Different Forms of Vitamin E and Their Biopotencies|
There are several distinct forms of vitamin E in dietary supplements vying for consumers' attention. While tocotrienols, the form of vitamin E that features an unsaturated side chain, is becoming more popular, tocopherols (mainly alpha-tocopherol) still remain the most widely used and researched form. Tocopherol supplements usually feature either alpha-tocopherol, alpha-tocopheryl acetate or alpha-tocopheryl succinate, the latter two thought to confer enhanced stability.
Alpha-tocopherol can be derived from natural sources or can be synthetically produced. Natural source alpha-tocopherol is a single stereoisomer and is designated by the prefix "d-" or "RRR," while the synthetic form is a combination of eight stereoisomers and is identified by the prefix "dl-" or "all-rac."
Naturalsource d-alpha-tocopherol has been shown to have a higher biopotency than the dl-rac form. The currently accepted ratio is 1.36:1 (d:dl). A recent proposal has been made to the National Academy of Sciences to change this biopotency factor to 2:1 based on the results of more recent research studies. Yet, some researchers challenge this proposition believing it is based on erroneous conclusions extrapolated from these studies. 
1. Hoppe PP, Krennrich G.
Bioavailability and potency of natural-source and all-racemic a-tocopherol in the human: a dispute.
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