Is Soy Bad for You? Debunking Myths with Science

Soy is one of the most debated foods out there. For years, it's been praised as a plant-based powerhouse - a complete protein with heart-health benefits and a sustainability profile that puts many animal products to shame. But it's also been wrapped in controversy, especially when it comes to women’s health and hormones.

Maybe you’ve heard claims like "soy causes hormonal imbalances", "soy promotes breast cancer", or - an all-time favourite: "soy isn’t safe for men". These myths have been repeated so often, they feel like facts. But are they actually true?

At WAM, we use non-GMO soy protein isolate in our products because it's sustainable, complete, and effective. And we know that for many people, choosing a protein powder isn’t just about nutrition - it’s about trust. That’s why we’re tackling these concerns head-on, using science to set the record straight.

Myth #1: Soy Disrupts Your Hormones

The biggest fear around soy comes from its isoflavones - natural compounds found in soybeans that are sometimes called "phytoestrogens". But soy isn’t the only source: isoflavones are also found in smaller amounts in foods like chickpeas, fava beans, peanuts, alfalfa sprouts, and even drinks like coffee and beer. Yet soy continues to be singled out, despite being one of many plants that naturally contain these compounds.

That term alone can be misleading. Phytoestrogens aren’t hormones. They are plant-derived compounds that can bind to estrogen receptors in the body, but they behave very differently from the hormone estrogen itself. In fact, they bind preferentially to a specific receptor subtype (called ER-beta), which is typically linked to protective effects in the body rather than hormonal overstimulation. 

The fear that soy mimics estrogen and causes hormonal havoc largely stems from animal studies in the 1990s. But here’s the issue: rodents metabolise isoflavones very differently than humans do. They often receive unrealistically high doses in lab settings, far beyond what a human would consume from diet or supplements.

So what happens when actual humans eat soy? A 2025 systematic review and meta-analysis of 40 clinical trials found that soy isoflavones had no significant impact on estrogen levels, endometrial thickness, or vaginal maturation in postmenopausal women. In plain terms: soy didn’t increase estrogen activity or behave like hormone replacement therapy.

Instead of disrupting hormones, soy seems to support balance, especially during hormonal transitions like menopause.

Myth #2: Soy Is Feminizing Men

It’s not just women who are targeted by soy myths. Over the past two decades, there’s been growing anxiety about soy causing "feminizing effects" in men - things like lowered testosterone, reduced sperm quality, or even gynecomastia (enlarged breast tissue).

These fears have been fuelled by a few isolated case reports and a heavy dose of media hype. But again, the actual clinical evidence paints a very different picture.

A comprehensive 2010 review examined over a dozen high-quality human trials and found that neither soy foods nor isoflavone supplements had any significant effect on testosterone or estrogen levels in men. Sperm count, semen quality, and other markers of male reproductive health were also unaffected.

Even when soy intake was significantly higher than what’s typical in Asian diets or isolated isoflavones were supplemented, no feminizing effects were observed. So unless you're consuming gallons of soy milk a day (which we wouldn’t recommend for many reasons), there's no reason to worry.

Myth #3: Soy Increases the Risk of Breast Cancer

This myth has understandably created a lot of fear, particularly among women with a history of breast cancer or a family predisposition.

The concern stems from the fact that many breast cancers are hormone-sensitive, and since soy contains phytoestrogens, people feared it might stimulate tumour growth. But what we now know is that soy isoflavones do not behave like estrogen in breast tissue.

A technical review of over 400 human studies concluded that isoflavones do not increase breast density, estrogen levels, or markers of cancer risk in women. In fact, observational data from countries with high soy intake (like Japan and China) show lower rates of breast cancer, not higher.

Some studies even suggest that soy consumption during adolescence or early adulthood may offer a protective effect against breast cancer later in life.

Myth #4: Soy Hurts Your Thyroid

Some people worry that soy interferes with thyroid function. This concern, again, comes from lab studies and animal research showing that isoflavones may block an enzyme involved in making thyroid hormones. But here's the key point: humans are not rats, and studies in people tell a much more reassuring story.

A large clinical review and more recent human studies found that soy has no negative effect on thyroid function in people with normal thyroid levels. Even when soy intake was high, thyroid hormone production remained stable.

If you take thyroid medication (like Synthroid), soy may slightly reduce how much is absorbed - but this also happens with many other foods. That’s why thyroid meds should be taken on an empty stomach. Soy is still fine to eat at a different time of the day - when in doubt, ask your healthcare provider!

What About Soy Protein Isolate?

Soy protein isolate is a highly refined form of soy used as an ingredient in many food products and also directly in protein powders and supplements. It’s made by removing most of the carbohydrates and fats from defatted soy flour, leaving a concentrated source of high-quality, complete protein - typically around 90-95% by weight.

Some people worry that soy protein is overly processed or "unnatural". But the reality is that it’s been safely used for decades in sports nutrition, clinical settings, and food applications. It’s a complete protein, meaning it contains all nine essential amino acids - and it’s particularly rich in branched-chain amino acids (BCAAs), which support muscle recovery and growth.

Processing also reduces the isoflavone content. During the production of soy protein isolate, the soybeans are defatted and then subjected to multiple washing and extraction steps using water or alcohol. These steps are designed to isolate the protein - but they also remove a significant portion of the isoflavones along with the carbohydrates and other non-protein components. As a result soy protein isolate contains far fewer isoflavones than whole soy foods.

For women who lift, train hard, and want plant-based gains, soy protein isolate is a powerful ally.

So, Should You Eat Soy?

If you enjoy soy foods and tolerate it well, there is really no reason to avoid it. In fact, there are many reasons to include it:

• High-quality protein: complete, digestible, and effective for muscle-building.

• Heart-health benefits: linked to improved cholesterol profiles.

• Sustainable nutrition: growing soy requires fewer resources than most animal proteins.

• Affordable and versatile: from edamame to soy milk to protein powder.

The fear around soy has been disproportionate to the evidence. Decades of research - in both men and women - support its safety. For most people, moderate soy consumption is not only safe, but beneficial.

Let’s Lift the Soy Stigma

It’s time to stop letting outdated myths hold women back from strong, evidence-based choices.

Soy isn’t an endocrine disruptor. It isn’t a feminizing food. And it isn’t something to fear. Instead, it’s a well-studied, safe, and powerful plant-based protein that can support strength, health, and recovery - without compromise.

Sources

[1] Viscardi G, Back S, Ahmed A, Yang S, Mejia SB, Zurbau A, Khan TA, Selk A, Messina M, Kendall CW, Jenkins DJ, Sievenpiper JL, Chiavaroli L. Effect of Soy Isoflavones on Measures of Estrogenicity: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. Adv Nutr. 2025 Jan;16(1):100327.

[2] Messina M. Soybean isoflavone exposure does not have feminizing effects on men: a critical examination of the clinical evidence. Fertil Steril. 2010 May 1;93(7):2095-104.

[3] Messina M, Mejia SB, Cassidy A, Duncan A, Kurzer M, Nagato C, Ronis M, Rowland I, Sievenpiper J, Barnes S. 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.

[4] Fritz H, Seely D, Flower G, Skidmore B, Fernandes R, Vadeboncoeur S, Kennedy D, Cooley K, Wong R, Sagar S, Sabri E, Fergusson D. Soy, red clover, and isoflavones and breast cancer: a systematic review. PLoS One. 2013 Nov 28;8(11):e81968.

[5] Messina M, Messina V. The role of soy in vegetarian diets. Nutrients. 2010 Aug;2(8):855-88.