What Are The Side Effects Of Soybeans?

It seems the perfect vegan option. But beware of the negative effects of consuming soy.

Medically reviewed by Lucas AounLucas Aoun, ND
By Sindhu KogantiSindhu Koganti, Certified Health & Nutrition Life Coach  • 

Soybeans are rich in nutrients and are touted for their multitude of benefits. However, concerns about the side effects of soy proteins are on the rise.

These widely consumed legumes are often found in many processed foods, and their protein is extracted from their outer shells (1), (2). Soybeans are also rich in various vitamins and minerals like calcium, zinc, iron, and amino acids (3).

But when you consume them in excess quantities, their protein content may interfere with thyroid regulation, cause testosterone imbalance and allergic reactions, and increase the risk of cancer proliferation.

This article further discusses the side effects of soybeans and their proteins in detail. Take a look.

protip_icon Know The Flip Side: Soybeans

Short-Term Effects
It may cause constipation, stomach ache, or diarrhea.

Long-Term Effects
It may lower sperm concentration and increase the risk of cancer and dementia.

Drug Interactions
It may interfere with anticoagulants and cancer and hypothyroidism medications.

When To See A Doctor
If you experience excessive skin and throat itching, vomiting, wheezing, or diarrhea.

protip_icon Trivia
While its origination is vague, most botanists claim that soybeans came from China and were domesticated around 7000 BCE.

What Is The Major Issue With Soybeans?

The major issue with soybeans is its isoflavone content.

Soybeans are reservoirs of phytoestrogens (isoflavones), which are structurally and functionally similar to the estrogen hormone in your body (3). Isoflavones are a class of phytoestrogens (also called soy proteins) found in soy and soy products. The following table should give you an idea about the isoflavone content in different soy foods.

Unfermented soy foodsIsoflavone content (mg)
soy milk, 1 cup6
tofu (bean curd), soft, 3 ounces20
soybeans, mature, boiled, ½ cup55
soybeans, dry roasted, 1 oz.40
edamame, boiled, ½ cup16
soy cheese, 1oz.2
soy burger, 1 patty5
Fermented soy foodsIsoflavone content (mg)
miso, 3 oz.37
natto, 3 oz.70
tempeh, cooked, 3 oz.30
soy sauce, 1 tbsp0.02

Source: Harvard T.H.Chan School of Public Health

Soy phytoestrogens have been used to compensate for estrogen hormone deficiency. Soy protein is a part of the estrogen replacement therapy given to women who are going through menopause (3).

Some epidemiological studies suggest that dietary intake of phytoestrogens may reduce the incidence of postmenopausal cardiovascular disease, osteoporosis, and hot flashes, among other symptoms. Contrasting data has also reported on the potential of phytoestrogens to prevent cancers of the breast and prostate (3).

However, the benefits of soy have been unclear. In fact, certain other studies also state that soy protein could cause potential harm (3).

What Happens When You Consume Soybeans? What Causes The Adverse Effects?

When you consume soybeans (soy protein) in any form, the soy isoflavones (the phytoestrogens) bind to the estrogen receptors in your body. These phytoestrogens compete with the natural estrogen to bind to the receptors. This results in a weak estrogenic or anti-estrogenic activity. In other words, soy isoflavones could make the natural estrogen in your body less effective (3), (4).

Such interactions can lead to a hormonal imbalance and cause gender-related changes in girls. They may also exert anti-androgenic effects in men. Men may start growing breasts (gynecomastia) and experience a dip in their sperm concentration when on a high-soy diet (3).

Isoflavone overdose over a long period may stimulate the endometrium (uterus) and breasts in women. Therefore, anyone who has been treated for breast cancer is usually recommended to avoid soy protein (3).

Phytoestrogens can affect several physiological and pathological processes in the body. High doses of soy isolates may cause adverse effects on reproduction, skin, bone remodeling, cardiovascular system, nervous system, immune system, and metabolism.

Let’s take a look at the adverse effects of soybeans (soy proteins) in detail.

What Are The Adverse Effects Of Soybeans (Soy Protein)?

1. May Interfere With Thyroid Regulation

Woman experiencing thyroid problems as a side effect of soy proteins

Image: Shutterstock

Soy foods may increase the risk of developing hypothyroidism in people with compromised thyroid function. Such individuals may develop goiter and autoimmune thyroid disease. This risk further increases when the individual’s iodine intake is low (5).

