Dietary phenolic compounds and vitamin e bioavailability

The human diet contains a vast number of dietary phenolic compounds of which vitamin E represents only one class. Vitamin E is a generic name for all substances exerting the biological functions of α-tocopherol. The two quantitatively most important E vitamers are α- and γ-tocopherol (α-T and γ-T). The fat soluble vitamin E is absorbed and transported in the circulation to the liver where α-T is preferentially re-secreted into the bloodstream while the other vitamers are degraded by cytochrome P450 enzymes to the water-soluble carboxyethyl hydroxychroman (CEHC) metabolites excreted in the urine. Thus, α-T blood concentrations are usually 4-10 times higher than those of γ-T. Vitamin E is mainly recognized to protect cell components from oxidative damage, but has also been reported to inter alia control gene expression and cellular signalling pathways. This thesis aimed at investigating the effects of dietary phenolic compounds on the bioavailability of vitamin E in model studies. To this purpose, polyphenols were incorporated into standardized, semi-synthetic diets and fed to male Sprague-Dawley rats for 4 weeks. Blood plasma, liver and lung tissue concentrations of α-T and γ-T were determined. The sesame lignan sesamin and cereal alkylresorcinols greatly increased the bioavailability of γ-T, but not α-T, in all tissues. In contrast, the flaxseed lignan secoisolariciresinol diglucoside reduced the bioavailability of both tocopherols. The flavanols (+)-catechin and (−)-epicatechin and the preservative butylated hydroxytoluene (BHT) markedly enhanced the bioavailability of α-T in all analysed tissues. Curcumin and the tested anthocyanins and phenolic acids exerted only minor, inconsistent effects in different tissues in the rat model. In order to study the impact of selected polyphenols on the enzymatic degradation of vitamin E, HepG2 cells were incubated together with phenolic compounds in the presence of tocopherols and the formation of metabolites was determined. Sesamin almost completely inhibited tocopherol side-chain degradation and cereal alkylresorcinols inhibited it, dose-dependently, by 20-80%. BHT and (+)-catechin had no effect on tocopherol-ω-hydroxylase activity in HepG2 cells. To verify the inhibition of γ-T metabolism by sesame lignans in humans, sesame oil or corn oil muffins together with deuterated d6-α-T and d2-γ-T were given to volunteers. Blood and urine samples were collected for 72 hours and analysed for deuterated and non-deuterated tocopherols and their metabolites. Consumption of sesame oil muffins significantly reduced the urinary excretion of d2-γ-CEHC. Overall, the findings from this thesis show that dietary phenolic compounds alter vitamin E bioavailability in humans and animals through various mechanisms. Frank, Jan (2004) Dietary phenolic compounds and vitamin e bioavailability : model studies in rats and humans. Doctoral diss. Dept. of Food Science, SLU. Acta Universitatis agriculturae Suecia. Agraria vol. 446. Full text available as: PDF