The soy myths, and why we still cook with it

Where the thyroid worry came from

The thyroid story traces back to early studies, many of them in rats, which suggested that soy isoflavones might interfere with thyroid hormone production. Rats metabolise isoflavones differently than we do, and the doses were high.

A 2019 systematic review and meta-analysis in Scientific Reports pulled together the human trials. In healthy adults with enough iodine in their diet, soy protein and isoflavone supplements produced a small rise in TSH and no meaningful change in the actual thyroid hormones, fT3 and fT4. The authors describe the clinical significance as unclear, which is a scientist saying "we could measure it and it does not appear to matter".

The estrogen question

Soy contains phytoestrogens, which are plant compounds that can attach to the same receptors human estrogen uses. That much is true, and it is where the fear comes from. What the word "phytoestrogen" hides is that binding to a receptor is not the same as acting like estrogen. Depending on the tissue and the hormonal context, they can behave as weak agonists or as antagonists, which means in some places they nudge the signal up a little and in others they get in the way of it.

Human trials have found that soy does not raise circulating estradiol and does not change estrogen-responsive tissue the way actual estrogen does.

Soy and breast cancer

This is the one people ask about most, usually because someone told them soy was dangerous for anyone with a breast cancer history. The evidence points the other way.

Long-term observational data has not shown harm from soy in relation to breast cancer risk or recurrence. The American Cancer Society has concluded that soy food consumption before diagnosis is associated with lower overall mortality, and that soy intake, whether before or after diagnosis, is associated with a lower risk of recurrence. Intake at the level of a traditional Japanese diet, roughly two to three servings a day, may be protective.

The distinction researchers keep drawing is between soy foods and high-dose isoflavone supplements. Tofu, tempeh, edamame and soy milk are food. A concentrated 100 mg isoflavone capsule is not, and there is less evidence behind it. We are a food company, so we are talking about the first kind.

Why we actually cook with it

• It is a complete protein, meaning it carries all the essential amino acids, which is not true of most single plant sources.
• It is low in saturated fat, and swapping it in for higher-fat protein sources is associated with lower LDL cholesterol.
• Soy foods are often calcium-set or fortified, which matters if you are off dairy.
• Some evidence suggests the isoflavones ease menopausal symptoms like hot flashes, with milder effects than hormone therapy.
• It does more different jobs than almost anything else in the kitchen. Tofu, tempeh, edamame, soy milk, tamari, miso and soy curls are all the same bean wearing different hats.

That last one is the real reason it is on our menu every week. Soy is the most versatile protein in a plant-based kitchen, and after decades of people looking hard for a problem with it, they mostly have not found one.