People avoid soy for a mix of reasons, some rooted in real biology and others in widespread misunderstanding. The most common concerns involve hormones, thyroid function, genetic modification, allergies, and environmental impact. Some of these worries hold up under scrutiny, while others have been largely debunked by clinical research. Here’s what’s actually going on with each one.
The Estrogen Fear
This is the big one. Soy contains compounds called isoflavones, which are structurally similar to estrogen. Because of that resemblance, a persistent belief has taken hold that eating soy will raise estrogen levels, feminize men, or disrupt hormonal balance. The internet amplified this into a cultural phenomenon, complete with the term “soy boy” used to mock men who consume it.
The clinical evidence tells a different story. A 2025 systematic review and meta-analysis of randomized controlled trials found that soy isoflavones had no statistically significant effect on any measure of estrogenicity in the human body. Estradiol levels, the primary form of estrogen, didn’t meaningfully change. Neither did other hormonal markers. Separate research has confirmed that isoflavone supplementation had no effect on androgens (the family of hormones that includes testosterone) in postmenopausal women. The weak estrogenic activity of isoflavones in a lab dish simply doesn’t translate to hormonal disruption at normal dietary levels.
Concerns About Men’s Fertility
Closely tied to the estrogen worry is the idea that soy harms male reproductive health. One study from a fertility clinic found that men with the highest soy food intake had, on average, 41 million sperm per milliliter less than men who ate no soy. That’s a striking number, and it got a lot of attention. But it came with important context: the association didn’t affect sperm motility, morphology, or ejaculate volume, and the study population was men already seeking fertility treatment, not a representative sample. A separate trial that gave young men 40 milligrams of isoflavones daily for two months found no appreciable changes in semen quality or reproductive hormones.
The picture is mixed enough that men actively trying to conceive sometimes choose to limit soy as a precaution. But for general consumption, the weight of evidence doesn’t support the idea that moderate soy intake impairs male fertility or testosterone levels.
Thyroid Function
Soy’s relationship with the thyroid is more nuanced and represents one of the more legitimate reasons some people steer clear. Isoflavones can interfere with thyroid hormone production, particularly in people who are already iodine-deficient. Iodine is essential for making thyroid hormones, and if your intake is marginal, soy compounds can tip the balance toward underactive thyroid function.
For people already taking thyroid medication, there’s a practical concern: soy can reduce absorption of the medication, meaning the dose may need to be increased. This isn’t dangerous, but it does require monitoring. If you have healthy thyroid function and adequate iodine intake (common in countries that use iodized salt), moderate soy consumption is unlikely to cause problems. But for the roughly 5% of the population with some form of hypothyroidism, this concern is worth paying attention to.
GMO and Pesticide Worries
The vast majority of soybeans grown in the United States are genetically modified to tolerate glyphosate-based herbicides. For many people, “soy” and “GMO” are practically synonymous, and avoiding one means avoiding the other. The concern is twofold: that the genetic modification itself is harmful, and that the crops carry higher pesticide residues.
On the genetic modification side, a controlled feeding study found that rats consuming GM soybeans showed no significant changes in body weight, blood chemistry, or gut bacteria composition compared to rats eating conventional soybeans. These crops have been in cultivation for over 20 years, and major scientific bodies have consistently found no evidence of harm from the genetic modification itself.
Glyphosate residue is a separate question, and one that generates more scientific debate. People who want to avoid it entirely can choose organic soy products, which are grown without glyphosate. But the decision to avoid all soy because of GMO concerns goes further than the evidence warrants for most people.
Soy Allergies
Soy is one of the eight major food allergens, and for people with a true soy allergy, avoidance isn’t a lifestyle choice. It’s a medical necessity. Soy allergy is most common in infants and young children. An estimated 5 to 14% of infants who are allergic to cow’s milk also develop soy allergy when soy-based formula is introduced. Symptoms can range from atopic dermatitis (a form of eczema) to more serious reactions like asthma, wheezing, and throat swelling.
Most children outgrow soy allergy by age 10, but it can persist into adulthood. Because soy is used as an ingredient in an enormous range of processed foods, from bread to canned tuna to chocolate, people with soy allergies often develop a deep-seated wariness of the ingredient that extends to their families and social circles. This likely contributes to the broader cultural suspicion of soy beyond the allergic population itself.
Breast Cancer Confusion
For years, the estrogen-like properties of soy isoflavones led to warnings that women with breast cancer or at high risk should avoid soy entirely. The logic seemed straightforward: many breast cancers are fueled by estrogen, so anything resembling estrogen must be dangerous. This recommendation persisted for decades and still circulates widely.
Current evidence points in the opposite direction. A meta-analysis found that soy isoflavone consumption actually reduces the risk of breast cancer in both pre-menopausal and post-menopausal women. One large study reported that high soy intake during adolescence and adulthood was associated with a 47% reduction in pre-menopausal breast cancer risk. Women consuming more than 15 milligrams per day of soy isoflavones had significantly lower rates of breast cancer diagnosis compared to those consuming less. The isoflavones appear to act as weak estrogen-blockers in breast tissue, competing with the body’s own stronger estrogen for receptor sites.
Soy Formula for Infants
Parents sometimes avoid soy because of concerns about soy-based infant formula. Babies fed soy formula consume relatively high amounts of isoflavones for their body weight, which raises questions about developmental effects during a sensitive growth period. The American Academy of Pediatrics has stated there is no conclusive evidence that dietary soy isoflavones adversely affect human development, reproduction, or endocrine function. However, the National Toxicology Program took a more cautious position, saying the overall evidence was insufficient to conclude whether soy formula does or does not produce developmental effects in boys or girls.
That ambiguity is enough to make many parents uncomfortable, and pediatricians generally recommend soy formula only when there’s a specific reason to avoid cow’s milk formula, such as galactosemia or a strong cultural preference for plant-based feeding.
Environmental and Ethical Reasons
Some people avoid soy not because of what it does to their body but because of what its production does to the planet. Soy farming is a major driver of deforestation, particularly in South America. Nearly 80% of the world’s soybean crop goes to livestock feed for beef, chicken, egg, and dairy production rather than directly to human plates.
This creates an ironic dynamic: people who eat soy directly as tofu or soy milk consume far less of it than people who eat animal products raised on soy feed. Still, some environmentally motivated consumers avoid soy products altogether, concerned about supporting an industry tied to habitat destruction. Others specifically choose organic or domestically grown soy to sidestep the deforestation issue while still including it in their diet.