Ingredients and Uses

Is Soy Protein Bad For You? Research And Reasons

Medically reviewed by Lucas Aoun, Naturopathic doctor
Is Soy Protein Bad For You? Research And Reasons Hyderabd040-395603080 July 8, 2019

Let’s start with the fundamental question…

What Is Soy Protein?

Soy protein a form of protein found in soybeans. It can be used to replace animal proteins in your diet. It is obtained after removing the outer shell of soybean and its fatty acids (1), (2).

This form of protein isolate comes with the right amounts of calcium, iron, zinc, phosphorus, vitamins, and amino acids (3).

It is not only cruelty-free but also healthy. Soy protein can boost heart health and regulate body weight and diabetes. It has also been proven that this plant-derived protein can reduce the risk of cancer (2).

But research states that it has certain adverse effects.

What Is The Major Issue With Soy Protein?

Soybeans are reservoirs of phytoestrogens. Phytoestrogens are structurally and functionally similar to the estrogen hormone in our body (3). Find out the isoflavone content of each soy product below:

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 hormone deficiency. Soy protein is a part of estrogen replacement therapy given to women going through menopause (4).

Some epidemiological studies suggest that dietary intake of phytoestrogens may reduce the incidence of postmenopausal cardiovascular disease, osteoporosis, hot flashes, and other symptoms (4).

Despite having sound scientific evidence, the scientific community is not convinced of these benefits. Most good studies show no clear benefits offered by phytoestrogens. In fact, they show more potential for harm (4)!

Contrasting data has been reported on the potential of phytoestrogens to prevent breast and prostate cancers (4).

Wondering why there is such a contrast in the data? Keep reading.

What Happens When You Consume Soy Protein? What Causes Adverse Effects?

When you consume soy protein in any form, soy isoflavones bind to the estrogen receptors in your body. These phytoestrogens compete with estrogen to bind to the receptors and cause either weak estrogenic or anti-estrogenic activity (4), (5).

Such interactions can lead to a hormonal imbalance and cause changes in gender-related behavior in girls.

They may also exert anti-androgenic effects in men.

This means, they may start growing breasts (gynaecomastia) and experience a dip in their sperm concentration when on a high-soy diet (4).

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 (4).

What’s worse is that phytoestrogens can affect a number of physiological and pathological processes. Reproduction, skin, bone remodeling, cardiovascular system, nervous system, immune system, and metabolism – any or all of these can be affected by high doses of soy isolates.

Quite scary, isn’t it?

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

What Are The Adverse Effects Of Soy Protein?

1. Interferes With Thyroid Regulation

Soy foods may increase the risk of developing hypothyroidism in people with compromised thyroid function. They may develop goiter and autoimmune thyroid disease. This risk increases when the individual’s iodine intake is also low (6).

Soy isoflavones have been found to inhibit the activity of thyroid peroxidase. This enzyme is required for the synthesis of the thyroid hormone. This is why you face a risk of hypothyroidism when you eat a lot of soy protein (7).

Soy products also interfere with the absorption of levothyroxine (L-thyroxine), a drug used to treat thyroid hormone deficiency (8). Since it alters the availability of correcting drugs, you might 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. It happens only when coupled with an inadequate consumption of dietary iodine. Therefore, the influence of soy protein on the thyroid gland is debatable.

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% (9). 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. Animal studies demonstrate feminization, erectile dysfunction, and infertility in males when given soy foods/isoflavones (7).

It is noteworthy that most of these observations are based on lab and animal studies. Hence, the relationship between soy isoflavones and testosterone is still inconclusive (10).

3. May Elicit Hypersensitivity (Allergy)

May Elicit Hypersensitivity Pinit


Soy products can cause allergies or hypersensitivity in children and adults. Often, soy allergy starts in infancy, with 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) (11).

If you experience any of these symptoms, you may have a soy allergy. Get tested to confirm a soy allergy. If tested positive, you may be advised against having soy products/isoflavones (11).

4. Promotes Cancer Proliferation

Soy isoflavones – like genistein – may stimulate the proliferation of cancer cells in your body, especially estrogen-dependent breast cancer. This happens because soy isoflavones have estrogenic effects (12).

Genistein may deregulate the cell cycle and trigger tumor development. As per animal studies, it may act on the estrogen receptors to do so (13).

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

The critical factors here could be the amount and source of soy isoflavones as they greatly impact breast cancer. Also, having whole soy foods versus soy protein makes a significant difference (13), (14).

5. Might Trigger Alzheimer’s Dementia

Might Trigger Alzheimers Dementia Pinit


Traditionally, soy foods were fermented and only then used in cooking. 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 and cooked/uncooked soy foods, these anti-nutrients may affect the vital systems of your body, including your brain (15).

