Soy foods

Also called: tofu, soya, edamame, isoflavones.

Last reviewed 2026-09-16. How this works.

Safe to eat after a breast cancer diagnosis Grade B

Good human evidence, with a limit worth knowing.

Many women are told to avoid soy after breast cancer, and the studies looking for that harm did not find it.

The feared harm was looked for repeatedly and not found, including alongside tamoxifen.

The claim

The claim here runs the other way from most of this book. It is the widespread advice that women with breast cancer should avoid soy, because its plant oestrogens will feed a hormone-sensitive tumour or block tamoxifen.

What it does

Isoflavones do bind oestrogen receptors, which is why the worry arose and why it sounds convincing. What matters is what happens in a person, and that was measured rather than assumed.1

The evidence

A systematic review screened 4,179 records and included 131 articles, among them 40 randomised controlled trials. It reports a lack of evidence of harm from soy with respect to breast cancer risk or recurrence.1

The mechanism was tested directly rather than argued about. Human trials show that soy does not increase circulating estradiol and does not affect oestrogen-responsive target tissues.1

The specific worry about tamoxifen was also examined. Prospective data on soy in women taking tamoxifen do not indicate an increased risk of recurrence, and a later review reports that the observational data do not show isoflavones interfering with tamoxifen or aromatase inhibitors.12

This entry is in the book as a correction. A great deal of distress has been caused by advice that was never tested, and when it was tested it did not hold.2

The studies this rests on

  • Randomised trials of soy and isoflavones pooled in a systematic review1

    A meta-analysis, summarising randomised trials, measured a laboratory marker, not an outcome.

    soy did not increase circulating estradiol and did not affect oestrogen-responsive target tissues.

  • Observational follow-up of soy intake after breast cancer diagnosis12

    An observational study, measured a clinical outcome.

    no indication of increased recurrence risk, including in women taking tamoxifen.

What is absent

No randomised trial has assigned women with breast cancer to eat soy or not and counted recurrences. The absence of harm rests on randomised measurement of the mechanism and on observational follow-up of outcomes.

Scope: soy intake with a recurrence endpoint.

Established by trial scope, checked 2026-09-16. Result: the randomised trials pooled in this record measured hormone levels and tissue effects; the recurrence data are observational.12

What to ask

Is there any reason for me to keep soy out of my diet?

The dish-to-person gap

Soy is eaten as food rather than taken as a measured compound, and the register holds no human pharmacokinetic measurement for it.

When to call your team — no warning signs are recorded

If you have changed how you eat because of something you have read, or are thinking about it, the people treating you can only weigh that if they know about it. That is true when nothing has been written down here, and it is most true then.

The register records no warning signs for this entry. That is not a statement that nothing can go wrong.

Interactions

This is one of the few entries where the register can record that somebody looked. Human trials found that soy does not raise circulating estradiol or affect oestrogen-responsive tissues, and observational data in women taking tamoxifen or aromatase inhibitors do not show it interfering with those treatments.12

The whole interaction list, and how to turn it into a pharmacy review request.

Find a trial instead

This is an ordinary food rather than an intervention with recruiting trials attached.

References

  1. Soy, red clover, and isoflavones and breast cancer: a systematic review. PLOS One 2013;8(11):e81968.

    doi:10.1371/journal.pone.0081968

    How this was reached: transcribed from chapter note, via docs/CHAPTER-NOTES.json, chapter 10 note 4.

  2. A review of the clinical and epidemiologic evidence relevant to the impact of postdiagnosis isoflavone intake on breast cancer outcomes. Current Nutrition Reports 2025.

    doi:10.1007/s13668-025-00640-5

    How this was reached: transcribed from chapter note, via docs/CHAPTER-NOTES.json, chapter 10 note 5.