Editing Tocopherol (section)

===α-Tocopherol===
{{Main|α-Tocopherol}}
α-Tocopherol is the form of vitamin E that is preferentially absorbed and accumulated in humans.<ref>{{cite journal | vauthors = Rigotti A | title = Absorption, transport, and tissue delivery of vitamin E | journal = Molecular Aspects of Medicine | volume = 28 | issue = 5–6 | pages = 423–436 | year = 2007 | pmid = 17320165 | doi = 10.1016/j.mam.2007.01.002 }}</ref> The measurement of "vitamin&nbsp;E" activity in [[international unit]]s (IU) was based on fertility enhancement by the prevention of miscarriages in pregnant rats relative to α-tocopherol.

Although the mono-methylated form ddd-γ-tocopherol is the most prevalent form of vitamin E in oils, there is evidence that rats can methylate this form to the preferred α-tocopherol, since several generations of rats retained α-tocopherol tissue levels, even when those generations were fed only γ-tocopherol through their lives.

There are three [[stereocenter]]s in α-tocopherol, so this is a [[Chirality (chemistry)|chiral]] molecule.<ref name=Jensen/> The eight [[Stereoisomerism|stereoisomers]] of α-tocopherol differ in the arrangement of groups around these stereocenters. In the image of ''RRR''-α-tocopherol below, all three stereocenters are in the ''R'' form. However, if the middle of the three stereocenters were changed (so the hydrogen was now pointing down and the [[methyl group]] pointing up), this would become the structure of ''RSR''-α-tocopherol. These stereoisomers also may be named in an alternative older nomenclature, where the stereocenters are either in the ''d'' or ''l'' form.<ref name=Traber/>

[[File:Alpha-Tocopherol Structural Formulae V.1.svg|thumb|250px|class=skin-invert-image|''RRR'' [[Stereoisomerism|stereoisomer]] of α-tocopherol, bonds around the [[stereocenter]]s are shown as dashed lines (pointing down) or wedges (pointing up).]]

1 IU of tocopherol is defined as {{frac|2|3}} milligrams of ''RRR''-α-tocopherol (formerly named d-α-tocopherol or sometimes ddd-α-tocopherol). 1&nbsp;IU is also defined as 1&nbsp;milligram of an equal mix of the eight stereoisomers, which is a [[racemic mixture]] called [[tocopheryl acetate|''all-rac''-α-tocopheryl acetate]]. This mix of stereoisomers is often called dl-α-tocopheryl acetate, even though it is more precisely {{chem name|dl,dl,dl-α-tocopheryl acetate}}). However, 1&nbsp;IU of this racemic mixture is not now considered equivalent to 1&nbsp;IU of natural (RRR) α-tocopherol, and the [[Institute of Medicine]] and the [[USDA]] now convert IU's of the racemic mixture to milligrams of equivalent RRR using 1&nbsp;IU racemic mixture = 0.45 "milligrams α-tocopherol".{{r|USDA-NDL|pp=20–21}}