Myths about soy protein

Soy protein has been accused of everything from lowering testosterone to causing cancer. Our editorial team weighed the most common claims against meta-analyses and clinical trials to separate genuine cautions from gym folklore.

Why soy has such a bad reputation

Few foods divide opinion in the gym like soy. For some, it is a sensible plant protein used for centuries across East Asia. For others, it is a hormone disruptor that will lower testosterone and undo months of training. Much of this fear centres on isoflavones, compounds such as genistein and daidzein that have a structure loosely similar to oestrogen.

Isoflavones are often called phytoestrogens, and the name itself does a lot of damage. They can bind to oestrogen receptors, but far more weakly than human oestrogen, and they show a preference for the beta receptor subtype. Their effects in the body depend on dose, tissue and the hormonal background of the person eating them.

Laboratory studies in cells and rodents, sometimes using very high doses or injected isoflavones, fuelled concern. A handful of case reports, such as individuals consuming extraordinary quantities of soy milk, added to the narrative. These do not reflect what happens when ordinary people eat normal amounts of soy foods or a protein shake.

Soy protein isolate, the form used in most powders, contains isoflavones, though the amount varies with processing. Some manufacturing methods remove a substantial proportion. Below we test the most common myths against human data.

Myth 1: soy lowers testosterone in men

This is the most persistent claim, and it has been examined directly. A meta-analysis by Hamilton-Reeves and colleagues pooled clinical studies and found no significant effect of soy protein or isoflavones on testosterone, free testosterone or sex hormone-binding globulin in men.

An expanded and updated meta-analysis by Reed and colleagues, which included more recent trials, reached the same conclusion: neither soy nor isoflavone intake affected male reproductive hormones at intakes well above typical Asian diets.

The studies included a range of soy foods, protein powders and isoflavone supplements, and durations from weeks to months. While very extreme intakes cannot be ruled out as problematic, the amounts found in a few servings of soy foods or a daily protein shake do not appear to alter hormones.

Concerns about "man boobs" from soy similarly lack support in controlled data. Gynaecomastia has many recognised causes, including puberty, obesity, certain medicines and anabolic steroids, which are far more likely explanations.

Myth 2: soy is useless for building muscle

Soy is a complete protein, supplying all nine essential amino acids, though it contains somewhat less leucine than whey. In a well-known acute study, Tang and colleagues found that whey stimulated muscle protein synthesis more than soy after resistance exercise, while soy outperformed casein in that particular test.

Acute differences in protein synthesis do not always translate into differences in long-term gains. A meta-analysis by Messina and colleagues comparing soy with animal protein supplementation during resistance training found no significant difference in gains in muscle mass or strength.

The practical takeaway is that total daily protein intake and consistent training matter much more than whether a particular scoop is soy or whey. Vegans and people with dairy intolerance can confidently use soy as a primary protein source.

Myth 3: soy harms the thyroid and causes cancer

Soy isoflavones can interfere with an enzyme involved in thyroid hormone production in laboratory settings. A review by Messina and Redmond concluded that in people with normal thyroid function and adequate iodine intake, soy does not cause hypothyroidism.

There is one practical point: soy may reduce the absorption of levothyroxine, the medicine used to treat an underactive thyroid. People on this medicine are typically advised to keep a gap between taking their tablet and eating soy foods, and to keep their intake consistent so that doses can be adjusted.

On breast cancer, earlier fears were based on the oestrogen-like effect. Human evidence has been reassuring. A pooled analysis of cohort studies from the US and China by Nechuta and colleagues found that soy food intake after breast cancer diagnosis was not associated with worse outcomes and may even be linked to lower recurrence.

Anyone with a hormone-sensitive cancer should still discuss diet with their oncology team, particularly regarding concentrated isoflavone supplements, which are different from whole soy foods.

Real considerations worth knowing

Debunking myths does not mean soy suits everyone. Soy is one of the 14 major allergens that must be declared on food labels in the UK, and some people, particularly children, are allergic.

• Check labels if you have a soy allergy; it appears in many processed foods and sports products.
• If you take levothyroxine, keep timing consistent and let your GP know about dietary changes.
• Choose powders with clear protein content per serving and independent testing.
• Vary your protein sources across the week rather than relying on a single food.

A review by Messina summarising clinical and epidemiological evidence describes soy as a reasonable part of a healthy diet, with potential benefits for cholesterol and menopausal symptoms in some studies.

For athletes, soy protein is simply one option among many. It is not magic and not dangerous at normal intakes.

Editorial conclusions

The fear that soy lowers testosterone or feminises men is not supported by meta-analyses of human trials.

Soy protein supports muscle growth comparably to animal protein when total intake is adequate, making it a solid choice for vegans and those avoiding dairy.

Real cautions are limited to allergy and timing with thyroid medicine; whole soy foods appear safe for most people, including those with a history of breast cancer, though personalised advice is wise.

For related reading, see our guides to plant-based protein for athletes, comparing whey and casein, and leucine and the muscle-building threshold.

References

1. 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.
2. 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.
3. 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.
4. 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.
5. 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.
6. Nechuta SJ, Caan BJ, Chen WY, et al. Soy food intake after diagnosis of breast cancer and survival: an in-depth analysis of combined evidence from cohort studies of US and Chinese women. Am J Clin Nutr. 2012;96(1):123–132.
7. Messina M. Soy and health update: evaluation of the clinical and epidemiologic literature. Nutrients. 2016;8(12):754.