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  • Joanna Injore

Phytoestrogen and cancer: can I eat soy/soya?

What are phytoestrogens?

Phytoestrogens are a compound that occur naturally in plants the term ‘phyto’ means plant so they are a type of plant-oestrogen.

Phytoestrogens have a similar chemical structure to our human hormone oestrogen BUT it actually has a much weaker effect on the human body.

There are 3 main types of phytoestrogens:

1. Isoflavones,

2. Coumestans

3. ligans (and smaller group prenylflavonoids)

phytoestrogen types

Isoflavones are the most common type in found in soya bean, chickpeas, legumes. Soya is the main plant that produces isoflavones (1-2mg/g); generally, only small amounts are found in other edible plants 1.

Lignans include enterolactone and enterodiol. They are found in oilseeds (primarily linseed), cereal bran, legumes, grains and some vegetables.

Coumestans include alfalfa and red clover.

Most plants contain a mixture of all these phytoestrogens

Gut bacteria

The gut bacteria in some individuals can mean they metabolise/break down some types of isoflavones. For example the soy isoflavone daidzein can be metabolised into another product called equol. Research in equol has showed some benefits of reducing hot flushes

Phytooestogens are they safe in breast cancer?

is soya safe for cancer patients?

This is one of the most well debated topics in cancer and nutrition particularly for specific types of cancer such as oestrogen positive breast cancers.

Concern is whether consuming a plant oestrogen could have an oestrogen type effect binding to oestrogen receptors in the body promoting cancer development.

Research suggests they do the opposite and can prevent oestrogen from binding to the receptors 2

Epidemiological research in Chinese women who generally have a high soy diet (30g / day) showed they had lower risk of breast cancer. This adds to research that eating a soy rich diet in childhood can reduce girls’ risk of breast cancer later in life.

So the research overall shows soya isoflavones have a protective effect against cancer 3.

What about phytoestrogen and isoflavone supplements?

The difficulty in this area is that there is limited evidence of soya supplements compared to natural whole food soya. The safety of soya supplements and the risk of cancer breast cancer is inconclusive too.

Overall the evidence on isoflavone supplements on breast cancer risk and recurrence is too variable and inconclusive to recommend in groups at risk of breast cancer.

So general advice is not to have supplements but consuming natural products is safe 4

How you can get natural phytoestrogen from your diet?

Top food sources:

Soya milk and products, Soya flour, soya beans

Legumes: Beans: Edamame beans, fava beans, chickpeas

Seeds: Linseeds, pumpkin seeds, sesame seeds, nuts

Tofu, tempeh

Are there any benefits for including phytoestrogens in your diet?

Including phytoestrogens in your diet can help relieve some menopausal symptoms such as hot flushes 5. Foods containing phytoestrogens are generally high in fibre and a good protein alternative. Soya products are also low in saturated fat and have a good range of vitamins, minerals and plant nutrients which will have the added benefit of improving cholesterol levels.

How much food soy isoflavones should I be eating?

Aim for 45mg-90mg soy isoflavones a day which is approximately 2-3 portions, spread out through the day. (help with hot flushes and bone health)

It can take 3-4 months for benefits to be seen. The effect is also variable possibly due to the effect of how isoflavones are broken down by the gut bacteria.

Extra information for breast cancer ladies

Tamoxifen or raloxifene work by binding to oestrogen receptions so could be concern that isoflavones could reduce the effect of these medications. This is thought to be more of a concern with supplements rather than food sources 6-7

NICE (National Institute for Health and care Excellence) who provide national advice and guidance for health have the following advice:

The following herbal supplements are not recommended with menopausal symptoms who have breast cancer:

Red clover,

Black cohosh


Vitamin E

Take home message:
Include some phytoestrogen foods in your diet is safe in cancer and breast cancer patients but avoid supplements. Including phytoestrogen foods particularly soy isoflavones is linked to other health benefits so consider adding them to your daily diet!

  1. BMJ 2002; 324 doi: (Published 05 January 2002) BMJ 2002;324:52

  2. Vitale DC, Piazza C, Melilli B, Drago F, Salomone S. Isoflavones: estrogenic activity, biological effect and bioavailability. Eur J Drug Meta Pharmacokinet. 2013;38(1):15-25.

  3. Jargin SV. Soy and phytoestrogens: possible side effects. Ger Med Sci. 2014;12:Doc18. Published 2014 Dec 15. doi:10.3205/000203. Available at

  4. Marsden J, Marsh M, Rigg A. British Menopause Society consensus statement on the management of estrogen deficiency symptoms, arthralgia and menopause diagnosis in women treated for early breast cancer. Post Reproductive Health. 2019;25(1):21-32. doi:10.1177/2053369118824920 available at


  6. Déchaud H, Ravard C, Claustrat F, de la Perrière AB, Pugeat M. Xenoestrogen interaction with human sex hormone-binding globulin (hSHBG). Steroids. 1999;64(5):328-334.

  7. Oseni T, Patel R, Pyle J, Jordan CV. Selective estrogen receptor modulators and phytoestrogens. Planta Med. 2008;74(13):1656-1665.)

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