1994) and white blood cells (Tsuchiya et al., 1992). Plasma vitamin E is not routinely depleted in cigarette smokers relative to nonsmokers. However, vitamin E supplementation of smokers has been shown to reduce indicators of lipid peroxidation (Mezzetti et al., 1995; Morrow et al., 1995; Pratico et al., 1998; Reilly et al., 1996), suggesting that plasma vitamin E may not be indicative of tissue stores.
It is unknown if any adjustment in vitamin E requirements is needed in those who smoke or are routinely exposed to smoke.
As stated earlier, the Dietary Reference Intakes (DRIs) for vitamin E are based on α-tocopherol only and do not include amounts obtained from the other seven naturally occurring forms historically called vitamin E (β-, γ-, δ-tocopherol and the four tocotrienols). Because the different forms of vitamin E cannot be interconverted in the human, the Estimated Average Requirements (EARs), Recommended Dietary Allowances (RDAs), and Adequate Intakes (AIs) apply only to the intake of RRR-α-tocopherol from food and the 2R-stereoisomeric forms of α-tocopherol (RRR-, RSR-, RRS-, and RSS-α-tocopherol) that occur in fortified foods and supplements. Although both α- and γ-tocopherols are absorbed, only α-tocopherol is preferentially secreted by the liver into the plasma (because α-tocopherol transfer protein [α-TTP] recognizes only α-tocopherol) for transport to tissues while γ-tocopherol is preferentially metabolized and excreted. This implies that the body requires α-tocopherol for some special (as yet undefined) need and other forms of vitamin E do not qualify.
Currently, most nutrient databases, as well as nutrition labels, do not distinguish among the different tocopherols in food. Thus, the data below on intakes from surveys and nutrient content of foods are presented as α-tocopherol equivalents (α-TE) and thus include the contribution of all eight naturally occurring forms of vitamin E, after adjustment for bioavailability using previously determined equivalency (e.g., γ-tocopherol was assumed to have only 10 percent of the availability of α-tocopherol) based on fetal resorption assays (see earlier section on “ Units of Vitamin E Activity ”). Because other forms of vitamin E occur in foods (e.g., γ-tocopherol is present in widely consumed oils such as soybean and corn oils), the intake of α-TE is greater than the intake of α-tocopherol alone.