Soy isoflavones have been found to inhibit the activity of an enzyme called thyroid peroxidase. This enzyme is required for the synthesis of the thyroid hormone. This is why you may face a risk of hypothyroidism when you eat a lot of soy protein (6).

Soy products also interfere with the absorption of levothyroxine (L-thyroxine), a drug used to treat thyroid hormone deficiency (7). Since soy proteins seem to alter the availability of the drugs, you may be advised against consuming soy protein if you have a thyroid imbalance.

However, just a high intake of soy isoflavones does not appear to increase the risk of hypothyroidism, unless coupled with inadequate consumption of dietary iodine. Therefore, the influence of soy protein on the thyroid gland is debatable. More research is warranted in this regard.

2. May Cause Testosterone Imbalance

A study was conducted on 12 male subjects who consumed 56 g of soy protein isolate daily for four weeks. As a result, their serum testosterone levels dropped by 19% (8). Though the data was inconsistent, it was found that soy protein decreased serum testosterone levels in healthy men.

Soy protein is said to have adverse effects on the male reproductive function. However, there are no specific studies in this regard. Some animal studies, in fact, state that soy isoflavones do not exert any feminizing effects on men (9).

Most of the observations are based on lab and animal studies. Hence, the relationship between soy isoflavones and testosterone is inconclusive (10).

3. May Elicit Hypersensitivity (Allergy)

Woman experiencing allergic reaction as a side effect of soy proteins

Image: Shutterstock

Intolerance of soy products can cause allergies or hypersensitivity in children and adults. Often, soy allergy starts in infancy, with reaction to soy-Soy products that can cause allergies or hypersensitivity in children and adults. Often, soy allergy starts in infancy with a reaction to soy-based infant formula. However, most children outgrow soy allergy (11).

Generally, soy allergy is uncomfortable but not severe. An allergic reaction to soy is rarely frightening or lethal. except in the case of asthma. Symptoms of soy allergy may include tingling in the mouth, eczema or itchy skin, rashes, wheezing, diarrhea, stomach ache, headache, vomiting, and skin redness (flushing) (12).

If you experience any of these symptoms, you may have a soy allergy. Get tested to confirm the allergy. If tested positive, you may be advised against having soy products/isoflavones.

4. May Increase The Risk Of Cancer Proliferation

Soy isoflavones (one of them being genistein) may stimulate the proliferation of cancer cells in your body. This is especially true in the case of estrogen-dependent breast cancer, as soy isoflavones tend to have estrogenic effects (13).

As per animal studies, genistein may deregulate the cell cycle and trigger tumor development. It acts by triggering the estrogen receptors (14).

Contrarily, human studies show an inverse relationship between cancer and isoflavones. Soy intake was also found to reduce the incidence and death rate caused due to breast cancer. This could be because of the anti-estrogenic effect exerted by phytoestrogens (13).

The amount and source of soy isoflavones also greatly impact breast cancer risk (14).

5. May Trigger Alzheimer’s Dementia

Woman experiencing Alzheimer’s dementia as a side effect of soy proteins

Image: Shutterstock

Traditionally, soy foods were used in cooking only after fermenting. This two-step routine would destroy most of the anti-nutrients in soy, such as isoflavones (like genistein and daidzein) and DNA-altering enzymes (like topoisomerases) (15).

When you eat unfermented, cooked/uncooked soy foods, these anti-nutrients may affect the vital systems of your body, including your brain (15).

These anti-nutrients in soy could have detrimental effects on many individuals. Soy isoflavones could increase the risk of Alzheimer’s dementia (15).

If you are dealing with dementia or have a family history of this condition, it is safe to reduce your soy consumption (15).

6. May Cause Infant Health Issues

Doctor examining infant experiencing health issues due to soy proteins

Image: Shutterstock

Infant food formulas contain fair amounts of soy protein/isoflavones. Infants who are fed these formulas are exposed to 5.7–11.9 mg isoflavones/kg body weight during the first four months of life (16).

These kids are exposed to 6–11 times higher levels of isoflavones than adults. This could lead to disturbances in the reproductive health and endocrine function in the child. The major isoflavones, daidzein and genistein, preferentially bind to the estrogen receptors in the body (16).

