Is Soy Bad for You?

Soybeans and soy foods are considered a healthy source of high-quality complete protein and other nutrients. Soy protein intake has multiple health benefits, including lowering the risk of heart disease and stroke, reducing the risk of breast and prostate cancer, improving bone health, and reducing menopausal symptoms such as hot flashes.

But, there are also concerns that the estrogen-like effects of soy phytoestrogens could be harmful. Soybeans are a rich source of plant compounds called isoflavones or soy phytoestrogens, which are similar to the female hormone estrogen. Isoflavones have several health benefits but are also thought to be endocrine disruptors that interfere with hormone function in the body.

Common Concerns About Soy

Some of the potential risks of consuming soybeans and soy foods include:

Disruption in hormone levels in the body due to the estrogen-like effects of soy.
- Fertility problems in men and women.
- Decreased concentration of sperm in the ejaculate in men.
- Drop in testosterone levels and feminization in men.
- Worsening thyroid function and increased need for thyroid replacement medicine in hypothyroid patients.
Increased risk of certain cancers.
- Growth of estrogen-sensitive breast cancer cells in existing tumors triggered by isoflavones in soy.
- Increased risk of endometrial cancer and growth of existing endometrial cancer cells by isoflavone compounds in soy foods.
Pro-inflammatory effect due to high omega-6 content of soybean oil.
Cognitive impairment in late life.

What is Soy?

Soybeans (Glycine max) are a type of legume native to China, Japan, and Southeast Asia. They grow in pods with seeds or beans and are classified as oil seeds because of their high fat content.

Types of Soy and Common Foods Containing Soy

Mature raw soybeans.
Edamame (80% mature soybeans in the pod that retain more moisture - edamame can be steamed or boiled and served with condiments).
Soymilk (made by soaking and grinding soybeans and boiling and filtering the mixture).
Tofu (soybean curd made by coagulating soy milk).
Soy sauce (a liquid condiment made from the fermented paste of soybeans).
Soy flour (made from ground, roasted soybeans).
Miso (made by fermenting soybeans and turning them into a paste - miso can be added to soups and marinades).
Natto (a traditional Japanese breakfast food made from fermented soybeans).
Tempeh (a traditional Indonesian food which is a cake made of fermented soybeans).
Soy protein concentrate and soy protein isolate (made from dehulled and defatted soy flakes and soy flours after extracting the oils and fats).

Health Benefits of Soy Foods

Soy is a complete protein that contains all nine essential amino acids. It is equal in quality to animal protein. Soy protein can therefore be an important protein source for vegetarians and vegans.

According to the United States Department of Agriculture, 100 grams of raw soybeans contain:

446 calories
19.9 grams of fat
36.5 grams of protein
30.2 grams of carbohydrate
9.3 grams of fiber
Micronutrients (calcium, iron, zinc, potassium, B vitamins)

Potential Benefits for Heart Health

Research suggests that consuming soy foods can lower cholesterol levels by about 4% to 6%, specifically LDL (low-density lipoprotein) cholesterol, also known as “bad” cholesterol. The decrease in cholesterol is more in people with high cholesterol levels compared to those with normal cholesterol levels.

In addition, soy protein also lowers triglyceride levels by about 5% and increases HDL (high-density lipoprotein) cholesterol levels by about 1% to 3%. HDL cholesterol is also known as “good” cholesterol. In contrast, high triglyceride levels increase the risk of stroke, heart attack, and heart disease.

High protein diets are linked to a modest reduction in blood pressure. Studies have found that regular consumption of soy protein can lower systolic blood pressure by 2 to 5 mmHg.

High blood sugar, high cholesterol, and high blood pressure are major risk factors for stroke and coronary heart disease. Studies suggest that consuming soy protein can potentially lower the risk of cardiovascular disease by 5-15%.

In addition, soy and soy foods have also been found to improve endothelial function (the endothelium is the inner lining of blood vessels) and reduce arterial stiffness.

May Relieve Menopausal Symptoms

Research has shown that isoflavones in soy products interact with estrogen receptors and mimic the effects of estrogen hormone in the body. This has led to the hypothesis that soyfoods can prevent the onset of hot flashes and/or reduce the severity of hot flashes in menopausal women. The low prevalence of menopausal symptoms among women in Japan (where eating soy foods is common) supports this theory.

A systematic review and meta-analysis found that taking soy isoflavone supplements led to a 20% reduction in the frequency and severity of hot flashes compared to the placebo group. However, further analysis of the data found that the effects of soy were twice as high when the amount of genistein isoflavone was 19 milligrams (mg) compared to lower amounts. (To get 19 mg of genistein you would need to take 40 mg of total isoflavone supplements).

