Influence of dietary phenolic compounds on vitamin E status

J. Frank, Kiel

Vitamin E has many health effects and is recognized as the most important lipid-soluble, chain-breaking antioxidant in the human body. Vitamin E has also been reported to play a regulatory role in cell signalling and gene expression. Epidemiological studies have shown that high blood concentrations of vitamin E are associated with a lower risk of cardiovascular diseases and certain cancers. In addition to vitamin E, the diet contains a great variety of phenolic compounds, i.e. potent antioxidants which may interact with vitamin E in the human organism, influencing its status and biological activities.

To identify dietary phenolic compounds capable of increasing blood and tissue concentrations of vitamin E, selected polyphenols were incorporated into standardized, semi-synthetic diets and fed to male Sprague-Dawley rats for 4 weeks. Blood plasma and liver tissue concentrations of α-tocopherol and γ-tocopherol were determined. The flavanols -catechin and (–)-epicatechin, the flavonol quercetin, and the synthetic preservative butylated hydroxytoluene (BHT) clearly increased α-tocopherol concentrations in plasma and liver.

The three flavonoids also protected α-tocopherol against oxidation to LDL isolated from human volunteers and recycled the vitamin from its α-tocopheroxyl radical form in micelles of sodium dodecyl sulfate, suggesting that antioxidant interactions caused the α-tocopherol sparing observed in the rat model. The sesame lignan sesamin and cereal alkyl resorcinols substantially increased the concentrations of γ-T., but not α-T., in the liver. Sesamin also increased γ-T. concentrations in plasma. In order to study the influence 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, at concentrations as low as 2 μM, almost completely inhibited tocopherol side-chain degradation, while cereal alkylresorcinols inhibited it, dose-dependently (5–20 μM), by 20–80 %. In order to confirm the inhibition of γ-T. metabolism by sesame lignans in humans, sesame oil or corn oil muffins together with deuterium-labelled d6-α-T and d2-γ-T were given to volunteers. Urine samples were collected for 72 hours and analysed for deuterated and non-deuterated tocopherol metabolites (CEHC). Consumption of sesame oil muffins significantly reduced urinary excretion of d2-γ-CEHC and total (sum of labelled and non-labelled) γ-CEHC. These studies have shown that the dietary phenolic compounds tested increase vitamin E concentrations via different mechanisms and, thus, have the potential to improve the vitamin E status without use of vitamin E supplements.

Key words: alkylresorcinol, antioxidant, butylhydroxytoluol, catechin, epicatechin, polyphenols, tocopherol, tocopherol metabolites, vitamin E, vitamin E status, quercetin, sesamin

Sie finden den Artikel in deutscher Sprache in Ernährungs-Umschau 02/07 ab Seite 52.