However, these conclusions are based on animal studies. Human studies may give a different picture. Moreover, currently available soy-based formulas show no overt toxicities in healthy infants (16), (17). Hence, check with your pediatrician before giving soy-based formulas to your child.

7. Unsure Osteoprotective Effects

Soy protein has shown to conserve bone mineral density, particularly in postmenopausal women. In perimenopausal women, the soy isoflavones were shown to prevent bone loss from the lumbar spine. These compounds had no significant effect (neither positive nor negative) on bone formation and/or bone resorption (18).

In some cases, the mineral balance could be impaired with soy consumption. Soybeans contain (by weight) about 1–3% phytic acid (also called phytate). Phytic acid is an anti-nutrient that binds to minerals like zinc, iron, and calcium. Along with soy isoflavones, phytic acid also may decrease their bioavailability (18).

However, a major chunk of literature highlights the osteoprotective effect of soy protein. Further research is needed to determine the precise impact of soy protein/foods on bone density and other related diseases (19).

Most of the research on soybeans and soy protein is still unclear. While in some studies they seem to promote health, certain other studies tend to state the potential harm they may cause. However, this does not mean you have to totally be off soy. There are a few specific soy products you may want to avoid.

Which Soy Products Should You Avoid?

Avoid soy protein isolate

Image: Shutterstock

Moderation is important, and so is eating right. Choosing the right kind of soy products can protect you from the adverse effects mentioned above.

When given a choice between natural soy foods and soy protein isolate, go for the natural options. Avoid industrial soy foods if you have iodine deficiency or thyroid imbalance (15).

Whole soy foods are mostly fermented and cooked in a healthier manner. You can take tofu, edamame, soy milk, tempeh, miso, and natto a few times every week.

protip_icon Fun Fact
By the 17th century, British colonists brought soybeans to Georgia, a British colony.

How Much Soy Is Safe To Eat In A Day?

According to the US FDA, daily consumption of 25 g of soy seems to be safe. This amount of soy could also help lower cholesterol levels (20).

It also is believed that the intake of 50 g of soy protein per day could help prevent heart disease, diabetes, and estrogen-dependent cancers. However, more research is warranted.

There is limited information on the excess intake of soy protein/soybeans. But we don’t recommend you take more than 25 grams per day.

Is Eating Too Much Soy Harmful?

There is little information about the toxicity of soy protein. In limited doses, soy protein may be helpful for women who are at a high risk of breast cancer (21).

Some studies reported very few drug-related adverse events, which were mild. In very few cases, high levels of soy isoflavones adversely affected blood pressure levels (22).

Hence, it is important you talk to your healthcare provider about a suitable dosage for you.

Soy protein side effects are attributed especially to soy isoflavones, which render natural estrogen activity in the body less effective, leading to hormonal imbalance. Soy foods may also increase the risk of hypothyroidism, especially if the iodine intake is low. Also, some individuals, including infants, may be allergic to soy foods and may experience itchy skin, diarrhea, vomiting, stomach ache, etc. So, avoid eating them once you encounter these symptoms. Moreover, soy isoflavones are said to aid in the multiplication of cancer cells and increase cancer risk. Hence, it is advised to consume soy in recommended dosages.

Frequently Asked Questions

Which protein is better – soy or whey?

It depends on your personal preference. Both proteins showed no significant differences in muscle growth. However, whey protein may have a slight edge due to its slightly higher leucine content (24).

Does soy cause gas and bloating?

Yes, consumption of soy products may result in excessive intestinal gas. Legumes contain high levels of the indigestible oligosaccharides raffinose and stachyose, which are fermented in the colon by bacteria (25).

Can soy cause sinus problems?

Possibly. If you are sensitive or allergic to soy, you may face sinus problems.

Is soy protein hard to digest?

It varies from individual to individual. For some people, it may cause indigestion, and they may experience gas, bloating, nausea, stomach pains, or discomfort.

Key Takeaways

  • Soy products may affect the synthesis of essential hormones like thyroid and testosterone.
  • Consuming soybean-based foods may cause allergic reactions with symptoms like itchy skin, diarrhea, and vomiting.
  • Unfermented soy foods may affect the brain and increase the risk of dementia.
  • Instant food formulas with soy protein may also affect reproductive health and endocrine function in children.
  • Daily consumption of 25g of soy is considered safe by US FDA.


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