The soya bean has some interesting nutritional properties. As well as providing high quality protein almost equal in quality to that of animal protein, the soya bean also contains a number of phytochemicals that are important in human nutrition. Of these the isoflavones daidzein and genistein have been extensively studied for their oestrogenic effects in humans. Epidemiological studies suggest that those who regularly consume soy products have a lower risk of developing sex hormone related cancers such as prostate and breast cancer. This has been explained by the modulatory effect of the isoflavones in soy to inhibit the growth of hormone positive cancers. Studies into the effects of the soy isoflavones have identified a metabolite of daidzein called s-(-)-equol (equol) as a possible candidate for its beneficial effects on health. Equol is produced in the distal region of the colon by gut bacteria, following which it is thought to be absorbed to the circulation where it can exert its biological effect on cells and tissues.
However, only around 25 to 30 % of adults in Western nations produce equol. This is in comparison to the 50 to 70 % of adults in asian countries such as Japan, Korea and China that produce equol. The reasons for this are not clear, but research suggests that only equol producers benefit from the health effects of soy. Researchers are therefore interested in the reasons for the production of equol, and studies have started to be published as to why this might be, For example, in one study1 a group of researchers investigated the dietary factors that influence equol production in 159 health subjects from the United States and Australia. In particular, the study authors investigated the effects of 143 nutrients on equol production by analysing the food intake of the subjects and also measured their production of equol through analysis of urine. Because equol might be affected by microbial populations in the gut, the analysis of equol was repeated following administration of an antibiotic (metronidazole, 500 mg) to 5 subjects.
The results of the study showed that 30 % of the subjects from the United States and 29 % of the subjects from Australia were equol producers. There were no differences in the protein, carbohydrate or fat content of the diet in equol producers compared to non-equol producers. However, higher intakes of polyunsaturated fatty acids, maltose, vitamin A and vitamin E in the equol producing subjects. In addition, the equol producers consumed less cholesterol. In the follow up period, the inability of non-equol producers to synthesise equol did not change, but 20 % of the equol producers ceased production of equol. Interestingly the administration of metronidazole to subjects in order to sterilise their colons, resulted in only 20 % participants, (1 of the 5 subjects) ceasing production of equol. These results therefore suggest that subtle dietary factors can influence equol production in humans, and that bacterial populations in the gut might not play as an important role as was previously considered.
Dr Robert Barrington’s Nutritional Recommendation: Although epidemiological studies show an inverse association between soy consumption and reproductive cancers, the nature of the relationship is not fully understood. The role of equol is also not fully understood. For example, those that eat large amounts of soy also consume other foods that may be partly responsible for the health effects. In addition, soy is eaten mainly as fermented products in asian countries such as tofu, miso, natto and tempeh. Consuming products containing soy but not in these forms may therefore not confer the same effects. Further, some studies have shown that isolated isoflavones from soy may increase breast cancer risk in certain subgroups of women. If soy is consumed it is therefore recommended that it is eaten in its traditional fermented forms, as part of a healthy diet, with moderation. Supplements of isolated isoflavones should not be used to treat reproductive cancers until further research verifies their beneficial effects.
RdB