May Improve Bone Health

Soy foods promote bone health in postmenopausal women due to the estrogen-like effects of isoflavones. Studies have shown that women with moderate to high soy intake have a one-third reduction in the risk of fractures related to osteoporosis (weak bones).

Studies have also found that among Seventh-Day Adventists (40% of whom are vegetarians), postmenopausal women who report soy milk consumption have a significantly lower osteoporosis risk. However, consumption of cow’s milk has a similar protective effect on bone health, suggesting that the calcium content of soy milk may be responsible rather than the isoflavone content.

Nonetheless, soy isoflavone supplements at 105 mg/day were found to increase bone mineral density by about 7.5%, approximately half the increase obtained with risedronate, a drug used to treat osteoporosis. Doubling the dose of isoflavones had a lower effect, suggesting moderate doses of isoflavones (50 to 100 mg/day) are the most effective.

May Lower Cancer Risk

Soy-food-consuming countries in East Asia have a significantly lower breast cancer risk compared to Western countries. Studies have found that higher soy consumption results in reduction in breast cancer risk by about one-third. However, the data suggests that soy consumption must take place during childhood and/or adolescence to lower breast cancer risk. High soy intake in early life can lower the risk of breast cancer by 25% to 60%.

Similarly, there are significantly lower rates of prostate cancer among men in Asian countries where soy food consumption is much higher than the West. Case-control studies show that high consumption of soy can lead to a 50% reduction in the risk of developing prostate cancer.

Potential Anti-Inflammatory Effects

Systematic review and meta-analysis of more than a dozen clinical trials showed a 22% reduction in C-reactive protein (CRP) in postmenopausal women with high baseline CRP concentrations after soy intake compared to controls. C-reactive protein is an important indicator of inflammation in the body. Inflammation plays a key role in many chronic health conditions. More research is needed to understand this link better.

May Benefit Kidney Function

There is some evidence that soy protein places less stress on the kidneys than other types of proteins. This could potentially lower the risk of developing kidney disease in people who are at high risk, such as people with diabetes. Specifically, replacing animal protein with soy protein may protect against kidney damage in people with diabetes. However, in studies involving patients with chronic kidney disease, dietary soy protein did not affect the glomerular filtration rate.

May Benefit Skin Health

Isoflavones in soy products bind to estrogen receptors in the skin and may improve skin elasticity and hydration and reduce pigmentation and fine wrinkles.

Health Risks and Concerns: Why is Soy Considered Bad For You?

Soy and Cancer Risk: Does Soy Actually Increase Estrogen?

Breast Cancer

There are concerns that the isoflavones in soy products, which resemble the female hormone estrogen, can adversely affect patients with breast cancers. Further research is needed to understand if the effects of soy can stimulate the growth of existing tumors in breast tissue.

Available research shows that soy intake post-diagnosis of breast cancer reduces breast cancer recurrence. The American Cancer Society and the American Institute for Cancer Research say that eating soy foods is safe for women with breast cancer.

Endometrial cancer

The use of unopposed estrogen therapy (estrogen without progesterone) is linked to a 2-3 times higher risk of endometrial cancer (cancer of the lining of the uterus) compared to never-users.

There is a concern that the presence of isoflavones in soy products, which resemble human estrogen hormone, could potentially increase the risk of developing endometrial cancer and stimulate the growth of existing endometrial tumors.

However, current research suggests that isoflavones have little to no effect on the endometrium, and that soy intake is associated with a lower endometrial cancer risk.

Should Men Avoid Soy Consumption?

The concerns about soy’s effects on male fertility and feminization stem from the fact that soy isoflavones are structurally similar to the female reproductive hormone estrogen. However, despite the similar structure, soy isoflavones don’t have the same effects as estrogen. Clinical studies have shown that soy foods and isoflavone supplements do not affect testosterone levels in men, despite concerns that soy isoflavones reduce testosterone production.

A key factor to remember is that the feminizing effects of soy are associated with men with soy consumption nine times greater than those with an average intake. It is therefore unlikely that a moderate intake of soy will result in fertility issues or feminization in men.

Male Fertility

A small study found that eating soy foods can lead to a decrease in sperm concentration; however, this was found to be due to an increase in the volume of the ejaculate and not a decrease in the sperm count.

In addition, the same research group found that in couples undergoing in vitro fertilization (IVF), soy food intake by the male partner did not affect fertilization rates, the quality of the embryos, implantation rates, or pregnancy and live birth rates.

Other studies have similarly found that soy food consumption and isoflavones do not have an effect on sperm quality or concentration. There are case reports of the male partner of an infertile couple with low sperm count achieving normal sperm quality and quantity with daily isoflavone supplementation for 6 months.

