly processed soy contained in many packaged energy bars, protein powders, meat alternatives, soy cheeses and soy ice creams. Be sure to check the labels.
If you are looking to avoid GMOs, choose organic soy. Organic products, by law, must be GMO-free. Also, you may want to heed Patisaul's recommendation about moderation with regard to your soy consumption.
- Frontiers in Neuroendocrinology: "The Pros and Cons of Phytoestrogens"
- Thyroid: "Effects of Soy Protein and Soybean Isoflavones on Thyroid Function in Healthy Adults and Hypothyroid Patients: a Review of the Relevant Literature"
- Archives of Toxicology: "Effects of Isoflavones on Breast Tissue and the Thyroid Hormone System in Humans: a Comprehensive Safety Evaluation"
- Journal of Clinical Endocrinology and Metabolism: "The Effect of Soy Phytoestrogen Supplementation on Thyroid Status and Cardiovascular Risk Markers in Patients With Subclinical Hypothyroidism: a Randomized, Double-Blind, Crossover Study"
- American Cancer Society: "Common Questions About Diet and Cancer"
- Annals of Pharmacotherapy: "Effects of Soy Phytoestrogens Genistein and Daidzein on Breast Cancer Growth"
- Carcinogenesis: "Adolescent and Adult Soy Intake and Risk of Breast Cancer in Asian-Americans"
- JAMA: "Soy Food Intake and Breast Cancer Survival"
- Fertility and Sterility: "Clinical Studies Show No Effects of Soy Protein or Isoflavones on Reproductive Hormones in Men: Results of a Meta-Analysis"
- PLoS One: "Short-Term Soy Isoflavone Intervention in Patients with Localized Prostate Cancer: A Randomized, Double-Blind, Placebo-Controlled Trial"
- Illinois Aces: "U of I Scientists Aim to Overcome Allergic Reactions to Soy"
- USDA: "Genetically Engineered Crops in the United States"
- Semantic Scholar: "All Around the (Genetically-Modified) Mulberry Bush: Information-seeking and Consumer Preferences for Genetically Modified Food Labeling in Vermont"
- GMO-FREE Regions: "GMO-Free Regions By Country"
- WHO: "Frequently Asked Questions on Genetically Modified Foods"
- Journal of the American Heart Association: "Cumulative Meta‐Analysis of the Soy Effect Over Time"
- Frontiers in Neuroendocrinology: The Pros and Cons of Phytoestrogen
- Thyroid: Effects of Soy Protein and Soybean Isoflavones on Thyroid Function in Healthy Adults and Hypothyroid Patients: A Review of the Relevant Literature
- Genistein aglycone does not affect thyroid function: results from a three-year, randomized, double-blind, placebo-controlled trial.
- Jouranl of the American College of Nutrition: The Symbolization of Eggs in American Culture: A Sociologic Analysis
- American Cancer Society
- Effects of soy phytoestrogens genistein and daidzein on breast cancer growth.
- Soy Isoflavones and Breast Cancer
- Soy isoflavones consumption and risk of breast cancer incidence or recurrence: a meta-analysis of prospective studies.
- Soy food intake after diagnosis of breast cancer and survival: an in-depth analysis of combined evidence from cohort studies of US and Chinese women.
- The role of early life genistein exposures in modifying breast cancer risk.