Soybeans are a type of legume native to Asia.
Soy has been part of traditional Asian diets for thousands of years. In fact, there’s evidence that soybeans were grown in China as early as 9,000 B.C. (
Today, soy is widely consumed, not only as a source of plant-based protein but also as an ingredient in many processed foods.
However, soy remains a controversial food — some praise its health benefits, while others claim it could be bad for you.
This article examines the evidence for and against eating soy.
Soybeans are a type of legume that can be eaten whole or processed into a variety of forms.
Whole soy products
While mature soybeans are rarely eaten whole in the Western diet, edamame is a favorite high-protein appetizer in Asian cuisines.
Soy milk is made by soaking and grinding whole soybeans, boiling them in water, and then filtering out the solids. People who cannot tolerate dairy or wish to avoid milk commonly use it as a milk alternative.
Tofu is made by coagulating soy milk and pressing the curds into blocks. It’s a common source of plant-based protein in vegetarian diets.
Soy sauce is a liquid condiment made from:
- fermented soy
- roasted grains
- salt water
- a type of mold
Tempeh is a fermented soy cake that originated in Indonesia. Though not as popular as tofu, it’s also commonly eaten as a source of protein in vegetarian diets.
Miso is a traditional Japanese seasoning paste made from:
- a type of fungus
Soy-based processed foods
Soy is used to make several processed foods, including:
- vegetarian and vegan meat substitutes
Many packaged foods contain soy flours, texturized vegetable protein, and soybean oil.
Soy protein isolate is a highly processed derivative of soy made by grinding soybeans into flakes and extracting the oil.
The flakes are then mixed with alcohol or alkaline water, heated, and the resulting soy concentrate is spray-dried into a powder (
Soy protein isolate is available in many protein powders and also added to many processed foods, such as protein bars and shakes.
Other soy supplements include soy isoflavones, which are available in capsule form, and soy lecithin, which can be taken in capsules or as a powder.
Soy includes a wide variety of foods, including edamame, products made from whole soybeans, fermented soy foods, more processed soy-based foods, as well as supplements.
Soy foods are a good source of many important nutrients.
For example, 1 cup (155 grams) of edamame contains (4):
- Calories: 189
- Carbs: 11.5 grams
- Protein: 16.9 grams
- Fat: 8.1 grams
- Fiber: 8.1 grams
- Vitamin C: 16% of the Reference Daily Intake (RDI)
- Vitamin K: 52% of the RDI
- Thiamine: 21% of the RDI
- Riboflavin: 14% of the RDI
- Folate: 121% of the RDI
- Iron: 20% of the RDI
- Magnesium: 25% of the RDI
- Phosphorus: 26% of the RDI
- Potassium: 19% of the RDI
- Zinc: 14% of the RDI
- Manganese: 79% of the RDI
- Copper: 19% of the RDI
Soy is high in plant-based protein and a good source of many nutrients and phytochemicals.
The unique phytochemicals in soy may offer several health advantages.
May help lower cholesterol
Several studies suggest that soy may improve cholesterol levels, especially LDL (bad) cholesterol.
In an extensive review of 35 studies, researchers found that eating soy products reduced LDL (bad) cholesterol and total cholesterol while raising HDL (good) cholesterol.
These improvements were greater in people with high cholesterol levels (
However, the researchers observed that soy supplements didn’t have the same cholesterol-lowering effect as eating soy foods (
In another older review of 38 studies, researchers noted that an average soy intake of 47 grams per day was linked to a 9.3% decrease in total cholesterol and a 13% decrease in LDL (bad) cholesterol (
Fiber seems to play an important role in cholesterol-lowering effects of soy.
In one study, 121 adults with high cholesterol took 25 grams of soy protein with or without soy fiber for 8 weeks. The soy with fiber reduced LDL (bad) cholesterol more than twice as much as soy protein alone (
May affect fertility
Studies have turned up conflicting results on the relationship between soy intake and fertility.
For example, one study found that soy consumption was associated with improved outcomes for women undergoing fertility treatments with assisted reproductive technology (
Another study demonstrated that soy had a protective effect against BPA, a chemical found in plastic, which may have negative effects on fertility.
Women who ate soy before in vitro fertilization (IVF) were more likely to have a successful pregnancy than those who did not (
Furthermore, soy intake by the prospective father does not seem to affect pregnancy rates in women receiving IVF (
On the other hand, some studies have found that soy intake may actually negatively affect fertility.
For instance, one review reported that consuming very high amounts of soy could alter levels of reproductive hormones and negatively affect ovarian function (
Another study in 11,688 women found that higher soy isoflavone intake was associated with a lower likelihood of having been pregnant or giving birth to a live child (
What’s more, an animal study showed that feeding rats a diet rich in soy phytoestrogens induced several symptoms of polycystic ovary syndrome (PCOS), which may negatively affect reproductive health (
Therefore, more research is needed to examine the complex relationship between soy intake and fertility.
May reduce menopause symptoms
Isoflavones are a class of phytoestrogens found naturally in soy that act like a weak estrogen in the body.
Estrogen levels decrease during menopause, leading to symptoms like hot flashes. Since soy acts as a natural estrogen, it may help reduce these symptoms.
