A walk into any restaurant, coffee shop or grocery store confirms that there are few foods as versatile as soy. We can sub soy milk in a morning latte, toss tofu in a stir fry for dinner, throw a veggie burger on the grill or enjoy edamame while waiting on sushi. Soy’s presence in our diets has grown over the last several decades due to scientific research recognizing its potential health benefits and shifts in consumer demand toward more plant-based diets.
Despite its popularity – or maybe because of it – understanding the health effects of soy has become increasingly complicated. As one of the most researched foods in nutrition science, studies on soy are regularly published on topics ranging from its effects on cardiovascular health to how it impacts our gut microbiome. Interpreting the extensive amount of soy research can be a challenge, so let’s walk through it together.
What’s in a Bean?
Soybeans are technically classified as legumes, a group of plants whose seeds grow in enclosed pods, like peas or peanuts. However, the nutritional profile of a soybean is much different than most others in the legume family: soybeans are much higher in protein and fat and lower in carbohydrate. Unlike other plant-based protein sources, soy is considered a complete protein since it contains each of the essential amino acids our body needs for cell metabolism, building and repairing tissues and providing energy. Soy protein is found in whole soybean products such as tofu, edamame, miso and tempeh. It’s also present in soybean extracts like soy protein isolate, soy protein concentrate or soy protein flours. These extracts are used to make foods like dairy alternatives (e.g., soy milks, yogurts and cheeses), soy-based infant formulas, gluten-free breads and meat alternatives like veggie burgers.
In addition to its high protein content, soy is a source of fiber, polyunsaturated fats, vitamins and minerals. It’s also a plant source of omega-3 fatty acids, which are known for their cardioprotective effects. Compared to other legumes, soybeans contain more calcium, iron, magnesium, and potassium, all of which were identified as under-consumed nutrients in the American diet.
Soy foods are relied upon by people following plant-based diets, not only for their health benefits but also for ethical and environmental reasons. Depending on whether you follow a US, Mediterranean or Vegetarian eating pattern, the Dietary Guidelines for Americans recommend eating between 5 and 14 ounces of nuts, seeds, and soy products every week as part of a 2,000 calorie/day meal plan. It’s recommended that vegetarians and vegans consume more of these foods, since they’re key sources of protein in plant-based diets.
Soy Research Through the Years
While soy has been consumed for centuries in many Asian countries, it wasn’t until the 1960s that it began to emerge as a common food source in the American diet. Research on its health benefits took off in the 1970s and 80s, with particular interest in its association with cardiovascular disease sharpening in the 1990s. In 1999, the Food and Drug Administration (FDA) approved an authorized health claim touting research to support the relationship between soy consumption and reduced risk of coronary heart disease (CHD), and manufacturers of soy products meeting certain criteria could print the phrase, “25 grams of soy protein a day, as part of a diet low in saturated fat and cholesterol, may reduce the risk of heart disease” on packaging. Studies on soy and cardiovascular disease continued, and many showed more moderate effects than those used as evidence to support the FDA health claim. In 2006 the American Heart Association (AHA) published a paper concluding that, “many soy products should be beneficial to cardiovascular and overall health because of their high content of polyunsaturated fats, fiber, vitamins, and minerals and low content of saturated fat”, while at the same time questioning the strength of the evidence supporting a strong link between soy and decreased low-density lipoprotein (LDL) cholesterol. This prompted a review of the literature by the FDA, which was completed in 2017.
What’s the Latest Research on Soy and Health?
Thousands of studies have been conducted in hopes of determining soy’s impact on human health, sometimes with contrasting results. Much of the variation in soy research is a result of basic differences in the ways that research studies are designed. For example, epidemiologic studies have shown stronger associations between soy and health than experiments like randomized controlled trials, which could be attributed to a few different factors. In observational studies most soy intake taken into consideration is in the form of traditional soy foods, like tofu, miso, tempeh and soy milk. On the other hand, quite a bit of soy research has focused on specific components like soy protein isolate or isoflavones, which are a class of polyphenols, compounds commonly recognized as antioxidants. We’ve learned from decades of nutrition research that isolating a nutrient from its natural form can change its effects on the body, and soy is no exception.
