Phytoestrogens, also known as phytosterols or phytochemicals, are natural plant compounds which have a similarity to human estrogens. "Phyto" is taken from Greek and is generally used as a prefix to describe substances derived from plants. Phytoestrogens, therefore, are substances from plants which have estrogen-like qualities.

The prevailing scientific opinion on phytoestrogens is favourable as there is substantial research indicating that they hold great potential for health benefits. Recent epidemiological studies have suggested that typical Asian diets, which have always been much higher in phytoestrogens than Western diets, appear to be associated with a significantly lower risk of breast, prostate and colon cancer as well as a reduced incidence of heart disease and osteoporosis. These degenerative diseases have in fact long been associated with modern Western diets and therefore the studies have truly caused no surprise by reinforcing this idea and favouring Asian diets for good health.

The benefits of phytoestrogens to good health are mainly due to the effects they have on the body's hormonal balance, acting as both agonists and antagonists. To understand how phytoestrogens help the body's hormonal balance, it is useful to recall what hormones are and how they work. Hormones are chemical substances produced by the body's endocrine glands and released into the bloodstream to act as chemical messengers, travelling through the body with instructions to trigger activity in their target tissues. These target tissues each contain receptor sites specific to a particular hormone and the required effect is initiated when the hormone in question arrives and docks at those receptor sites. For example, estrogens are released from the ovaries and travel through the blood to the breast area where they dock and deliver the instruction to initiate mammogenesis.

Phytoestrogens are thought to act as estrogen agonists by occupying estrogen receptor sites when natural estrogens are unavailable. For example, the body's natural estrogen levels inevitably decline with the onset of menopause and phytoestrogens may help to offset this decline if they can fill receptor sites instead. Once docked on the sites the phytoestrogens exert estrogen-like activity and can therefore initiate the required effect just as natural estrogen would have done.

Phytoestrogens are also thought to act as estrogen antagonists by occupying estrogen receptor sites ahead of the body's natural estrogens and equally importantly ahead of synthetic estrogens and also environmental estrogens derived from chemical products, otherwise known as bad estrogens or xenoestrogens. In order words, where estrogen levels are high, phytoestrogens are able to compete with the body's natural estrogens or the artificial estrogens present and may fill the receptor sites before they do. If this happens the phytoestrogens will in effect decrease estrogenic activity in the body, because the effect of the docked phytoestrogens on the target tissues will be less than if the available estrogens had been allowed to dock.

A limitation on the hormone balancing actions of phytoestrogens is that they do not in fact have estrogenic properties when still in the plant or even at the time they are consumed, but acquire them only during the digestive process through the actions of bacteria residing in the gastrointestinal tract. The bacteria cause the phytoestrogens to undergo complex metabolic conversions, leading to the formation of an estrogen-like metabolite which can then be absorbed by the body. What this means is that the biologically useful forms of phytoestrogens, the metabolites, are dependent for their existence upon a digestive system in good order and complete with adequate microflora capable of converting the basic plant compounds into the active forms.

This is a limitation on the effectiveness of phytoestrogens for the simple reason that there are a number of factors that can adversely affect the stability of the normal gastrointestinal flora. Poor or inappropriate diets, stress, and antibiotics can all significantly disrupt the ideal healthy balance of gastrointestinal organisms. Antibiotics in particular can quickly destroy friendly bacteria, as well as the invading organisms they are actually meant to destroy. High fat intake is another culprit. One thing that should have a positive effect though is fibre intake as a high-fibre diet is known to help the metabolism of phytoestrogens.

Further resources:

An Information Statement on Phytoestrogens from the Institute of Food Science & Technology, London UK.

An Information Sheet in Microsoft Word format PHYTOESTROGEN.doc or Adobe Acrobat format PHYTOESTROGEN.pdf from the National Osteoporosis Society, Bath UK.