The Truth About Soy: What You Need to Know

Soybeans

Soy has long been hailed as a superfood, especially in plant-based diets. From tofu to soy milk, edamame to soy protein powders, it’s everywhere. But not all soy is created equal — and depending on the form, it can either support your health or create problems.

Let’s break down the difference between natural vs. processed soy, its effects on thyroid health, potential benefits, and things to watch out for.

Natural vs. Processed Soy

Natural, minimally processed soy includes foods like:

• Edamame (whole soybeans)

• Soy nuts

• Traditional fermented soy (tempeh, miso, natto)

These forms are nutrient-dense, high in protein, fiber, and phytonutrients. Fermentation also reduces anti-nutrients like phytates and makes the soy more digestible.

Highly processed soy includes:

• Soy protein isolates (found in protein powders and bars)

• Soy protein concentrates

• Soy-based fillers in packaged foods

These often have fewer nutrients, more additives, and higher levels of compounds that can interfere with mineral absorption and digestion.

Soy and Thyroid Health

Soy contains isoflavones, a type of phytoestrogen. While these compounds can provide health benefits (more below), they may also interfere with thyroid hormone production, especially in people with low iodine or pre-existing hypothyroidism.

• Isoflavones can inhibit the enzyme thyroid peroxidase, which is needed to make thyroid hormones.

• For most healthy adults with sufficient iodine, moderate soy intake is unlikely to cause thyroid problems.

• For those with low thyroid function or iodine deficiency, it’s wise to keep soy consumption low and not cosume highly processed soy foods.

Fermented soy (like miso and tempeh) tends to have less impact on thyroid function than unfermented forms.

Health Benefits of Soy

Soy offers several potential benefits:

• Heart health: Isoflavones may help lower LDL cholesterol and support vascular function.

• Bone health: Isoflavones may help maintain bone density, especially in postmenopausal women.

• Cancer protection: Some studies suggest soy may reduce risk of hormone-related cancers (breast, prostate), though research is mixed.

• Protein source: Especially valuable in plant-based diets as a complete protein with all essential amino acids.

Potential Issues and Considerations

While soy can be beneficial, there are some caveats:

• Thyroid interference: As noted, particularly in those with iodine deficiency.

• Digestive discomfort: Soy contains oligosaccharides that may cause gas and bloating in some people.

• Hormonal effects: Isoflavones are weak phytoestrogens; while generally safe, very high intakes may have subtle hormonal effects in sensitive individuals.

• Processed soy: Often highly refined and may contain GMOs, additives, and less nutrient density.

Bottom Line

Soy can be a healthy part of your diet if consumed in natural, minimally processed forms. Fermented soy is especially easy on digestion and rich in nutrients. Highly processed soy products should be consumed sparingly, particularly if you have thyroid issues or are sensitive to phytoestrogens.

Moderation and form matter: choose whole or fermented soy, pair it with iodine-rich foods if you’re thyroid-sensitive, and watch for processed soy in packaged foods. As with most health topics or foods, there is no one right answer. There is no one-size-fits-all. What will work for one, may not work for another. Soy is not a bad food, nor is it necessarily a “superfood”. Whole soy foods do have health benefits for most people when consumed in a balanced diet.

References

• Messina M. Insights gained from 20 years of soy research. J Nutr. 2010;140(12):2289S–2295S.

• Messina M, Redmond G. Effects of soy protein and soybean isoflavones on thyroid function in healthy adults and hypothyroid patients. Nutr Rev. 2006;64(10):S61–S68.

• Sacks FM, et al. Soy protein, isoflavones, and cardiovascular health: an American Heart Association Science Advisory. Circulation. 2006;113:1034–1044.

• Setchell KD, Cassidy A. Dietary isoflavones: biological effects and relevance to human health. J Nutr. 1999;129:758S–767S.

• Hirose S, et al. Impact of fermented and unfermented soy foods on thyroid health. J Clin Endocrinol Metab. 2016;101:444–450.