Aside from differences in chemical composition of soy products, how long someone eats soy (lifelong intake versus just a few weeks), the average amount eaten, inconsistencies in study populations (men vs. women or healthy vs. diagnosed health conditions, for example) and genetic variation also bring layers of complexity to soy’s impact on health. The body of literature supports the safety of soy consumption in moderation. Nevertheless, associations between soy and certain health conditions have come to the forefront and remain hotly pursued areas of research.
More than 1 in 3 U.S. adults has higher-than-recommended levels of LDL cholesterol, a known risk factor for coronary heart disease. Many studies have demonstrated that consuming soy protein lowers LDL cholesterol by about 4-6%. While this is lower than initial estimates of an almost 13% reduction in LDL cholesterol, it’s important – for every 1% reduction in LDL cholesterol, there is a corresponding 1-2% reduction in cardiovascular events like heart attacks and strokes. A meta-analysis conducted by Health Canada in 2015 showed a statistically significant reduction in total and LDL cholesterol levels with soy protein consumption and no unfavorable effect on HDL cholesterol and triglyceride levels. This effect has proven especially beneficial in individuals with high LDL cholesterol (usually considered to be >130 mg/dL) as compared to people with normal levels and in people eating higher-fat and higher-cholesterol diets. Research has also demonstrated the capability of soy protein and soy isoflavones to lower blood pressure, especially in people with high blood pressure. It remains unclear if soy’s health benefits specifically arise from compounds in soy foods or if the benefits are due to replacing foods high in saturated fat and cholesterol (like red and processed meats) with soy foods.
Short-term clinical studies have found that soy isoflavones favorably affect bone turnover and/or bone mineral density in postmenopausal women, a group particularly at risk for bone fractures and osteoporosis. The calcium provided by many soy foods may also contribute to bone health.
Effects on the Gut Microbiome
The microbes living in our gastrointestinal tract, known as the gut microbiome, are becoming established as critical modulators of human health. Dietary components influence the composition of the gut microbiome and there are four major components in soy which impact the microbiota from a prebiotic standpoint: fiber, oligosaccharides, isoflavones and protein. While the body of research is still developing in this area, animal and human studies have demonstrated that eating soy can increase the abundance of beneficial bacteria like bifidobacteria and lactobacilli. Soy may also modify the ratio between Firmicutes and Bacteroides, the main types of bacteria found in the gut, in a direction thought to confer health benefits.
Reading through even the first page of a Google search is proof of the many confusing messages about soy-based foods. Evaluating the evidence behind any research study is a great first step in weeding out poorly conducted science and the stories that report on it, but there are a few soy-specific controversies that we’ll clear up here.
First up: soy’s effects on cancer. There is a perennial fear that soy may raise breast cancer risk, despite a strengthening body of research that concludes the opposite. Much of this links back to the isoflavone content of soy and its weakly estrogenic effects. Yet, according to the American Cancer Society, consuming soy isoflavones may actually lower cancer risk. In fact, a recent study of over 6,000 women with breast cancer found a reduction in mortality risk in women with the highest intake of soy compared to those with the lowest intake. These results mirror similar studies in Asian populations, who tend to eat more soy than Western countries like the U.S. and Canada.
In 2011 the American Institute for Cancer Research concluded that diets high in dietary fiber, including fiber found in soybeans, soy nuts and edamame, lower risk of colorectal cancer. Evidence also suggests that soy may be protective against endometrial and prostate cancer. Overall, human studies show that soy does not increase cancer risk. In some cases, research has shown that it may actually lower it.
Other common topics of discussion include concerns that soy’s isoflavones can lead to male feminization (they don’t, this is pure urban legend) or that it may be beneficial for hot flashes and other symptoms of menopause (more research is needed). Evidence about soy’s effects on many other health conditions is still inconclusive.
While research on soy is ongoing, a few things are clear: research has shown soy to confer a variety of health benefits. Soy is a high-protein alternative for those looking to incorporate more plant-based foods and healthy fats into their diet. It’s why the 2015-2020 Dietary Guidelines for Americans assert that a healthy eating pattern can include a multitude of soy products.