Here's a look at the controversial history of soy -- and what the science now says about it.
1940s: The root of the soy dispute
Soy contains a naturally occurring estrogenic compound called isoflavone
, which seems to have stirred a storm of confusion around how soy impacts public health.
"Isoflavones are considered phytoestrogens. In other words, plant estrogens," said Dr. Omer Kucuk, a medical oncologist at Emory University's Winship Cancer Institute, who has studied the benefits of soy isoflavones.
Isoflavones are a part of a larger group of compounds called flavonoids
, and "there are many, many flavonoids in nature," Kucuk said.
Yet evidence of isoflavones possibly having harmful effects appeared in the 1940s, spurring one of soy's first controversies.
"The breeding problem appears to be of purely nutritional origin," the authors of the study wrote. "A further possibility is the presence in the clover of some substance which potentiates the animal's natural estrogen or abnormally stimulates its production."
The infertility may have resulted from a hormone imbalance, triggered by the herbage
that the sheep ate, according to the study. Still, this was found only in sheep and not in humans.
Soon enough, researchers would conduct more studies on these estrogenic effects in various farm animals, and the animal feed industry would develop an interest in soy.
1950s: Soy used in animal feed
About 80% of the soy currently produced in the United States is used as animal feed, Kucuk said. Soy is a billion-dollar industry that spans continents and feeds millions of livestock worldwide, according to the World Wide Fund for Nature
, but it wasn't always that way.
In the 1950s, the soy industry was just beginning to explore how soy could be used in animal feed and how isoflavones could affect animals.
By 1959, soybean oil meal
was described as "an excellent source of growth promoting" for young farm animals, such as turkeys.
In the decades to come, soy studies would shift away from soy's role in farm animal diets and toward its role in human diets.
1960s: Soybean industry grows in the US
"Soy consumption is not an uncommon thing. More than 2 billion -- that's with a 'b' -- 2 billion people consume soy every day," Kucuk said.
"If you put China, Japan and Korea and southeast Asia together, you have more than 2 billion people, and these people consume 20 to 30 times more soy than the average American every single day of their lives," he said. "It's part of their diet, and this has been going not just for hundreds of years. It has been going on for thousands of years."
With a long history in Asia, soy slowly emerged as a common food source in the US in the 1960s. During this time, states formed soybean industry groups affiliated with the American Soybean Association
, which was funding research to find uses for soybeans and ways to reduce production costs.
1970s: The rise of soy in the American diet
By the early 1970s, more studies shed light on the use of soy proteins in baked foods
and the functional properties of soy proteins
The American Soybean Association established its headquarters in St. Louis
Around that time and later, contradicting studies about the potential health benefits of soy arose, creating confusion.
1980s: Setting the record straight in animal, human studies
Two animal studies -- one on monkeys published in 1986
and another on rats published in 1987
-- suggested that soy diets caused an enlarged pancreas and were associated with the growth of pancreatic cancer in those animals.
In response to those studies and others, the National Cancer Institute's Division of Cancer Etiology organized a workshop to discuss the state of research on soy in relation to cancer risk.
After the workshop, participants published a report in the journal Cancer Research in 1989
(PDF). They wrote that there was no evidence that soybean-derived foods had adverse effects on the human pancreas.
Rather, it was observed that human populations with high levels of soy in the diet had decreased rates of pancreatic cancer
In the years to come, cancer researchers would tout the potential cancer prevention benefits of soy foods.
1990s: The breast cancer, soy connection explained
Among the first of many studies to suggest that soybeans contain potentially anti-carcinogenic benefits
, providing something of a protective effect against cancer
, was a 1991 study published in the Journal of the American Dietetic Association.
Things changed in 1996. That was the year a pilot study, published in the journal Cancer Epidemiology, Biomarkers, & Prevention
, suggested that consuming soy protein might actually stimulate the growth of breast cancer cells.
This confusion about whether soy is good or bad for cancer risk stems from the fact that soy isoflavones can mimic estrogen in the body and can bind to estrogen receptors, said Kucuk, the medical oncologist.
"Some people naively thought, 'Well, since they're estrogenic, they must be bad, because estrogen causes breast cancer.' We all know that in women, estrogen levels are associated with a higher risk of breast cancer," he said.
"There are two estrogen receptors (in the human body): alpha and beta. Alpha is the bad one. That's the one where, if something binds to alpha, it may increase the risk of breast cancer, because it makes breast cells grow. But beta, on the other hand, it causes the opposite effect," he said. "Soy isoflavones bind preferentially to estrogen receptor beta."
Even though soy isoflavones bind to both estrogen receptors, they preferentially bind to the good estrogen receptor
, Kucuk said.
Kucuk added that more studies would indicate that women in regions where soy is primarily consumed, such as Japan and China, tend to experience lower cancer rates
than women who consume Western diets.
However, those studies wouldn't end the soy and cancer debate -- which continued on into the 2000s.
2000s: More studies on soy and your health
Another study, published in the journal Cancer Research
in 2001, suggested that soy isoflavones
stimulated the growth of estrogen-dependent human breast cancer cells.
Then, a 2007 review paper published in the journal Cancer
suggested an apparent lack of association between soy and breast cancer. Rather, the paper suggested that avoiding weight gain and limiting the consumption of alcohol reduces the risk of breast cancer.
But there were other benefits too, including that replacing junk food in your diet with soy can aid in avoiding weight gain, according to research published in 2004
and in 2009
Around that time, soy protein and isoflavones also gained attention for having a potential role in improving cardiovascular health
An American Heart Association paper
published in the journal Circulation in 2006 found, "soy products should be beneficial to cardiovascular and overall health because of their high levels of polyunsaturated fats, fiber, vitamins and minerals and low levels of saturated fat."
