One of the newer dietary supplements to hit the United Kingdom market — isoflavones, which are derived mainly from soya — is generating an enormous amount of consumer interest. Yet, five or more years ago, few people outside the academic community had heard of them. Even now, most of the research in this area is relatively new. Few studies on isoflavones were conducted before 1993 but, during the past five years, well over 2,000 papers have appeared in the literature.
Soya has been consumed in China and other eastern countries for more than 5,000 years but it is relatively new to western countries. It was introduced into the United States less than 200 years ago, when it made up ballast aboard returning clipper ships. The recent interest in the potential health benefits of soya, and hence of isoflavones, arose as a result of epidemiological studies, primarily from Japan where soya consumption is high. These indicated that soya had a protective effect against sex-hormone-dependent cancers, including breast and prostate cancers.1 Currently, soya isoflavones are being investigated to see whether they have a protective role in a range of conditions, including cardiovascular disease, various cancers, osteoporosis and menopausal symptoms. To date, most research on these compounds has been conducted using soya products, as it is not yet clear whether isoflavones are the only potentially beneficial compounds in soya. Investigators are also assessing whether dietary supplements containing isoflavones are of value or not.
What are isoflavones?
Isoflavones belong to a class of compounds known as flavonoids. Products made from them include soya flour, soya milk, tempeh and tofu. They are present in varying amounts, depending on the type of soya product and how it is processed. Isoflavones are also found in dietary supplements, most of which are based on soya, but some come from other plant sources, such as red clover.
The principal isoflavones in the soya bean are genistein, daidzein and glycetin. These are usually found in the form of glycosides, which after ingestion are hydrolysed in the large intestine by the action of bacteria to release isoflavones.
What do they do?
By virtue of their chemical structure, isoflavones are weak estrogens and are also known as phytoestrogens. They are capable of binding to estrogen receptors, where, depending on the hormonal status of the individual, they seem to exert either estrogenic or antiestrogenic effects. This has led to speculation that, premenopausally, isoflavones may be antiestrogenic, whereas postmenopausally they could act as estrogen receptor agonists.
The identification of a second estrogen receptor in the mid-1990s helped to shed light on the possible role of isoflavones. This “newer” estrogen receptor is known as the beta-receptor, while the “older” one is known as the alpha-receptor. Different tissues appear to have different ratios of each receptor type. Alpha-receptors appear to predominate in the breast, uterus and ovary, while beta-receptors occur more frequently in prostate, bone and vascular tissue. Phytoestrogens, although far less potent than endogenous or synthetic estrogens, have been shown to bind to beta-estrogen receptors, raising the possibility that they could produce beneficial effects on, for example, bone and vascular tissue, without causing adverse effects on the breast and ovary.
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Citation: The Pharmaceutical Journal URI: 10977606