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PatientPlus articles are written by UK doctors and are based on research evidence, UK and European Guidelines. They are designed for health professionals to use, so you may find the language more technical than the condition leaflets.

Phyto-oestrogens (often written as phytoestrogens) are plant substances with oestrogenic properties.

They first came to light about 50 years ago when it was observed that some plants could have an adverse effect on fertility in livestock. Phyto-oestrogens tend to have weaker effects than most oestrogens, are not stored in the body, and are readily metabolised and eliminated.

There are more than 20 compounds that can be found in more than 300 plants, such as herbs, grains, and fruits. The three main classes of dietary phyto-oestrogens are isoflavones, lignans, and coumestans:

  • Isoflavones (genistein, daidzein, glycitein, and equol) are primarily found in soy beans and soy products, chickpeas and other legumes.
  • Lignans (enterolactone and enterodiol) are found in oilseeds (primarily flaxseed), cereal bran, legumes, and alcohol (beer and bourbon).
  • Coumestans (coumestrol) can be found in alfalfa and clover.

Most food sources containing these compounds include more than one class of phyto-oestrogens.

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Much of the evidence about phyto-oestrogens in bone metabolism is based on animal studies. Soybean protein, soy isoflavones, genistein, daidzein and coumestrol have all been shown to have a protective effect on bone in animals after oophorectomy.

In humans the evidence is conflicting:

  • Phyto-oestrogens have been associated with a decreased risk of osteoporosis, but results from intervention and observational studies have been inconsistent.[1] 
  • Soy isoflavone supplements have been shown to significantly increase bone mineral density and decrease bone resorption.[2] 
  • A Cochrane review concluded there was no evidence of any effect on postmenopausal vasomotor symptoms.[3] 
  • There is some evidence of soya protein and/or isoflavones having beneficial effect on both blood lipid profile and bone density in postmenopausal women.[4] 
  • A systematic review of randomised controlled trials (RCTs) was unable to give unequivocal support for the benefit of phyto-oestrogens for the prevention of osteoporosis.[5] There is some evidence of adverse effects on laboratory animals, but studies to assess the problem in humans are not yet forthcoming.[6] There is also suggestion that phyto-oestrogens may enhance osteogenesis at low concentration and inhibit it at higher concentrations.[7]

Some studies suggest that, unlike oestrogens, phyto-oestrogens do not appear to increase the risk of breast cancer or uterine cancer. They seem more like selective oestrogen receptor modulators (SERMs) such as raloxifene and tamoxifen.

However, in other studies, high isoflavone levels have been linked to an increased risk of breast cancer. Clearly, additional research is needed and is currently being undertaken. The effect of consumption of soy as a risk for breast cancer is controversial:

  • A review concluded that the evidence was not strong in any direction and that even women who have had breast cancer may safely consume soy.[8] 
  • Studies have indicated that countries with the highest phyto-oestrogen consumption have the lowest rates of breast cancer, but other epidemiological studies suggest the lack of a causative relationship.[9] Some foodstuffs are beneficial.[10] No studies have found an increased risk of breast cancer with increased soy consumption. It may be dangerous to read too much into epidemiological studies on diet for a multifactorial condition.
  • Isoflavones exhibit some antioxidant activity, which may contribute to cancer prevention.

Phyto-oestrogens are also recommended as anti-androgens and hence are of possible protective benefit with regard to carcinoma of prostate and benign prostatic hyperplasia.[11] Whether they also reduce sperm count has not been examined.

