Vitamin E (a-tocopherol) occurs in nature as a single compound in which carbon atoms 2, 4' and 8' are in the R position and the phytyl side chain is straight (RRR, Fig. 10.1). Synthetic vitamin E (all-rac-a-tocopherol) is a mixture of eight stereoisomers including RRR. Half of the material is in the 2R-conformation, half in the 2S. It is believed that biopotency depends largely on the 2R-position; this is evident from the relative potencies in Fig. 10.1 (in brackets) as determined in the rat gestation-resorption test (1).

The bioavailability and biopotency of natural (RRR) and synthetic (all-rac) a-tocopherol have been discussed controversially for decades. Studies in humans that tried to assess potency of vitamin E are rare and the methods used do not appear to be sensitive enough to allow measurement of graded dose responses. Therefore bioavail-ability studies have been used in lieu of potency studies. The terms bioavailability and biopotency have often been used interchangeably, resulting in potentially misleading and invalid conclusions. The confusion in terminology is widespread in the literature and has hampered progress in determining potency. The aim of this presentation is a critical review of the studies on potency and bioavailability.

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