Vitamin E comprises of eight isomers (α-, β-, γ-, and δ-tocopherol and α-, β-, γ-, and δ-tocotrienol) that share a common biological activity. Vitamin E is fat soluble, and this has important implication for its absorption, storage and mechanisms of action in vivo. Vitamin E’s lipophilic nature ensures that it accumulates in the lipid membranes of cells, where it acts as an antioxidant and prevents lipid peroxidation. Absorption of vitamin E requires bile acids, which emulsify the vitamin E with dietary fats and allow absorption by increasing miscibility. Once absorbed the vitamin E is packaged in chylomicrons for transport to the circulation. Increased solubility of vitamin E can be accomplished by bonding of the tocopherol (alcohol) molecule to an organic succinate, acetate or nicotinate molecule, producing a tocopheryl ester, although there is no evidence that this increased absorption.
A number of water soluble vitamin E supplements have become commercially available in recent years. Water soluble forms of vitamin E are created by incorporating a molecule of polyethylene glycol 1000 by ester linkage onto the end of α-tocopheryl succinate, which results in the formation of an amphipathic structure (D-alpha-tocopheryl polyethylene glycol 1000 succinate; TPGS). This structure does not need bile acids to transverse the unstirred water layer of the enterocyte brush border because the amphipathic structure allows the formation of micelles in the absence of an emulsifier. Evidence suggests that these compounds are absorbed into the enterocytes and hydrolysed to form the alcohol tocopherol form. Clinical data suggests that those individuals with fat malabsorption conditions are able to absorb TPGS and that they may be able to increase the absorption of other fat soluble vitamins because of its ability to create micelles.
Differences in plasma concentrations of α-tocopherol have been studied in healthy individuals after supplementation with either TPGS or D-α-tocopheryl acetate1. Individuals were given 400, 800 or 1200 iu of water soluble or fat soluble vitamin E preparation, in both a single dose or multiple doses protocols. In the single dose studies, TPGS supplementation resulted in increases in the plasma concentration of D-α-tocopherol, with peak concentrations occurring between 6 and 15 hours post ingestion. Plasma levels of α-tocopherol remained elevated for up to 24 hours. However, supplementation with tocopheryl acetate resulted in greater increases in plasma α-tocopherol concentrations in all subjects. As with the water soluble form, plasma levels remained elevated for only 24 hours before dropping to baseline. Plasma levels of α-tocopherol showed a dose response after supplementation with fat soluble forms, but not after the water soluble form.
In the multiple dose studies water soluble and fat soluble vitamin E were given for 28 consecutive days in divided doses. Mean α-tocopherol plasma elevations for the 400, 800 and 1200 iu doses of water soluble vitamin E were significantly different from baseline, but there was no significant difference between the treatment doses (5.6, 3.9 and 5.8 µmol/L, respectively). Supplementation with fat soluble tocopheryl acetate caused significant elevations in plasma α-tocopherol from baseline. There was also a dose response between 400 iu (12.1 µmol/L elevation) and 800 and 1200 iu (19.8 and 19.3 µmol/L elevations, respectively). The difference in α-tocopherol elevations between the TPGS and the tocopheryl acetate was 6.5 µmol/L after 400 iu, 15.8 µmol/L after 800 iu and 13.4 µmol/L after 1200 iu. There results suggest that fat soluble forms of vitamin E are better absorbed in healthy individuals.
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