Male Feminization

Two case reports have been published describing the feminizing effects of soy products in men. However, in both, soy intake was 360 mg per day of isoflavones, which is 9 times higher than the average Japanese man. A rise in circulating estrogen levels was noted in one of these cases. However, many clinical studies have found that taking up to 150 mg per day of isoflavone supplements does not affect estrogen levels in men.

A fall in testosterone levels has been noted in one case and linked to the effects of soy. But this is inconsistent with the majority of clinical data that shows neither soy products nor soy isoflavone supplementation affects testosterone levels.

Pro-Inflammatory Effects: Is Soy Inflammatory to the Body?

There are claims that the high amounts of omega-6 fatty acids in soybean oil have pro-inflammatory effects, but these are not supported by clinical evidence. In fact, studies have shown that high soy intakes are linked to lower levels of circulating inflammatory markers such as interleukins, tumor necrosis factor, and C-reactive protein.

Thyroid Function

Euthyroid Individuals (People With Normal Thyroid Function)

There are concerns that isoflavone-rich soy protein has adverse effects on thyroid function. However, these concerns are primarily based on laboratory studies in animals.

A meta-analysis of randomized clinical trials showed that neither soyfoods nor isoflavones have a harmful effect on thyroid function in people who are euthyroid (have normal thyroid function). Studies have also shown that isoflavone supplements do not affect thyroid function in post-menopausal women.

Hypothyroid Individuals (People With Decreased Thyroid Function)

In people with hypothyroidism (underactive thyroid gland), high soy intakes may increase the amount of thyroid medication needed to treat the condition. This is not because of a direct effect on the thyroid gland by dietary isoflavones in soy products, but because soy protein can interfere with the absorption of levothyroxine (thyroid hormone replacement therapy).

Healthcare providers do not recommend that hypothyroid individuals avoid soy products since thyroid medication is taken on an empty stomach and the dose can be easily adjusted to compensate for any decreased absorption of the medicine related to soy isoflavone intake.

The exception for soy intake is infants with congenital hypothyroidism who should avoid soyfoods.

In the past, several cases of goiter were linked to the use of soy infant formula. But, after iodine fortification of infant formula was started in the 1960s, this is no longer a concern.

Subclinical Hypothyroid Individuals (People With Compromised Thyroid Function)

There is concern that soy intake could worsen thyroid function in those with compromised thyroid function but no symptoms of hypothyroidism - this is called subclinical hypothyroidism.

This concern is based on the potential for soy isoflavones to take up iodine instead of the amino acid tyrosine, thereby preventing the synthesis of thyroid hormone. However, research has found that iodination of isoflavones is negligible and clinically insignificant.

Choosing Soy Products: Is Soy Milk Bad For You?

Whole Soy vs. Processed Soy

Whole soy foods are generally healthier than processed soy foods. Minimally processed soy foods are a complete protein with all nine essential amino acids. They also contain fiber, vitamins, minerals, and other beneficial compounds. Examples of whole soy foods include edamame, tofu, tempeh, soy milk, and miso.

Processed soy foods and soy extracts are dehulled and defatted, which removes some of their nutritional value. In addition, processed soy foods often contain added salt, sugar, and other additives. Examples of processed soy products include soy protein isolate, soy isoflavone extracts, soy energy bars, and meatless soy burgers.

Fermented Soybean vs. Non-Fermented Soy Products

Fermented soy foods, like soy sauce, tempeh, miso, and natto, may be more beneficial than non-fermented soy products because the fermentation process improves the quality of soybeans by increasing isoflavone content and improving digestibility. Fermented soy foods are also probiotics and increase the levels of healthy gut bacteria.

Soy Foods vs. Soy Protein Supplements

Most healthy people do not need to take soy protein supplements. Experts in clinical nutrition say the best way to get the health benefits of soybeans is to eat minimally processed, whole soy-rich foods. Research suggests that bioavailability of isoflavones is higher with eating dietary soy proteins compared to soy supplementation in pill form.

Moderation and Dietary Balance

Eating soy protein has many health benefits. Soybeans are high in fiber, vitamins, minerals, and polyunsaturated fats, and low in saturated fat. Replacing red meat and other animal protein sources with natural soy products like tofu or edamame can be especially beneficial. Soy is an excellent source of dietary protein for vegetarians and vegans.

Nutrition experts at Harvard University, the U.S. Department of Agriculture, the European Food Safety Authority recommend eating whole soy foods (tofu, edamame, soy milk) in moderation, several times a week, to get the beneficial health effect of soy.

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