What Are The Side Effects Of Soybeans?

Medically reviewed by Lucas Aoun, Naturopathic doctor
Written by Sindhu Koganti

Soybean is a widely consumed legume. It is a staple of the Asian diet and is used as an ingredient in several processed foods. The protein in soy (soy protein) can replace the animal proteins in your diet and is obtained after removing the outer shell of soybeans and their fatty acids (1), (2).

Though soybeans are rich in nutrients like calcium, iron, zinc, and amino acids, they could cause certain side effects (3).

When consumed in excess, they may interfere with thyroid regulation medication and cause testosterone imbalance, allergies, and cancer proliferation.

Also, long-term usage of soy products in excess could be unsafe. In this post, we discuss soybeans and their possible adverse effects. Keep reading.

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

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)

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. Symptoms of soy allergy may include tingling in the mouth, eczema or itchy skin, wheezing, diarrhea, stomach ache, 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

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

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?

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.

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 could be the reason Asians have historically had lower rates of cardiovascular disease. Their menopausal symptoms and incidence of breast cancer, diabetes, and obesity are also lower than their Western counterparts (23).

There is substantial yet unclear evidence stating the adverse effects of soybeans and soy protein/foods. Soy isoflavones may disturb the hormonal balance, stimulate cancer proliferation, and trigger allergies.

Hence, choose your soy portions carefully. Pick fermented soy foods over protein or isoflavone isolates. Follow the instructions by your physician/nutritionist and stick to the recommended intake limit.


Articles on StyleCraze are backed by verified information from peer-reviewed and academic research papers, reputed organizations, research institutions, and medical associations to ensure accuracy and relevance. Check out our editorial policy for further details.

Recommended Articles:

Was this article helpful?
The following two tabs change content below.
Sindhu Koganti is a Biotechnology graduate and has been in the writing field for over 4 years now. She specializes in writing on Health and Wellness. She has hands-on experience in writing articles and press releases on Life Sciences and Healthcare, Food and Beverages, and Chemicals and Materials. When she’s not writing, she loves watching movies and listening to music. She also enjoys traveling.