Myths About Soy Protein
Few food products are surrounded by as many fears as soy. In gyms you can still hear that soy protein 'lowers testosterone', 'grows breasts' and 'does not build muscle'. The editorial team has broken down the most popular claims and compared them with what clinical studies and meta-analyses show.
Where the myths about soy came from
Most fears around soy are related to isoflavones — genistein, daidzein and glycitein. They are classified as phytoestrogens because they are capable of binding to estrogen receptors. The word 'estrogen' in the name alone was enough for a persistent image of soy as a 'female hormone in a jar' to form.
In reality, isoflavones are not estradiol. They have a much weaker affinity for the receptors and predominantly bind to the beta estrogen receptor, whereas the effects of estradiol that lead to feminization are related primarily to the alpha receptor. Because of this they are classified as selective estrogen receptor modulators rather than 'estrogen substitutes'.
The second source of myths is animal studies. Rodents metabolize isoflavones differently from humans, and in many experiments received doses unattainable with ordinary nutrition. The technical review by Messina and colleagues (2022) explains in detail why results obtained in rats and mice cannot be directly transferred to humans.
The third source is individual clinical cases, which spread well on social media. A single case may be a signal for further research, but it gives no idea of the risk to the population. Assessing risk requires controlled studies and their meta-analyses — it is precisely on them that we will rely further.
Myths about hormones: testosterone and gynecomastia
Myth 1: soy protein lowers testosterone.This is the most common claim, and it is precisely the best tested. The meta-analysis by Hamilton-Reeves and colleagues (2010) combined clinical studies and found no effect of soy protein or isoflavones on total and free testosterone, sex hormone-binding globulin and the free androgen index in men. The updated meta-analysis by Reed and colleagues (2021), which already included over 40 studies, confirmed this conclusion.
Importantly, these analyses included studies with an amount of isoflavones exceeding typical consumption even in the countries of Asia. That is, even 'large' doses of soy did not demonstrate a hormonal effect in men. Individual studies with a decrease in testosterone were not confirmed when the data were combined.
Myth 2: soy causes gynecomastia.The literature describes several clinical cases of gynecomastia in men who consumed extreme amounts of soy products — on the order of several litres of soy milk a day. After stopping such a diet, the symptoms disappeared. These cases are interesting, but they describe doses many times higher than an ordinary serving of protein and do not reflect the typical situation.
In controlled studies where men received soy protein together with strength training, no signs of feminization were observed. For example, Kalman and colleagues (2007) compared soy and whey supplements and found no clinically significant changes in sex hormones. So for a person who drinks 1–2 servings of soy shake a day, the risk of gynecomastia from soy remains theoretical.
Myths about muscles and protein quality
Myth 3: soy protein is 'incomplete'.Soy contains all nine essential amino acids. The PDCAAS score for soy isolate is close to the maximum, and the newer DIAAS scale, recommended by the FAO in 2013, places soy protein below whey but above most other plant proteins. That is, soy is one of the highest-quality plant proteins.
Myth 4: it is impossible to build muscle on soy.Acute studies do indeed show that after training a single serving of whey stimulates muscle protein synthesis more strongly than the same serving of soy (Tang et al., 2009). The reasons are the faster absorption of whey and its higher leucine content. However, protein synthesis after a single intake is not yet muscle gain over months of training.
Long-term studies give a different picture. The meta-analysis by Messina and colleagues (2018) found no difference between soy and animal protein supplements regarding the gain of muscle mass and strength against the background of strength training. And in the study by Hevia-Larraín and colleagues (2021), vegans who received enough protein predominantly from soy gained muscle the same as omnivorous participants.
| Claim | What the data show | Verdict |
|---|---|---|
| Soy lowers testosterone | Meta-analyses of clinical studies — no effect | Myth |
| Soy causes gynecomastia | Only individual cases with extreme consumption | A myth for ordinary doses |
| Soy is an incomplete protein | Contains all essential amino acids, high PDCAAS | Myth |
| It is impossible to build muscle on soy | Training studies — results comparable to animal protein | Myth |
| Soy is 'worse' than whey in acute tests | The one-time stimulation of protein synthesis is somewhat lower | Partly true |
Practical conclusion: soy's lag in acute studies can be compensated by a slightly larger serving of protein and a sufficient total amount of protein per day. For a person who gets 1.6 g/kg of protein per day and trains systematically, the choice between soy and whey will not be a decisive factor.
