General Description and Scientific Name

Dietary components with vitamin E antioxidant activity include a-, 7-, and ¿-tocopherols and a-, 7-, and ¿-tocotrienols. These compounds all have a chromanol ring with a phytyl tail (tocopher-ols) or an unsaturated tail (tocotrienols) (Figure 1) and vary in the number of methyl groups on the chromanol ring: a-tocopherol or a-tocotrienol has three methyl groups, or 7- have two, and ¿- tocopherol and ¿-tocotrienol have one.

The naturally occurring form of a-tocopherol is called RRR-a-tocopherol; on labels it is called d-a-tocopherol and it is more formally known as 2,5,7,8-tetramethyl-2R-(4'R,8'R,12 trimethyltridecyl)-6-chromanol. At positions 2, 4', and 8'of a-tocopherol are chiral carbon centers that are in the R-conformation in naturally occurring a-tocopherol, but theoretically can take on either the R- or the S-conformation. Position 2 is the most important for biologic activity. Therefore, the

DRIs for vitamin E are given in milligrams of 2R-a-tocopherol (Table 1 see below for discussion).

The chemical synthesis of a-tocopherol results in an equal mixture of eight different stereoisomers (RRR, RSR, RRS, RSS, SRR, SSR, SRS, SSS) or, more formally, 2,5,7,8-tetramethyl-2RS-(4'RS,8'RS,12 trimethyltridecyl)-6-chromanol. To indicate that synthetic a-tocopherol is a racemic mixture, it is called a//-rac-a-tocopherol, or on labels, dl-a-tocopherol. The first letter of the three-letter combination is the 2 position; therefore, only half of the synthetic a-tocopherol is in the 'active' 2R-a-tocopherol conformation. Table 2 lists the factors used to convert international units (IU) to milligrams. For example, if a vitamin E supplement is labeled 400 IU and it is dl-a-tocopheryl acetate, then 400 times 0.45 equals 180mg2R-a-tocopherol, but if it is labeled d-a-tocopheryl acetate, then 400 times 0.67 equals 268 mg2R-a-tocopherol.

Vitamin E Supplements

Most vitamin E supplements and food fortificants contain all rac-a-tocopherol, but can contain mixtures of tocopherols or tocotrienols. Supplements often are sold as esters, which protect a-tocopherol from oxidation. These can be acetates, succinates, or nicotinates of a-tocopherol. Either the natural stereoisomer (RRR-a-tocopherol) or the synthetic (all rac-a-tocopherol) can be sold as an ester, e.g., d- or dl-a-tocopheryl acetate, respectively.

Dietary Vitamin E

Vitamin E can be readily obtained from food. Generally, the richest sources are vegetable oils. Wheat germ oil, safflower oil, and sunflower oil contain predominantly a-tocopherol, while soy and corn oils contain predominantly 7-tocopherol. All of these oils are polyunsaturated. Good sources of monounsaturated oils, such as olive or canola oils, also contain predominantly a-tocopherol. Whole grains and nuts are also good sources of vitamin E. Fruits and vegetables, although rich in water-soluble antioxidants, are not good sources of vitamin E.

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