In 2008, the AHA then stated that there was not enough evidence to claim a strong link between soy and reduced risk of coronary heart disease
"Soy has the antioxidant properties that can lower our LDL
, so basically the bad cholesterol," said Jenna Stangland, a registered dietitian at Twin Cities Orthopedics in the Minneapolis area.
"It doesn't necessarily increase our HDL, or our good cholesterol," she said. "But we know that it is able to lower that bad cholesterol, and it prevents the bad cholesterol from being oxidized. When bad cholesterol gets oxidized, it then ends up clogging our arteries."
In the mid-2000s, research offered more information on how soy intake may impact thyroid function
The evidence suggested that soy can interfere with the body's ability to absorb a synthetic thyroid hormone often used to treat hypothyroidism. Generally, it is recommended to wait four hours before consuming any soy products after taking thyroid medication, according to the Mayo Clinic
Additionally, some studies have suggested that eating soy foods can help reduce certain menopause symptoms
such as hot flashes. But others have suggested otherwise
A review paper in the journal Menopause
, released in October 2010, called for more clinical studies to evaluate the role of soy isoflavones in menopausal health.
"There are mixed results of the effects on midlife women," the authors wrote about soy.
Meanwhile, as the types of soy foods being consumed by Americans continued to expand and diversify, experts noticed that how soy was being eaten impacted how beneficial it could be for your health.
2010s: Natural, processed and fermented soy
"Where we stand is it being a beneficial protein and healthy fat source, with more research showing the benefits of a plant-based diet," Stangland, the registered dietitian, said of soy.
"Twenty-five grams a day
is a good estimate or a good guideline to follow in trying to incorporate it into what we are eating," she said.
Yet she added that while natural soy food products offer health benefits, processed soy foods provide a different story.
"I think soy milk is one that would be OK, and tofu and tempeh and edamame. Again, those being more on the natural side," Stangland said.
"As we go into a lot of vegan and vegetarian products, like when they use that soy to make a meat patty and it's not just soybeans that are in there but it's also additional soy product to make it into a meat patty, then that's where I would say it's more processed," she said. "Any of the bars and the powders out there that are manipulated are not even giving us the same benefits that we are finding in the research."
In other words, the health boost you would get from soy foods might depend on how the soy was prepared, said Dr. Cate Shanahan, a Newtown, Connecticut-based family physician who also consults as a nutritionist with the Los Angeles Lakers.
"In Asian countries, much of the time, they are using soy in a traditionally fermented, cultured dish, which to some degree has a different nutritional profile," said Shanahan, author of the book "Deep Nutrition: Why Your Genes Need Traditional Food."
"Fermenting just means allowing microbes to work on the food to grow in it. ... Microbes make amino acids that we need; they make a variety of fatty acids; they also make a lot of vitamins," she said. "So, when you have a fermented soy product, you're getting more of those nutrients in massive quantities than the non-fermented product."
Now, as Americans are gaining a better understanding of soy, uncertainty over the cancer and soy connection finally seems to have been settled in new research.
2017: 'This study puts that argument to rest'
Soy may not pose a risk for women with breast cancer after all, according to a study published in the journal Cancer
On the contrary, it actually might be associated with a reduced risk of death over a nine-year period in some women, said Dr. Fang Fang Zhang, a cancer epidemiologist and assistant professor at Tufts University's Friedman School of Nutrition Science and Policy.
"Results from our study and those from others are consistent that soy food consumption in women with breast cancer does not have a detrimental effect on prognosis or survival," said Zhang, lead author of the new study.
The study involved data on 6,235 American and Canadian women with breast cancer, including their diets, and cancer survival and death rates, between 1995 and 2015. The data came from the Breast Cancer Family Registry
, an international database funded by the National Cancer Institute.
Zhang and her colleagues analyzed the data, taking a close look at each woman's diet and survival outcome, which were tracked over 113-month (or about 9.4-year) follow-up periods in the data.
The researchers controlled for other factors that might influence death rates, such as socioeconomic status, exercise, weight and habits such as smoking or drinking alcohol.
Then, the researchers sorted the women into four groups based on the amount of isoflavones they consumed through soy foods.
The researchers found that the women in the highest quartile -- who consumed the most isoflavones, about a half to one serving a week -- were 21% less likely to die compared with the lowest quartile over the nine-year period in which mortality data were measured.
"I would say this study is probably the strongest one that we have right now in North American women, showing that soy consumption in breast cancer patients is not only safe but also beneficial," Emory's Kucuk said.
"Previous studies were in Asian women in China, Japan and Korea," Kucuk said. "And one of the things that people criticizing soy will say is that, 'Oh, well, soy may prevent breast cancer in China and Japan because they eat it all their lives, but in the US, it may not prevent it because US women don't start eating it as a child, they may start it later, and this may not be beneficial.' Well, this study puts that argument to rest."
Kucuk added that more research is needed to investigate the impact of soy foods on public health.
"Where there is a great need is more clinical studies looking at both prevention of cancer, not just breast cancer but other cancers as well, and also one area that soy isoflavones can be beneficial is preventing the side effects of the cancer treatments we use, like chemotherapy, radiation therapy, hormone therapy," Kucuk said.
In his editorial, Kucuk noted that most Americans don't consume appreciable levels of soy foods, even though the US is the top soy producer in the world, producing about 40% of the global soy supply.
"Imagine in areas of low socioeconomic status where a lot of women have a high risk of breast cancer, especially African-American women, and, for example, there could be some large public health type of studies where people can be given vouchers to obtain soy milk or have soy products given to them at a discount or some kind of health policy," Kucuk said.
"Imagine, this could result in huge health care savings," he said. "So far, we know that soy foods are good, soy foods are safe, soy foods prevent breast cancer, and also improve treatment results and decrease mortality in breast cancer patients."