Children with lactose intolerance or atopic eczema are often given a soya-based feed instead of milk:

  • These children may have plasma phyto-oestrogen concentrations of up to 7,000 nm/L, which compares with an average of 744 nm/L in adult Japanese women.[12]
  • The daily exposure to phyto-oestrogens from baby formula is much higher than a hormonally active dose in adults and plasma concentrations of isoflavones are much higher than endogenous oestrogen concentrations in infants. However, the only readily identified problem in these children is allergy in a small minority and also possibly inadequate intake of calcium.
  • Studies following children through adolescence have not reported any obvious adverse reproductive effects.[12] Nevertheless, the Chief Medical Officer (CMO) advises that soya-based infant formulas should not be used as the first choice for the management of infants with proven cow's milk sensitivity, lactose intolerance, galactokinase deficiency and galactosaemia.[13] As an alternative to soya-based products, more appropriate hydrolysed protein formulas are available and can be prescribed. Soya-based formulas should only be used in exceptional circumstances to ensure adequate nutrition. For example, they may be given to infants of vegan parents who are not breast-feeding or infants who find alternatives unacceptable.

Whether or not early exposure to high doses of isoflavones has any positive or negative effects on cancer rates or cognitive and neurological parameters in later life is not yet known.

The American National Institute for Health funded a cohort study which followed 3,734 Japanese men in Hawaii who had been tracked since 1965 for a cardiovascular longitudinal study:[14]

  • Cognitive function was assessed according to standard parameters in the living participants and their wives (aged 71-93 years).
  • MRI and later autopsies looked for changes in brain tissue.
  • Those who had consumed the greatest quantity of tofu in midlife had lower cognitive test performance and lower brain weight than those who had consumed the least tofu.
  • The authors noted that the degree of impairment in the highest consumption versus the lowest consumption group was "roughly of the magnitude as would be caused by a four-year difference in age or a three-year difference in education."

They postulated that the observed effect might be due to isoflavones inhibiting key enzymes in oestrogen synthesis pathways. Oestrogen is known to be involved in repair of neural structures that degenerate over time, and it has been observed that higher levels of oestrogen are associated with lower incidence of Alzheimer's disease in women. Around the menopause some women become forgetful. Another study found a beneficial effect of phyto-oestrogens on mood and cognitive function in postmenopausal women.[15]

Isoflavones in soy and flavonoids from other sources inhibit the enzyme thyroid peroxidase, which is involved in the synthesis of thyroxine. It would appear only to be a problem where there is borderline iodine deficiency.

This is a substance that is recommended by herbalists for a variety of conditions and it is highly recommended for menopausal symptoms, although some authorities say that it has anti-oestrogenic effects. There are many side-effects associated with it and the European Medicines Agency has given warning about the possibility of hepatitis resulting from its use.[16]

There is some evidence of a protective effect against the progression of chronic kidney disease by phyto-oestrogens.[17]

There is a great deal of evidence about phyto-oestrogens, but there is also much confusion:

  • Much of the evidence relates to animals and in vitro studies and there are questions as to how readily it should be extrapolated to humans.
  • Oestrogens and similar substances are highly complex and variable. Some may have an agonistic effect on bone, an antagonistic effect on breast tissue and a neutral effect on the endometrium as we have seen with selective oestrogen receptor modulators (SERMs). There may be considerable variation with regard to the effect of plant chemicals too.
  • When a substance is not given as a drug in a predetermined dose but is part of the diet, the amount ingested may be highly variable and there may also be problems such as an agonist effect at low dose and an antagonist effect at higher doses.
  • Some of the results, such as those related to cognition, are contradictory. This may be due to agonists and antagonists working in different directions.
  • Epidemiological studies about multifactorial diseases must be interpreted with care. When comparing different populations we cannot be sure that only one parameter is changed.
  • There is very little good, long-term evidence about the effects of phyto-oestrogens in human populations. Many questions remain to be answered. If a diet high in phyto-oestrogens does protect women against breast cancer, osteoporosis and heart disease, does it also impair fertility? If it protects men against prostatic cancer, does it also impair spermatogenesis?
  • The evidence to suggest that phyto-oestrogens are protective against osteoporosis is at best poor. Long-term safety is not established.
  • There have been many studies on the efficacy and safety of mammalian HRT, including the enormous 'Million Women Study'.[18] The Million Women Study was basically very reassuring but it did outline some causes for concern. The assumption that plant oestrogens are as effective but safer is a very unsafe premise.

