Soya is the main source of dietary isoflavones which are naturally present in many plants including legumes such as chick peas and lentils.  Isoflavones are often described as phytoestrogens because their chemical structure shows similarities to the human hormone oestrogen. The term ‘phyto’ refers to the fact they are of plant origin. 

The other main types of dietary phytoestrogens are coumestans, prenylflavonoids and lignans. Coumestans are found in very small quantities in vegetables, such as peas, Brussel sprouts and mungbean sprouts. Prenylflavonoids are found in hop and its derived product beer. Lignans are found in whole-grain cereals such as flaxseed and in some fruits and vegetables (including celery, asparagus and broccoli).

Soya is recognised as the main dietary source of naturally occurring isoflavones. The three isoflavones genistein, daidzein and glycitein and their respective glycosides account for approximately 50%, 40% and 10%, respectively, of the total isoflavone content of soya beans. Soya foods contain approximately 3.5mg of isoflavones per gram of protein. Consequently one serving of 200 mL soya drink (approx. 6g protein), typically provides about 21mg isoflavones. 

The difference between phytoestrogens and oestrogen

Although similar in chemical structure, isoflavones behave differently to oestrogen in the body. 

Two types of oestrogen receptors (ER) exist in the human body: alpha (ERα) and beta (ERβ). At the molecular level, isoflavones preferentially bind to and transactivate the ERβ whereas oestrogen binds to and transactivates both ERα and ERβ equally. 

This difference in binding and transactivation between isoflavones and oestrogen is significant because the two ERs have different tissue distributions and, when activated, can have different and sometimes opposite physiological effects. The preference of isoflavones for ERβ is the reason they are classified as selective oestrogen receptor modulators (SERMs). SERMs exert oestrogenic effects in some tissues, anti-oestrogenic effects in others and in some tissues have no effect at all.

The tissue distribution and relative ligand binding affinities of the ER beta and ER alpha differ, and this finding may help to explain the selective action of isoflavones in different tissues. 

ER alpha is mainly expressed in tissues such as the breast and uterus whereas ER beta is present in the brain, bone, bladder and cardiovascular system.

Very weak estrogenic activity

Isoflavones are natural compounds found in many plants, belonging to the large family of polyphenols. Isoflavones are also referred to as phytoestrogens as their chemical structure show similarities to the human hormone oestrogen. However, their effects in the body are clearly different. Isoflavones have weak and selective affinity to the oestrogen receptors compared to oestrogen.

In foods, isoflavones are mainly present as inactive ‘glycosides’ i.e. they are bound to a sugar molecule. To be absorbed into the body, this sugar must be removed, through a process called deglycosylation, carried out by a specific intestinal enzyme. The ‘aglycon’ isoflavones (freed of their sugar) are then absorbed by the body.

After absorption, the majority of aglycon isoflavones are further metabolised in the liver: the majority of the aglycons (about 98%) are glucuronidated or sulphuronidated and most are thus inactivated again. Thus the majority of isoflavones circulating in our body are present in inactive glucuronidated or sulphuronidated form. Whereas the activity of circulating aglycon isoflavones has been shown under laboratory conditions to be about 1000 times lower than that of endogenous oestrogen.

Important to note: Much of the concern about isoflavones can be attributed to the results of rodent studies. Rodent are of limited and futile value for evaluating isoflavones and human health, as they metabolise isoflavones very differently to humans.

International bodies confirm safety of isoflavones

EFSA’s risk assessment concludes that in postmenopausal women, isoflavones do not adversely affect breast, thyroid or uterus.

The European Food Safety Authority (EFSA) conducted an extensive literature review to assess the risk for peri- and post-menopausal women taking soya food supplements containing isolated isoflavones.

The EFSA experts concluded that there was no increased risk of breast cancer and no effect on either the uterus or the concentrations of thyroid hormones in menopausal women when consuming a supplement of 150 mg of isoflavones a day. The amount of isoflavones naturally present in soya foods is much lower compared to the amount in food supplements. No effects have been observed by EFSA at the highest intake levels of isolated isoflavones in food supplements; this means the EFSA indirectly recognizes that the consumption of soya foods, naturally containing isoflavones in much lower quantities, are safe. 

The World Cancer Research Fund (WCRF) and the American Institute for Cancer Research (AICR) have also concluded that soya foods as part of a healthy diet are perfectly safe and do not pose a risk for women with breast cancer. 

Soya protein content of common UK soya foods and drinks with estimated isoflavone content

Please note: Isoflavone content of soya foods and drinks is based on the accepted average estimate of 3.5mg isoflavones per 1g soya protein (see Messina).  However, isoflavone content varies considerably depending on soil, growing conditions and production methods, where up to 80% of isoflavones can be lost.

Source: Dietplan7 – 2018 unless otherwise highlighted with an asterisk/s.

*Soya drinks nutrition information: the average of current soya drinks on the market (n=7) excluding specialised drinks (formulated to meet the nutrition needs of 1-3 year olds, ‘super-fortified ’ with protein, fibre or calcium and organic variants). **Alpro, leading manufacturer of soya custards, desserts and alternatives to yogurts – website nutrition information ***Average of current brands on the market.


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