Bioavailability of alpha-tocopherol stereoisomers in rats depends on dietary doses of all-rac- or RRR-alpha-tocopheryl acetate.

The biological function of the stereoisomers of alpha-tocopherol is believed to depend on their bioavailability. Assessment of bioavailability within the body is therefore considered to be a good and easy way to predict biological value. The separation of alpha-tocopherol methyl ethers by chiral column HPLC is a good and easy tool with which to study the distribution of alpha-tocopherol stereoisomers. The objective of this investigation was to evaluate the bioavailability and distribution of the stereoisomers of alpha-tocopherol in the plasma and tissue in growing rats fed 25, 50, 100 or 200 mg/kg diet of either RRR- or all-rac-alpha-tocopheryl acetate for 10 d. The ratio between the two vitamin E sources based on their alpha-tocopherol concentration in plasma and tissues varied in the plasma between 1.04 and 1.74 and in tissues, ratios of 0.84-1.24 for liver, 0.34-1.59 for lung and 0.75-1.50 for spleen were obtained. An increasing dietary level of all-rac-alpha-tocopheryl acetate decreased the proportion of RRR-alpha-tocopherol, whereas the other stereoisomers were not affected. RRS-alpha-Tocopherol was present in the highest proportion, followed by RSR-, RSS- and RRR-alpha-tocopherol. In contrast to the other tissues and plasma, the liver contained the highest proportion (29-33 %) of the four 2S stereoisomers of total alpha-tocopherol. Rats fed RRR-alpha-tocopheryl acetate for 10 d showed a significant increase in the plasma and tissue content of RRR-alpha-tocopherol and a simultaneous decrease in the other three 2R isomers, whereas the absolute content of the 2S isomers was unaffected. In adipose tissue, concentrations of the three synthetic 2R isomers remained constant, whereas there was a steep increase in the content of RRR-alpha-tocopherol.