Vitamin E metabolism in humans

    Research output: ThesisDoctoral Thesis

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    Abstract

    [Truncated abstract] Vitamin E is comprised of a family of tocopherols (TOH) and tocotrienols. The most studied of these is [alpha]-tocopherol ([alpha]-TOH), as this form is retained within the body and any deficiency of vitamin E is corrected with this supplement. [alpha]-TOH is a lipid-soluble antioxidant required for the preservation of cell membranes and potentially acts as a defense against oxidative stress. Individuals who have a primary vitamin E deficiency such as low birth weight infants, secondary vitamin E deficiency due to fat malabsorption such as in abetalipoproteinaemia, or a genetic defect in TOH transport require supplementation. There is debate as to whether vitamin E supplementation in other patient groups is required. Vitamin E supplementation has been recommended for persons with FHBL, a rare disorder of lipoprotein metabolism that leads to low serum [alpha]-TOH and decreased LDL cholesterol and apolipoprotein B concentrations. We examined the effect of truncated apoB variants on vitamin E metabolism and oxidative stress in persons with heterozygous FHBL. We used HPLC with electrochemical detection to measure [alpha]- and [gamma]-TOH in serum, erythrocytes, and platelets, and GC-MS to measure urinary F2-isoprostanes and TOH metabolites as markers of oxidative stress and TOH intake, respectively. Erythrocyte [alpha]-TOH was decreased, but we observed no differences in lipid-adjusted serum TOHs, erythrocyte [gamma]-TOH, platelet [alpha]- or [gamma]-TOH, urinary F2-isoprostanes, or TOH metabolites. Taken together, our findings do not support the recommendation that persons with heterozygous FHBL should receive vitamin E supplementation. ... Sesame lignans are natural components of sesame seed oil and there is evidence that these lignans can inhibit CYP450 enzymes, in particular, those responsible for vitamin E metabolism. We hypothesised that sesame seed ingestion would increase serum [gamma]-TOH, lower plasma lipids and inhibit platelet function in human subjects with at least one cardiovascular risk factor. We used HPLC with electrochemical detection to measure [alpha]- and -TOH in serum and GC-MS to measure F2-isoprostanes and TOH metabolites as markers of oxidative stress and TOH intake, respectively. We used high-sensitive C-reactive protein as a measure of systemic inflammation. Platelet function was assessed using the PFA-100 platelet aggregation assay. Although serum [gamma]-TOH increased by 17%, we observed no effect on lipid metabolism, markers of inflammation, oxidative stress or platelet function following treatment with ~25 g/day sesame seeds for five weeks. Our findings challenge the hypothesis that sesame seed ingestion provides beneficial cardiovascular effects. In summary, we have studied the metabolism and transport of both [alpha]- and [gamma]-TOH in humans to evaluate the requirements for supplementation and the effects of vitamin E on platelet function and CYP3A4 activity. Specialised techniques using HPLC were developed to measure serum and cellular TOH concentrations both in supplemented and un-supplemented individuals. We also used GCMS to provide a sensitive, accurate assessment of TOH metabolites and midazolam pharmacokinetics in humans after vitamin E supplementation. We have examined the role vitamin E has on important biochemical endpoints, with emphasis on the implications for TOH supplementation in subjects at risk of CVD.
    Original languageEnglish
    QualificationDoctor of Philosophy
    Publication statusUnpublished - 2008

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