The contribution of genetic factors to inter-individual variability in dietary isoflavone absorption and metabolism

Lower incidence of some diseases, including cancer and cardiovascular disease, in populations who consume diets with a high content of soyabean products may be due to components of the soyabean known as isoflavones. Studies demonstrating biological activity of soyabean and soyabean extracts and isolated isoflavones in animal models and cultured cells provide evidence consistent with population-based studies that indicate such protective effects. Studies examining how individual subjects absorb and metabolise soyabean isoflavones reveal considerable variability between different individuals, and this variability is likely to determine the degree to which consumption of a soyabean-rich diet may offer health benefits to particular individuals. To be able to give informed advice on the potential beneficial effects of a soyabean-rich diet, or of taking isoflavone supplements, therefore, it is essential to understand factors that determine why different individials differ in their handling of dietary isoflavones. One major source of variability is likely to be differences in individuals' genetic make up with respect to the transporter proteins involved in the absorption of the isoflavones across the intestine and in enzymes responsible for metabolism of the isoflavones once absorbed. The aim of this project, therefore, is to study how specific genetic differences between individuals may affect the efficiency of absorption and the metabolism of isoflavones taken as a single oral dose. Subjects will be 150 healthy, premenopausal, Caucasian females selected from an initial group of 300 screened using DNA prepared from blood samples to obtain a sample group who vary with respect to differences in the genes we wish to study. The use of this restricted population group will limit variability in isoflavone absorption and metabolism resulting from non-genetic factors or from genetic differences additional to those we will study. Volunteers will collect all urine for 24 hours following the ingestion of capsules containing soyabean concentrate and will also give a blood sample 3 hours after taking the supplement. Isoflavone concentrations will be measured in these samples to indicate the efficiency of absorption and metabolism. There is evidence to indicate that some metabolism of the isoflavones takes place in the mouth and may be particularly important in terms of protection against oral cancer. We will also, therefore, investigate if there is any relationship between variability in some of the genes we will study that are related to this form of metabolic activity and rates of metabolism of a marker compound by extracts prepared from cells collected form the epithelial surface of the mouth by all volunteers. A link between genetic variability between individuals at specific locations in the DNA sequence and efficiency of isoflavone absorption and metabolism, should this be established, will add to a body of accumulating data concerning interindividual beneficial effects of dietary isoflavone intake and will have important implications for the design of future studies aimed at determining how genetic variability affects the influence of dietary isoflavones on markers of disease risk.

The apparent benefits to health, including protection against cancer and cardiovascular disease, of consuming a diet with a high content of soybean may be attributable to the isoflavones, which have biological actions commensurate with such benefits. To exploit fully these potential health-beneficial effects, knowledge of the extent to which particular individuals differ in their handling of these compounds is essential. The project is aimed at determining if polymorphisms in components of the pathway involved influence the absorption and/or metabolism of isoflavones ingested as a single oral bolus dose of soyabean concentrate. Extending a pilot study currently in progress, a total of 300 healthy, premenopausal, female volunteers will undergo screening for genotype with respect to key components of the pathway and 150, selected according to genotype, will take a single, oral dose of soyabean concentrate, after which the urinary excretion and plasma concentration of the isoflavones genistein, daidzein, glycitein, their major glucuronide metabolites and the daidzein metabolite equol will be measured. In addition, beta-glucosidase activity, which is required for the intestinal absorption of the isoflavone beta-glucosides, the form in which they are present in the unfermented soyabean, will be measured in oral epithelial cell samples from all volunteers to determine if there is any correlation with genotype or with measures of isoflavone absorption. The outcomes of the study will inform research designed to investigate effects of inter-individual genetic variability on measures of the health outcomes of isoflavone consumption, in which variability in absorption and/or metabolism may be a confounding variable, unless taken into account, and will contribute to a body of data that may be, ultimately, sufficient to model of the effects of gene polymorphisms on inter-individual health benefits of isoflavone consumption using a systems biology approach.