Myths about health: cancer, thyroid gland, antinutrients
Myth 5: soy increases the risk of breast cancer.This fear arose on the basis of studies on animals and cells. Epidemiological data in humans do not confirm it: in the Shanghai Breast Cancer Survival Study (Shu et al., 2009), women with breast cancer who consumed more soy food had lower mortality and rarer recurrences. The review by Messina (2016) summarizes that soy food is compatible with the prevention and treatment of breast cancer, although regarding concentrated isoflavone supplements the editorial team advises caution.
Myth 6: soy destroys the thyroid gland.According to the review by Messina and Redmond (2006), in people with normal gland function and sufficient iodine intake, soy does not cause hypothyroidism. The real issue lies elsewhere: soy can reduce the absorption of levothyroxine, so people taking this medication should space them out in time.
Myth 7: soy protein is full of 'antinutrients'.Whole soybeans contain trypsin inhibitors, phytates and lectins. However, in the process of producing the isolate, particularly through heat treatment, the activity of trypsin inhibitors is significantly reduced, and lectins are inactivated. Phytates partly remain and can reduce the absorption of iron and zinc, but with a varied diet this rarely has practical significance.
Myth 8: all soy is dangerous GMO.A significant part of the world's soy harvest is indeed genetically modified. However, GM crops permitted in the EU undergo a safety assessment by EFSA, and products made from them are subject to mandatory labelling. The 'non-GMO' choice is a consumer's right, but it does not change the nutritional value of the protein.
Where the myth has a grain of truth
Not all fears about soy are groundless. First, soy is a real food allergen, and for people with allergy, soy protein is contraindicated. This is not a myth but a medical fact reflected in the legislation on allergen labelling.
Second, concentrated isoflavone supplements are not the same as soy food or protein. EFSA assessed the risks of isolated isoflavones for women in peri- and postmenopause and found no harmful effect on the mammary gland, thyroid gland and uterus at the doses studied, but the data for other groups, particularly pregnant women, are limited. Therefore the conclusions about soy food should not be transferred to isoflavone tablets.
Third, product quality depends on the manufacturer. Unscrupulous 'amino spiking', contamination with heavy metals or undeclared impurities are risks that concern any proteins, not only soy ones. Choosing a product with independent testing reduces them.
- Real restrictions: allergy, interaction with levothyroxine, protein control in kidney diseases.
- Exaggerated fears: testosterone, feminization, protein 'incompleteness'.
- Requires caution: isoflavone concentrates in risk groups.
Editorial conclusions
The most common myths about soy protein — about a decrease in testosterone, feminization and protein 'incompleteness' — are not confirmed by meta-analyses of clinical studies in humans.
Soy is indeed inferior to whey in the acute stimulation of muscle protein synthesis, but in long-term training studies, with a sufficient amount of protein, this difference does not turn into less muscle mass.
The real restrictions are allergy, interaction with levothyroxine and questions related to concentrated isoflavone supplements. It is precisely on them, not on myths, that attention should be paid when choosing.
If the topic interests you, we recommend reading our articles 'The Benefits of Soy Protein for Athletes: The Evidence Base', 'Who Should Not Take Soy Protein' and 'Side Effects of Soy Protein'.
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, 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.
- Messina M, Lynch H, Dickinson JM, Reed KE. No difference between the effects of supplementing with soy protein versus animal protein on gains in muscle mass and strength in response to resistance exercise. Int J Sport Nutr Exerc Metab. 2018;28(6):674–685.
- Tang JE, Moore DR, Kujbida GW, Tarnopolsky MA, Phillips SM. Ingestion of whey hydrolysate, casein, or soy protein isolate: effects on mixed muscle protein synthesis at rest and following resistance exercise in young men. J Appl Physiol. 2009;107(3):987–992.
- Shu XO, Zheng Y, Cai H, et al. Soy food intake and breast cancer survival. JAMA. 2009;302(22):2437–2443.
- 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.
- Hevia-Larraín V, Gualano B, Longobardi I, et al. High-protein plant-based diet versus a protein-matched omnivorous diet to support resistance training adaptations: a comparison between habitual vegans and omnivores. Sports Med. 2021;51(6):1317–1330.
Andriy Melnyk
A strength-sports coach and author of programs for beginner and intermediate levels. Writes about training planning.
