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Several strands of indirect evidence suggest that dietary consumption of phytoestrogens may protect against prostate cancer, in particular the type of prostate cancer which carries a lethal threat. For example, the disease is less frequent in Japan and China where consumption of soy foods is high. In addition, comparisons of typical diets of men with and without prostate cancer in these high-consumption countries have confirmed that men without prostate cancer had higher consumption of these foods; similar results have been found when evidence of phyto-estrogens in blood serum of these men have been compared.
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Based on the evidence described above, our objective was to study a population of Scottish men in whom phytoestrogen intake was assumed to be low to determine whether we found similar associations. Our first objective was to validate for phyto-estrogens a food frequency questionnaire which had already been extensively validated and used in Scotland. The second objective was to determine, using the FFQ and blood serum measurements, whether evidence of current diet high in phytoestrogens was associated with reduced risk of prostate cancer, specifically of clinically important prostate cancer.
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The project has been conducted as a ‘case-control’ study; that is men recently diagnosed with clinically important prostate cancer (n=433) have been compared with a representative sample of men without evidence of prostate cancer (n=483). All these men have self-completed the FFQ. Following receipt of informed consent a small serum sample has been collected (n=263 cases, n=218 controls) and phytoestrogen content measured using mass spectrometry (isotope reduction gas-chromatography). Using food tables derived from another FSA-funded project, we have estimated phytoestrogen intake from the FFQ. Comparison of results derived from the FFQ and serum measurements for the subjects with serum samples measured control subjects has shown a satisfactory level of agreement and has served to validate the FFQ for genistein, daidzein, equol and total isoflavones. No estimation of enterolactone from the FFQ has been possible for enterolactone as food tables are not available. Case-control comparisons have been made using logistic regression with adjustment for potential confounders, in particular deprivation and total energy intake.
Outcome: <BR>
For isoflavones:<BR>
Comparison of serum levels in cases and controls showed no differences for genistein, equol, diadzein or total isoflavone. Comparison of intakes of total isoflavones estimated from the FFQ showed similar lack of evidence of any protective effect; the best estimate was, in fact, of an 18% higher risk of prostate cancer in men with intake in the highest 25% compared to those in the lowest 25%. Only 7% of men reported consuming soy products and there was a suggestion of a protective effect in these men; risk was 19% lower in men who consumed at least one soy product/week that in men who did not; this was statistically significant initially but lost statistical significance after adjustment for confounders.
<BR><BR>For enterolactones: <BR>
Alone of the serum measurements this one showed evidence of case-control differences. There was a reduction of over 50% in risk of prostate cancer in men whose serum level was in the highest quintile compared to those in the lowest quintile. The downward trend of risk across quintiles was statistically highly significant (p=0.001). It was not possible (see above) to confirm this by reference to the FFQ and since the biological interpretation is unclear it has not been pursued further.
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What it means and why it is important.<BR>This study shows firstly that standard FFQs can be easily modified to assess dietary intake of isoflavones for the purpose of epidemiological studies. It also shows that dietary consumption is low in Scottish men, so low that the isoflavone content of diet is unlikely to make direct impact on dietary risk of prostate cancer (several other nutrients were found to be significantly associated with prostate cancer, both increasing and decreasing risk). Several studies have shown that ingestion of larger quantities of phyto-estrogens are protective elsewhere and it remains likely that dietary supplementation by PEs could be an important contribution to male health. Our serum and FFQ data are available to explore the enterolactone data if and when food tables emerge.
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Find more about this project and other FSA food safety-related projects at the <a href="http://www.food.gov.uk/science/research/" target="_blank">Food Standards Agency Research webpage</a>.