Further reading & references

  1. Kuhnle GG, Ward HA, Vogiatzoglou A, et al; Association between dietary phyto-oestrogens and bone density in men and postmenopausal women. Br J Nutr. 2011 Oct;106(7):1063-9. doi: 10.1017/S0007114511001309. Epub 2011 May 17.
  2. Wei P, Liu M, Chen Y, et al; Systematic review of soy isoflavone supplements on osteoporosis in women. Asian Pac J Trop Med. 2012 Mar;5(3):243-8. doi: 10.1016/S1995-7645(12)60033-9.
  3. Lethaby A, Marjoribanks J, Kronenberg F, et al; Phytoestrogens for menopausal vasomotor symptoms. Cochrane Database Syst Rev. 2013 Dec 10;12:CD001395. doi: 10.1002/14651858.CD001395.pub4.
  4. Cassidy A, Hooper L; Phytoestrogens and cardiovascular disease. J Br Menopause Soc. 2006 Jun;12(2):49-56.
  5. Whelan AM, Jurgens TM, Bowles SK; Natural health products in the prevention and treatment of osteoporosis: systematic review of randomized controlled trials. Ann Pharmacother. 2006 May;40(5):836-49. Epub 2006 May 2.
  6. Reinwald S, Weaver CM; Soy isoflavones and bone health: a double-edged sword? J Nat Prod. 2006 Mar;69(3):450-9.
  7. Dang ZC, Lowik C; Dose-dependent effects of phytoestrogens on bone. Trends Endocrinol Metab. 2005 Jul;16(5):207-13.
  8. Messina MJ, Wood CE; Soy isoflavones, estrogen therapy, and breast cancer risk: analysis and Nutr J. 2008 Jun 3;7:17.
  9. Lof M, Weiderpass E; Impact of diet on breast cancer risk. Curr Opin Obstet Gynecol. 2009 Feb;21(1):80-5.
  10. Hanf V, Gonder U; Nutrition and primary prevention of breast cancer: foods, nutrients and breast Eur J Obstet Gynecol Reprod Biol. 2005 Dec 1;123(2):139-49.
  11. Holzbeierlein JM, McIntosh J, Thrasher JB; The role of soy phytoestrogens in prostate cancer. Curr Opin Urol. 2005 Jan;15(1):17-22.
  12. Badger TM, Ronis MJ, Hakkak R, et al; The health consequences of early soy consumption. J Nutr. 2002 Mar;132(3):559S-565S.
  13. Advice issued on soya-based infant formulas; CMO's update 37, Dept of Health, 2004
  14. White LR, Petrovitch H, Ross GW, et al; Brain aging and midlife tofu consumption. J Am Coll Nutr. 2000 Apr;19(2):242-55.
  15. Casini ML, Marelli G, Papaleo E, et al; Psychological assessment of the effects of treatment with phytoestrogens on postmenopausal women: a randomized, double-blind, crossover, placebo-controlled study. Fertil Steril. 2006 Apr;85(4):972-8.
  16. Black Cohosh. Risk of liver problems, Medicines and Healthcare products Regulatory Agency (MHRA), July 2006
  17. Ranich T, Bhathena SJ, Velasquez MT; Protective effects of dietary phytoestrogens in chronic renal disease. J Ren Nutr. 2001 Oct;11(4):183-93.
  18. Beral V, Bull D, Reeves G; Endometrial cancer and hormone-replacement therapy in the Million Women Study. Lancet. 2005 Apr 30-May 6;365(9470):1543-51.

Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical conditions. EMIS has used all reasonable care in compiling the information but make no warranty as to its accuracy. Consult a doctor or other health care professional for diagnosis and treatment of medical conditions. For details see our conditions.

Original Author:
Dr Hayley Willacy
Current Version:
Peer Reviewer:
Dr John Cox
Document ID:
1537 (v23)
Last Checked:
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