Flavonoids in health and disease
The initial aim of this thesis was to gather, critically assess and collate all available compositional data for flavonoid compounds belonging to 5 out of 12 major flavonoid sub-classes - flavanones, flavones, flavonols, catechins and procyanidins - in edible food sources. A flavonoid food composition database for application in dietary assessment investigations was subsequently compiled. The completeness of the database was tested using a healthy North East of Scotland population. The ability of the Scottish Collaborative Group food frequency questionnaire (SCG FFQ) to estimate flavonoid intake was compared to 4-day weighed dietary records. This study demonstrated that the major source of dietary flavonols, catechins and procyanidins was black tea with added milk. However, the bio-availability of tea flavonoids was questionable, as flavonoids were known to readily bind to milk proteins in vitro. As a review of the literature was inconclusive with 2 studies supporting the hypothesis and 2 refuting it, a bio-availability study was subsequently completed. This investigation assessed the uptake of flavonoids and their effect on plasma antioxidant status after drinking black tea, black tea with milk and water and milk as a control. Findings suggested, that the possible exception of the flavonol, kaempferol, addition of milk to tea did not significantly negate increases in plasma flavonoid concentration or antioxidant activity. Finally, the hypothesis that low dietary flavonoid intake is associated with colon or rectal cancer risk was tested using the SCG FFQ applied to a population-based case-control study. Only the flavonol quercetin obtained from sources other that tea was strongly related to reduced risk of colon cancer in the North East of Scotland. However, further investigation is required to discern if this observed effect is due to quercetin per se or to other as yet unidentified components of fruit and vegetables which are co-associated with this flavonoid.