Side Effects of Soy Protein
Soy protein is considered a safe product for most people, but in individual cases it can cause adverse reactions — from ordinary bloating to allergy. The editorial team has broken down which side effects have a real scientific basis, how likely they are and who should be more careful.
General safety profile
Soy protein is part of the diet of hundreds of millions of people, especially in the countries of East Asia, where soy products are consumed daily throughout life. This long-standing experience, as well as numerous clinical studies, gives grounds to consider soy protein a safe food product for most healthy adults.
At the same time, 'safe for most' does not mean 'without side effects'. Like any concentrated protein, soy isolate can cause adverse reactions in individual people. Some of them are related to digestion, some to allergy, and some to the biologically active substances of soy, primarily isoflavones.
It is important to distinguish side effects that are actually observed in studies from hypothetical risks that circulate on the internet. For example, fears about a decrease in testosterone in men were not confirmed in meta-analyses of clinical studies, whereas soy allergy is a well-documented and potentially serious condition.
Below, the editorial team examines the main groups of adverse effects, assesses their likelihood and explains who should be especially attentive.
Digestive reactions
The most frequent complaints when consuming soy protein concern digestion: bloating, gas, a feeling of heaviness, sometimes changes in stool. Whole soybeans contain oligosaccharides — raffinose and stachyose — which are not digested by human enzymes and are fermented by the microbiota of the large intestine.
In a quality isolate these carbohydrates are largely removed, so reactions to the powder are usually weaker than to soy flour or concentrate. If discomfort arises specifically from the isolate, it is worth checking the composition: the cause is often polyol sweeteners, gums, inulin or too large a volume of shake.
Practical steps for gastrointestinal symptoms are simple: start with a smaller serving, dilute the powder with a larger amount of liquid, do not drink the shake in one go and do not consume it immediately before intense exertion. If symptoms persist, it is worth trying a different type of protein.
Persistent abdominal pain, diarrhea or blood in the stool is not a typical reaction to protein and requires seeing a doctor rather than changing the brand.
Soy allergy
Soy is on the list of 14 major food allergens that must be highlighted on labelling in the EU (Regulation 1169/2011). Soy allergy occurs more often in young children and in many of them passes with time, but in some people it persists into adulthood as well.
The symptoms of allergy vary from itching in the mouth, hives and swelling of the lips to vomiting, difficulty breathing and anaphylaxis. Any difficulty breathing or facial swelling after consuming the product requires emergency medical care.
Cross-reactivity in people allergic to birch pollen is described separately. The soy protein Gly m 4 is structurally similar to the main birch allergen Bet v 1, and in sensitized people the consumption of poorly processed soy protein, particularly in the form of drinks, can cause a reaction. Severe cases are also described in the clinical literature.
People allergic to peanuts or other legumes do not necessarily react to soy, but the risk is higher than in the general population. In such cases the decision to consume soy protein is best made together with an allergist after examination.
Hormones and the thyroid gland
Soy isoflavones — genistein and daidzein — are capable of binding to estrogen receptors. Hence the fears about the 'feminization' of men. However, meta-analyses of clinical studies (Hamilton-Reeves et al., 2010; Reed et al., 2021) found no effect of soy protein or isoflavones on testosterone, free testosterone or estradiol in men.
In the medical literature there are isolated descriptions of gynecomastia or hormonal changes in men who consumed extremely large amounts of soy milk daily over a long time. Such cases do not transfer to ordinary servings of protein, but they are a reminder that extreme amounts of any product are undesirable.
For the thyroid gland two aspects are important. First, soy products can reduce the absorption of levothyroxine, so patients with hypothyroidism are advised to space out the intake of the medication and soy food in time. Second, in people with iodine deficiency soy can theoretically worsen gland function, whereas with normal iodine status no significant effect has been detected (Messina & Redmond, 2006).
For women in menopause, EFSA assessed supplements with isolated isoflavones and found no grounds to consider that at the doses studied they harm the mammary gland, thyroid gland or uterus. However, this assessment concerns isolated isoflavones, not protein, and does not cover women with a history of cancer, for whom the question of soy should be discussed with an oncologist.
Minerals, kidneys and product quality
Soy contains phytic acid, which binds iron and zinc and reduces their absorption. For people with a diet rich in a variety of foods this rarely matters, but for vegans with low iron status it is worth remembering the combination with vitamin C and monitoring ferritin.
High-protein diets, regardless of the protein source, can be undesirable for people with chronic kidney disease. For healthy people there is no convincing evidence of harm from moderately high protein intake, but in the presence of kidney disease the amount of protein is determined by a doctor.
| Adverse effect | Who is at risk | What to do |
|---|---|---|
| Bloating, gas | People with a sensitive gut | Smaller serving, isolate without polyols |
| Allergic reaction | People allergic to soy, some people allergic to birch pollen | Avoid the product, consult an allergist |
| Reduced absorption of levothyroxine | Patients with hypothyroidism | Space out intake, monitor TSH with a doctor |
| Poorer absorption of iron and zinc | Vegans with low iron stores | Vitamin C in the meal, monitor ferritin |
| Load on the kidneys | People with kidney diseases | Amount of protein — on a doctor's recommendation |
Another risk is the quality of the specific product. Unscrupulous manufacturers may overstate the protein content or fail to control contamination. Choose products with transparent labelling and, where possible, with laboratory test results of the batch.
Editorial conclusions
For most healthy adults, soy protein is a safe source of protein; the most frequent adverse effect is temporary digestive discomfort.
The most serious risk is soy allergy, particularly a cross-reaction in people allergic to birch pollen. Difficulty breathing or swelling after consumption is a reason to seek medical help immediately.
Fears about a decrease in testosterone in men are not confirmed by meta-analyses, and patients with hypothyroidism should space out the intake of levothyroxine and soy products.
Our articles 'Who Should Not Take Soy Protein', 'Myths About Soy Protein' and 'Can Soy Protein Lower Testosterone' will help continue the topic.
References
- Hamilton-Reeves JM, Vazquez G, Duval SJ, et al. Clinical studies show no effects of soy protein or isoflavones on reproductive hormones in men: results of a meta-analysis. Fertil Steril. 2010;94(3):997–1007.
- Reed KE, Camargo J, Hamilton-Reeves J, Kurzer M, Messina M. Neither soy nor isoflavone intake affects male reproductive hormones: an expanded and updated meta-analysis of clinical studies. Reprod Toxicol. 2021;100:60–67.
- Messina M, Redmond G. Effects of soy protein and soybean isoflavones on thyroid function in healthy adults and hypothyroid patients: a review of the relevant literature. Thyroid. 2006;16(3):249–258.
- EFSA Panel on Food Additives and Nutrient Sources added to Food (ANS). Risk assessment for peri- and post-menopausal women taking food supplements containing isolated isoflavones. EFSA Journal. 2015;13(10):4246.
- Messina M. Soy and health update: evaluation of the clinical and epidemiologic literature. Nutrients. 2016;8(12):754.
- Messina M, Mejia SB, Cassidy A, et al. Neither soyfoods nor isoflavones warrant classification as endocrine disruptors: a technical review of the observational and clinical data. Crit Rev Food Sci Nutr. 2022;62(21):5824–5885.
- Regulation (EU) No 1169/2011 of the European Parliament and of the Council on the provision of food information to consumers. Official Journal of the European Union. 2011;L304:18–63.
Andriy Melnyk
A strength-sports coach and author of programs for beginner and intermediate levels. Writes about training planning.
