There is growing evidence that a-tocopherol, the active form of vitamin E, may help minimize the risk of certain chronic diseases such as cardiovascular disease, hypertension and cancer. Still, more research on vitamin E is needed to fully understand its metabolism and function(s). The IOM/NAS has listed key research recommendations for vitamin E. The list included quantifying vitamin E metabolism; determining whether mass balance is feasible and vitamin E requirements can be estimated using stable isotope-labeled vitamin E; determining whether vitamin E metabolism as it occurs in vivo in humans can be quantified; determining how, where, and how fast vitamin E is degraded; identifying the metabolic intermediates during metabolism; and determining whether they have biological function. The long term goal of this application is to quantify the dynamic and kinetic behavior of vitamin E metabolism as it might occur in vivo in humans using kinetic modeling of the fate of true tracer doses of 14C-vitamin E that ensure steady state conditions of vitamin E metabolism that are relevant and useful to nutrition, and where our study subjects are not asked to accept risks significantly greater than those experienced in everyday life; i.e., radiation exposures < those obtained in flying for 1-4 h, a common radiation exposure, even for young children and pregnant women. The following specific aims will achieve our long term goal.
Specific aim # 1: Determine the absorption, accretion, distribution, and elimination of RRR-a-TOC in humans. To accommodate this, we will quantify the transfer of 14C-RRR-a-TOC among various lipoprotein classes and its metabolites by time since dose.
Specific aim # 2: Construct a dynamic and kinetic model of 14C-RRR-a-TOC metabolism as it might occur in vivo in humans to test the hypothesis whether EHT-a-TOC is eliminated by a """"""""reverse a-TOC transfer"""""""" process. The null hypothesis is that EHT-a-TOC eliminated through """"""""reverse cholesterol transfer."""""""" Plasma lipoproteins play key roles in delivering RRR-a-TOC to and from EHT. For this purpose 12 volunteers, 6 women and 6 men, will be recruited and administered a single dose of 14C-RRR-a-tocopherol (100 nCi) that will be added to a cup containing a whipped mix of 10.5 g olive oil and 30 g skim milk. We will follow the 14C tracer over a 60-day period. The long term goal of this project is to quantify the dynamic/ kinetic behavior of vitamin E metabolism in vivo in humans of the RRR-a-TOC and of its 2R stereoisomer forms using a test re-test design. In addition, using a mechanistic model of tocopherol metabolism we can determine whether or not single nucleotide polymorphisms (SNPs) in tocopherol-relevant enzymes affects the kinetic behavior of human tocopherol metabolism as we recently show for folate Clifford et al. 2006. Baseline blood (10 mL) will be taken just before dosing and at various intervals until 60 days post dose to ensure that the 14C labeled a-tocopherol is fully equilibrated into the slowest turning over a-tocopherol pool that can affect the plasma kinetics late in a study. The lipoprotein classes will be separated using a new protocol. Erythrocytes, lymphocytes, and platelets will also be separated. Urine and feces will be collected until 30 days post dose. The 14C content of each biological sample will be measured at the Center for the Accelerator Mass Spectrometry at Lawrence Livermore National Laboratory (Livermore, CA). Chemical identity of the 14C in plasma and plasma lipoprotein samples will also be determined, 14C-RRR-a-tocopherol, a-CEHC, a-CPHC plus 4 more unknowns by using non-radioactive standards in an HPLC. We will construct a dynamic/kinetic model of a-tocopherol metabolism, we have already demonstrated the feasibility of our approach. We will use a high sampling density of plasma over a relatively long time since dosing to acquire a much more detailed/complete dataset than any already existing dataset. Then if specific metabolic functions for a-tocopherol can be stipulated, it will be possible to determine a minimum intake of a-tocopherol to sustain these critical functions, such as, preventing oxidation of lipoproteins, which has been linked to atherogenesis, and therefore minimizing the risk of atherosclerosis.
Vitamin E is a generic name for a group of lipid-soluble compounds known as tocopherols that have different formulas. These Vitamin E formulas occur naturally in many foods and act as biological antioxidants. Previous studies have shown differences in the way humans metabolize vitamin E. Since metabolism of the vitamin is poorly understood especially at normal physiological levels, in vivo studies under physiological conditions are needed in humans to understand it. Therefore, UC Davis Scientists are conducting a study with approximately 12 healthy volunteers to evaluate the metabolism of a single oral administration of tracer amounts of the RRR- (natural form) of vitamin E to learn more about how the human body utilizes this vitamin. Particular attention will be directed to the absorption phase. The results are expected to provide information about vitamin E biopotency and eventually improved Dietary Recommendations. ? ? ?
