Soy is a cornerstone of the American food supply, though most is not consumed directly. Nearly 80% of soy grown in the US is processed for oil, with the leftover protein-rich meal fed primarily to poultry, pigs, and cattle. As a human food, soy takes many forms: edamame (young soybeans), soy milk, tempeh (fermented soybeans), and tofu (made by curdling soy milk). Despite its long history in Asian diets, soy in the US has generated controversy pertaining to hormones, cancer risk, thyroid effects, and environmental sustainability. However, most of these concerns are overstated, while the benefits of soy are substantial.

Common Concerns and Their Reassurance

The most frequent worry is that soy’s phytoestrogens, plant isoflavones, might feminize men or lower testosterone. Yet studies consistently find no reduction in testosterone, no increase in estrogen, and no compromised sperm. Phytoestrogens are far weaker than natural estrogens and may actually block stronger endogenous estrogens in men. In women, this mild estrogenic effect may help alleviate hot flashes and modestly support bone health.

Soy has also been viewed with caution by breast cancer survivors, but large cohort studies show that moderate soy intake does not increase recurrence and may improve survival. Thyroid function is another concern, yet research shows no adverse effects in iodine-sufficient adults. The only caution is for those taking levothyroxine, as soy can reduce medication absorption if consumed at the same time.

True Concerns to Acknowledge

Soy is a recognized food allergen, though prevalence is relatively low, affecting 0.2 to 0.9% of U.S. adults, with higher rates in children. Concerns about soy protein in animal feed passing into meat, milk, or eggs and causing allergic reactions remain theoretical, with little clinical evidence. Overconsumption of soy supplements, particularly high-dose isoflavone extracts, has not been studied extensively and is not advised.

The Benefits of Soy

Soy contributes fiber, unsaturated fats, and phytochemicals associated with reduced risk of cardiovascular disease, diabetes, and obesity. Unique to soy, its protein modestly lowers LDL cholesterol and may help preserve bone in postmenopausal women.

Nutritionally, soy provides one of the highest-quality plant proteins. On a calorie basis, soy protein is more digestible than beans or lentils and nearly digestible as animal sources such as chicken breast or egg whites. However, although it contains all nine essential amino acids (EAAs), soy is low in methionine, making its protein somewhat less complete than animal-based protein.

Environmental Considerations

Soy is often implicated in deforestation, particularly in South America. However, the environmental burden is driven largely by soy grown for livestock feed, not soy consumed directly as food. For environmentally conscientious consumers aiming to protect rainforests and improve climate health, the greater impact does not come from avoiding tofu or soy milk but from reducing consumption of animal products like chicken, pork, beef, and dairy, which depend upon vast amounts of soy meal.

Conclusion

Soy is safe and beneficial for most people. Soy’s phytoestrogens do not disrupt male hormones, its interaction with thyroid medication can be managed by separating doses, and true allergy cases are uncommon. At the same time, soy supports cardiovascular and bone health, eases menopausal symptoms, and provides ample plant-based protein. From an environmental perspective, choosing soy foods directly rather than animal products raised on soy offers a double advantage for both human health and climate sustainability.

About the Author

Patrick Traynor, PhD, MPH, RD, CSOWM, CPT, is a registered dietitian and founder of MNT Scientific, LLC (MNTScientific.com), an insurance-based nutrition practice serving South Lake Tahoe, CA; Minden, NV; and Ashland, OR. He holds the Interdisciplinary Specialist Certification in Obesity and Weight Management (CSOWM) from the Commission on Dietetic Registration. Virtual appointments are available in-office or via telehealth. For inquiries or appointments, visit MNTScientific.com, dial (530)429-7363, or email securefoodphoto@mntscientific.hush.com.