Studies suggest soy’s beneficial role in menopause.
In a review of 35 studies, soy isoflavone supplements raised estradiol (estrogen) levels in postmenopausal women by 14% (
Lastly, in another review of 17 studies, women who took an average dose of 54 mg of soy isoflavones a day for 12 weeks had 20.6% fewer hot flashes.
They also experienced a 26.2% decrease in symptom severity compared to at the start of the study (
Some research suggests that soy may help lower cholesterol, improve fertility outcomes, and reduce menopause symptoms.
While soy has several health benefits, its impact on other conditions is unclear.
Effect on breast cancer is unknown
Soy contains isoflavones, which act like estrogen in the body. Since many breast cancers need estrogen to grow, it would stand to reason that soy could increase breast cancer risk.
However, this isn’t the case in most studies.
In fact, according to one review, higher soy consumption may be linked to a 30% lower risk of developing breast cancer in Asian women (
However, for women in Western countries, one study showed soy intake had no effect on the risk of developing breast cancer (
This difference may be due to the different types of soy eaten in the Asian diet compared to the Western diet.
Soy is typically consumed whole or fermented in Asian diets, whereas in Western countries, soy is mostly processed or in supplement form.
One review noted that soy isoflavones undergo structural changes during the fermentation process, which may significantly increase absorption (
Additionally, an animal study also found that fermented soy milk was more effective than regular soy milk at suppressing the growth and spread of breast cancer tumor cells in rats (
Therefore, fermented soy may have a more protective effect against breast cancer compared to many processed soy products.
In addition to protecting against breast cancer development, soy has also been linked to a longer lifespan after breast cancer diagnosis.
In a review of five long-term studies, women who ate soy after diagnosis were 21% less likely to have a recurrence of cancer and 15% less likely to die than women who didn’t eat soy (
Impact on thyroid function
Soy contains goitrogens, substances that may negatively affect the thyroid by blocking iodine absorption.
Some research has found that certain soy isoflavones, including genistein, may block the production of thyroid hormones. However, these findings are mostly limited to test-tube and animal studies (
On the other hand, studies on the impact of soy on thyroid function in humans suggest it may not have a significant effect.
One review of 18 studies showed that soy supplementation had no impact on thyroid hormone levels.
Although it slightly increased levels of thyroid stimulating hormone (TSH), it’s unclear whether this is significant for those with hypothyroidism (
However, according to another older review of 14 studies, soy had little to no effect on thyroid function.
The authors concluded that people with hypothyroidism do not need to avoid soy as long as their iodine intake is adequate (
Furthermore, another randomized trial found that consuming 66 mg a day of soy phytoestrogens had no effect on thyroid function in 44 people with subclinical hypothyroidism (
Effect on male sex hormones
Because soy contains phytoestrogens, men may worry about including it in their diet.
However, studies do not indicate that soy negatively impacts the production of testosterone in men.
In a review of 15 studies in men, intake of soy foods, protein powders, or isoflavone supplements up to 70 grams of soy protein and 240 mg of soy isoflavones per day did not affect free testosterone or total testosterone levels (
What’s more, soy may reduce the risk of prostate cancer in men.
In a review of 30 studies, high soy consumption was linked to a significantly lower risk of developing the disease (
Most soy contains GMOs
More than 90% of the soy produced in the United States is genetically modified (
There’s much debate over the safety of genetically modified organisms (GMOs). More long-term scientific studies are necessary to determine their effects in humans and in what quantity they’re safe (
Additionally, most genetically modified soy products withstand the pesticide glyphosate, which is controversial.
Certain GMO soy products have been found to contain glyphosate residues and have a poorer nutritional profile compared to organic soybeans (
Therefore, to avoid GMOs and exposure to glyphosate, stick with organic soy.
Impact on digestive health
Several recent animal studies show that certain compounds found in soy could negatively affect digestive health.
Soybean agglutinins, in particular, are a type of antinutrient that have been linked to several negative side effects.
According to one review, soybean agglutinins may impact digestion by influencing the structure and barrier function of the gut.
They may also disrupt the health of the microbiome, which is a group of beneficial bacteria housed in the digestive tract (
Another animal study showed that soybean agglutinins could increase intestinal permeability, making it easier for substances to pass through the lining of the digestive tract and into the bloodstream (
Soybeans may also contain several other antinutrients, including trypsin inhibitors, α-amylase inhibiting factors, phytates, and more (
Animal studies suggest soy negatively affects breast cancer, thyroid function, and male hormones, but human studies suggest otherwise.
Apart from organic soy, most soy is genetically modified. Most preparation methods can reduce antinutrients.
Some studies have suggested that soy may have positive effects on cholesterol levels, cancer risk, and menopause symptoms.
However, other studies have shown that soy intake may negatively impact certain aspects of health, including digestion and ovarian function.
What’s more, research has shown that the potential health benefits of soy likely depend on the form in which it’s consumed, with whole or fermented soy foods being superior to more processed forms of soy.
Although it’s clear that more high-quality research is needed to determine the effect of soy consumption on overall health, the majority of current studies suggest that consuming whole or fermented soy foods in moderation is likely safe and beneficial for most people.