These anti-nutrients induce biochemical changes at the DNA level, thus triggering cerebral atrophy and brain shrinkage. Soy isoflavones accelerate brain cell death, leading to Alzheimer’s dementia (15).

To make matters worse, they also generate free radicals in your body. If you are dealing with dementia or have a family history of this condition, it is safe to reduce your soy consumption (15).

6. Causes 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 on a body weight basis than adults. Such high levels can disturb reproductive health, fertility, and endocrine function. The major isoflavones – daidzein and genistein – preferentially bind to estrogen receptors in the body (16).

However, these conclusions are based on animal studies. Human studies might give a different picture. Moreover, currently available soy-based formulas show no overt toxicities in healthy infants (16), (17). So, ask your pediatrician if you can give soy-based formulas to your kid.

7. Unsure Osteoprotective Effects

Soy protein has shown to conserve bone mineral density, particularly in post-menopausal women. Soy isoflavones were shown to prevent bone loss from the lumbar spine in perimenopausal women. They can stimulate bone formation and/or inhibit bone resorption (18).

Despite such potential benefits, the mineral balance may be impaired with soy consumption. This is probably because soybeans contain (by weight) about 1–3% phytic acid. Phytic acid is an anti-nutrient that binds to minerals like zinc, iron, and calcium. Along with soy isoflavones, phytic acid may decrease their bioavailability (18).

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

In short, it is all unclear.

All the research conducted on soy protein and soy foods remains doubtful and unclear. What do we do then? Do we totally abstain from soy protein? Is it alright to eat any kind of soy products for protein?

Read the next section for the answers.

Which Soy Products Should You Avoid?

Well, you don’t have to abstain completely from eating soy. 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 whole natural foods. Avoid protein isolates, isoflavone supplements, or industrial soy foods, especially if you have iodine deficiency or thyroid imbalance (15).

Whole soy foods are mostly fermented and cooked comparatively healthier. No wonder Asians swear by it! Tofu, edamame, soy milk, tempeh, miso, and natto can be taken a few times a week (15), (20).

Here’s the next obvious question…

How Much Soy Is Safe To Eat In A Day?

According to the American Heart Association and US FDA, daily consumption of 25 g or more of soy protein with isoflavones is not harmful (21).

In fact, several clinical studies suggest that consumption of 50 g/day of soy protein is highly beneficial. It can protect you from cardiovascular disease, diabetes, and estrogen-dependent cancers (21).

What if you take more than 50 g/day of soy protein? Scroll down to find out!

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 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 (22).

That’s why it is mandatory that you talk to your healthcare provider about the intake limit.

In Hindsight…

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 soy protein/foods. Soy isoflavones may disturb the hormonal balance, stimulate cancer proliferation, and trigger allergies.

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

If you found this article interesting, please leave in your feedback, suggestions, and queries in the comments section below.

Have a ‘soy’ful meal today!


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  2. Soy Protein” The Journal of Perinatal Education, US National Library of Medicine.
  3. Soy protein isolate” National Nutrient Database for Standard Reference Legacy Release, Agricultural Research Service, United States Department of Agriculture.
  4. Soy and phytoestrogens: possible side effects” GMS German Medical Science, US National Library of Medicine.
  5. Straight Talk About Soy” The Nutrition Source, Harvard T.H.Chan School Of Public Health.
  6. Effects of soy protein and soybean isoflavones on thyroid…” Thyroid, US National Library of Medicine.
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  8. Unawareness of the effects of soy intake on the…” Pediatrics, US National Library of Medicine.
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  13. Dietary Isoflavones and Breast Cancer Risk” Medicines, US National Library of Medicine.
  14. Soy Consumption May Promote Overexpression…” On Cancer, Memorial Sloan Kettering Cancer Center, Sloan Kettering Institute.
  15. A possible cause of Alzheimer’s dementia- Industrial soy foods” Medical Hypotheses, Elsevier, Academia.
  16. Early Exposure to Soy Isoflavones and Effects on…” Nutrients, US National Library of Medicine.
  17. Concerns for the use of soy-based formulas in infant nutrition” Paediatrics & Child Health, US National Library of Medicine.
  18. The Effect of Soy Food Intake on Mineral Status…” Food Science and Human Nutrition Publications, Iowa State University Digital Repository.
  19. The Significance of Soy Protein and Soy Bioactive…” Journal of Osteoporosis, US National Library of Medicine.
  20. Confused about eating soy?” News briefs, Harvard Health Letter, Harvard Health Publishing, Harvard Medical School.
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  23. The pros and cons of phytoestrogens” Author manuscript, HHS Public Access, US National Library of Medicine.

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