Soy and hormones: sorting the claims from the rumours

Soy is the backbone of Vietnamese vegan cooking — tofu, soy milk, tofu skin, fermented bean curd, soy sauce. And because it's everywhere, it attracts more rumours than anything else.

This sorts it out: what has a basis, what doesn't, and where a doctor genuinely should be asked.

What isoflavones are

The rumours start with a group of compounds in soy called isoflavones. They're classed as "phytoestrogens" — plant estrogens — because their structure somewhat resembles human estrogen.

But "somewhat resembles in structure" doesn't mean "acts the same". Two points worth knowing:

• Far weaker effect than the body's own estrogen.
• Binds to receptors differently, with different preferences, so the effect isn't the same as real estrogen.

The word "estrogen" in the name is the source of most of the confusion.

Three common rumours

"Soy feminises men"

The most widespread claim, and it spread largely on the back of a few individual cases repeated endlessly — cases involving enormous daily quantities over long periods, far beyond ordinary eating.

At ordinary intakes — tofu with a meal, a glass of soy milk — this isn't an established effect. That vegetarians across Asia have eaten soy daily for generations is worth noting as context.

"Soy causes breast cancer"

This runs contrary to current understanding, which doesn't show that concern at ordinary intakes. But anyone in treatment for, or with a history of, hormone-related cancer should ask their treating doctor rather than looking it up — the answer depends on the type of disease, the stage, and the medication involved.

"Soy harms the thyroid"

This one has partial basis but gets overstated. The real issue is confined to two narrow cases:

• People already low in iodine. Soy may add difficulty for a thyroid already short of iodine. The answer is adequate iodine, not avoiding soy.
• People taking thyroid hormone medication. Soy can affect absorption of the medication. The usual handling is to take the medication well apart from soy-containing meals — and the specific gap is a question for the doctor.

For someone with a normal thyroid and adequate iodine, this isn't a concern at ordinary intakes.

What actually deserves attention

Instead of hormone worries, these are the practical points for Vietnamese vegans:

• Don't let soy be the only protein source. Not because it's harmful, but because variety covers nutrition better and is less monotonous. Pulses, nuts, mushrooms and wheat gluten are all separate sources.
• Whole soy differs from heavily processed soy. Tofu, soy milk and boiled beans are one group; vegan sausages and packaged mock meats made from concentrated soy protein with plenty of salt and oil are another.
• Soy allergy is real and entirely separate from the hormone question. Anyone allergic has to avoid it, and that's genuinely difficult in Vietnamese vegan cooking because soy is everywhere.

Who should ask a doctor first

• Anyone in treatment for, or with a history of, hormone-related cancer.
• Anyone taking thyroid hormone medication.
• Anyone with a thyroid condition under monitoring.
• Anyone in fertility treatment or on hormone medication.
• Infants on soy-based formula — a question for a paediatrician.

This is general information, not a substitute for diagnosis or medical advice. For anyone with a condition under monitoring, the right answer depends on their own records.

Quick answers

Are isoflavones in soy hormones?

No. They're plant compounds whose structure somewhat resembles estrogen, hence the name "plant estrogen", but the effect is far weaker and they bind to receptors differently. The word "estrogen" causes most of the confusion.

Does soy affect men?

At ordinary intakes this isn't an established effect. The rumour spread on a few individual cases involving enormous quantities over long periods, far beyond normal eating.

Does soy harm the thyroid?

Only in two narrow cases: people already low in iodine, and people taking thyroid hormone medication, where soy can affect absorption. The first calls for adequate iodine rather than avoiding soy; the second for asking a doctor about spacing medication and meals.

What about a history of breast cancer?

That's a question for the treating doctor, since the answer depends on the type of disease, the stage and the medication — not something to settle by searching online.

Should soy be my only protein source?

No, though not because it's harmful — variety covers nutrition better and is less monotonous. Pulses, nuts, mushrooms and wheat gluten are all separate sources.

Are tofu and vegan sausage the same thing nutritionally?

No. Tofu, soy milk and boiled beans are whole soy; vegan sausages and packaged mock meats are made from concentrated soy protein with plenty of salt and oil.