title: Complexity of vitamin E metabolism.
abstract: Bioavailability of vitamin E is influenced by several factors, most are highlighted in this review. While gender, age and genetic constitution influence vitamin E bioavailability but cannot be modified, life-style and intake of vitamin E can be. Numerous factors must be taken into account however, i.e., when vitamin E is orally administrated, the food matrix may contain competing nutrients. The complex metabolic processes comprise intestinal absorption, vascular transport, hepatic sorting by intracellular binding proteins, such as the significant α-tocopherol-transfer protein, and hepatic metabolism. The coordinated changes involved in the hepatic metabolism of vitamin E provide an effective physiological pathway to protect tissues against the excessive accumulation of, in particular, non-α-tocopherol forms. Metabolism of vitamin E begins with one cycle of CYP4F2/CYP3A4-dependent ω-hydroxylation followed by five cycles of subsequent β-oxidation, and forms the water-soluble end-product carboxyethylhydroxychroman. All known hepatic metabolites can be conjugated and are excreted, depending on the length of their side-chain, either via urine or feces. The physiological handling of vitamin E underlies kinetics which vary between the different vitamin E forms. Here, saturation of the side-chain and also substitution of the chromanol ring system are important. Most of the metabolic reactions and processes that are involved with vitamin E are also shared by other fat soluble vitamins. Influencing interactions with other nutrients such as vitamin K or pharmaceuticals are also covered by this review. All these processes modulate the formation of vitamin E metabolites and their concentrations in tissues and body fluids. Differences in metabolism might be responsible for the discrepancies that have been observed in studies performed in vivo and in vitro using vitamin E as a supplement or nutrient. To evaluate individual vitamin E status, the analytical procedures used for detecting and quantifying vitamin E and its metabolites are crucial. The latest methods in analytics are presented.
authors.affiliation: Lisa Schmölz, Stefan Lorkowski, Maria Wallert, Department of Nutritional Biochemistry and Physiology, Institute of Nutrition, Friedrich Schiller University Jena, Germany and Competence Center for Nutrition and Cardiovascular Health, Halle-Jena-Leipzig, 07743 Jena, Germany.
authors.forename: Lisa
authors.initials: L
authors.lastname: Schmölz
authors.affiliation: Lisa Schmölz, Stefan Lorkowski, Maria Wallert, Department of Nutritional Biochemistry and Physiology, Institute of Nutrition, Friedrich Schiller University Jena, Germany and Competence Center for Nutrition and Cardiovascular Health, Halle-Jena-Leipzig, 07743 Jena, Germany.
authors.forename: Marc
authors.initials: M
authors.lastname: Birringer
authors.affiliation: Lisa Schmölz, Stefan Lorkowski, Maria Wallert, Department of Nutritional Biochemistry and Physiology, Institute of Nutrition, Friedrich Schiller University Jena, Germany and Competence Center for Nutrition and Cardiovascular Health, Halle-Jena-Leipzig, 07743 Jena, Germany.
authors.forename: Stefan
authors.initials: S
authors.lastname: Lorkowski
authors.affiliation: Lisa Schmölz, Stefan Lorkowski, Maria Wallert, Department of Nutritional Biochemistry and Physiology, Institute of Nutrition, Friedrich Schiller University Jena, Germany and Competence Center for Nutrition and Cardiovascular Health, Halle-Jena-Leipzig, 07743 Jena, Germany.
authors.forename: Maria
authors.initials: M
authors.lastname: Wallert
keywords: Long-chain metabolites of vitamin E; Vitamin E bioavailability; Vitamin E metabolism; Vitamin E transport