Soy: The Pros and Cons

The Truth About Soy

Is soy a disease-preventing miracle food or a dangerous health threat disguised as diet-friendly protein? Here are both sides of the story, and why there's no clear picture of soy's health pros and cons.

By Annie Hauser, Senior Editor

The U.S. Food and Drug Administration announced in 1999 that diets including daily soy resulted in a significantly reduced risk of heart disease, the No. 1 cause of death in the United States, then and now.

With the announcement, soy officially became a darling of the health-food world and a cornerstone of American diets.

Fast-forward 13 years, and soy products are still widely seen as healthy alternatives to their animal counterparts. Soy protein saturates supermarket shelves — sneaking into everything from cereal to protein bars to crackers to frozen meals and even surprising places like some brands of canned tuna — not to mention the more recognizable forms of soy, including edamame, tofu, soy milk, soy cheeses, and soy meats.

But as soy's popularity wages on, some notable nutritionists and doctors now warn against consuming soy in any form, including the processed and packaged foods that contain types of soy protein.

Isoflavones: Soy's Potentially Dangerous Compound

The reason is soy's isoflavones, a naturally occurring plant compound that is present is all forms of soy, including organic soy, which is a type of phytoestrogen. Phytoestrogen is produced by plants and is slightly different from the estrogen hormone produced by the human body. But in excess, it can create some of the devastating effects of estrogen overload in both men and women. Although soy isn't the only source of isoflavones in the modern diet (all legumes have them), soy is the most concentrated, says Amy Gross MPH, RD, CDN of the NewYork-Presbyterian Hospital Weill Cornell Medical Center in New York City. To make things more confusing, while isoflavones harm some people, they have been found to be beneficial in others. 

"There are different theories as to why these things may affect us differently, or exacerbate one thing and improve another in our bodies," Gross says. "It could potentially bind to estrogen receptors in different cells in our body. It could act similarly to estrogen or take over where estrogen would usually take over when looking at that receptor itself that would be on that individual cell. There is a lot of research that tells different stories."

Soy's Health Benefits and Risks: Why the Jury Is Out

"Soy is very heavily marketed as a healthy food," says Kaayla Daniel, PhD, a nutritionist and author of the book The Dark Side of Soy, "but the fact is, we've got hundreds of studies that link soy consumption to malnutrition, ADHD, immune system, reproductive health problems, digestive disorders, certain types of cancers, and more." It is also important to note that non-genetically modified soy and organic soy has been linked to these health conditions as well, Daniel says. Israel and Germany have issued warnings against the consumption of soy foods for young children and infants. New Zealand also warns that soy formula for infants should only be used as a last resort. To date, these three countries are the only ones to come out forcefully against soy consumption.

Still, some studies indicates that soy in moderation, particularly tempeh, miso, and other soy sources with relatively low levels of isoflavones, can be beneficial — though many of these pro-soy studies have been funded by soy lobbying groups, such as the Soybean Board.

Here's a look at some of the conditions soy can affect. 

• Thyroid function. Hypothyroidism is often cited as the greatest risk of soy consumption: Just 30 grams of soybeans a day for 30 days produced thyroid issues in healthy adult men and women, a landmark study at Aichi Medical University in Japan found. "Soy is one of the many factors that puts the thyroid at risk," Daniel says, "and we know from many studies that just one glass of soy milk per day is sufficient to cause thyroid problems." 

• Allergies. The cause of itching, inflammation, hives, and even deadly anaphylaxis, soy is one of the top eight allergens in the food supply, the FDA reports. Because of these concerns and rising rates of soy allergies, the FDA began requiring food producers to print a "Contains: Soy" label on any food with any type of soy. "Because there's soy in just about everything that’s processed, people with soy allergies are really going to have to prepare foods from scratch — a blessing in disguise for overall health," Daniel says.

• Reproductive risks. Several studies, including one recently conducted at the University of Geneva Medical School, have linked the isoflavones in soy to a decreased sperm count and sexual side effects in mice. Soy-and-mice studies are numerous and controversial because critics argue that mice studies do not have a direct human application and that the mice studied are given much more concentrated doses of soy than any human would eat. "It’s true that mice and humans are different, but if soy adversely affect animals, it’s obvious that there’s going to be some effect on humans as well," Daniel counters.

• Cancer risk. Cancer and soy is one of the most hot-button health issues today. Some studies, including one that tracked almost 100,000 California women for eight years, have found that soy consumption decreases risk for ovarian or breast cancers. A second study that tracked 64,000 Japanese women for 15 years linked soybean curd, specifically tofu, consumption to a decreased mortality rate from ovarian cancer.
  
  On the flip side, multiple studies have linked soy consumption to breast cancer recurrence, meaning if a woman already has breast cancer, it's likely to come back if she eats a soy-heavy diet. Mark Messina, MD, who is an advisor to the U.S. Soybean Board, says studies have linked soy consumption to a decreased risk of prostate cancer, however a recent Swedish study concluded the opposite.

• Heart health and weight management. If there's one thing those on both sides of the soy debate can agree on, it's that soy products are an excellent source of lean protein. "If you’re choosing soy instead of a higher fat meat source as your protein, you will have less calories and less saturated fat, so it will be more of a heart-healthy choice," Gross says. "If you're doing it correctly, eating lower-calorie soy products instead of animal products could help with managing your weight." Studies have also found that regular soy consumption can lower bad LDL cholesterol.

• Menopause. In some women, soy's isoflavones improve symptoms of menopause, such as hot flashes. In others, soy only exacerbates symptoms, Daniel says."That’s why we need to respect soy as actually being a powerful medicine that is sometimes appropriate and many times not appropriate," she explains. Unfortunately, the only real way to know how it will affect you is to try it, Daniel says, because there's no test to determine soy's affect on individuals.

Should You Eat Soy?

Soy, like all foods, has pros and cons, so like all foods, the take-home message is to consume it in moderation. Gross recommends eating one to two servings a day, max, as part of a balanced diet, which includes plenty of fruits and vegetables, whole grains, low- or nonfat dairy, and other sources of lean protein.  Because soy often hides in processed foods under aliases such as "textured vegetable protein," it's important to read nutrition labels on anything you buy to make sure it doesn't have hidden soy along with other ingredients.

"Soy is in so many things you might not expect," Daniel warns. "Just about everything that comes in a package. If you’re eating a lot of processed and packaged foods, you're also eating additives and flavorings, high-fructose corn syrup, and who knows what else, so it's not just the soy that's the problem in those products."

Instead, Daniel and Gross both recommend preparing whole foods at home that you can make from scratch, and avoiding processed foods whenever possible for overall health.

"I think that it’s entirely appropriate to include soy in your diet," Gross says. "But I wouldn’t have it be everything that you’re eating. We need a variety of foods, and if you're focusing on one food, you might be missing out."

Soy Milk vs. Regular Cow's Milk

TELL US: Are you for or against soy? Sound off in the comments.

For more nutrition news, follow @weightloss on Twitter from the editors of @EverydayHealth.