Nomenclature Vitamin E
Eigh t na turally occurring compounds have vitamin E activity. They are fat-soluble alcohols of high molecular weight, closely related in structure, called tocopherols and tocotrienols. They all have the same basic ring structure but vary in. number and location of methyl substitutions. All tocopherols have a saturated phytyl side chain, whereas the side chain oftocotrienols possesses the three double bonds indicated by its name. Alphatocopherol (a-tocopherol) is the most active form. It deteriorates on exposure to light and decomposes upon irradiation with ultraviolet light. Contact \'lith lead and iron hastens destruction. Tocopherols, because they are readily oxidized themselves, have antioxidant properties and prevent deterioration of certain foods by oxidation. This characteristic probably exerts a protective action on vitamin A.
As with vitamins A and D, vitamin E activity was initially expressed in IU but the current preference is to give the individual compounds as milligrams (mg) and the total vitamin E activity as mg d-a-tocopherol equivalents. The IU was established as the biologic activity of 1 mg of dl-a-tocopheryl acetate. The biopotencies of the other available a-tocopherol compounds expressed as IV are as follows.
The other tocopherols (f3, y, and 8) and the tocotrienols (a, f3, y, and 8) have varied biologic activities from 1% to 50% of that of a-tocopherol. However, in mixed diets some may be present in amounts several times that of a-tocopherol, thereby contributing significantly to the total vitamin E activity. Only limited analytical data are available on the total vitamin E content of foods and, except for a-tocopherol, even less on the food values of individual tocopherols and tocotrienols. A compilation of the vitamin E content of foods has been published recently45.
For the purpose of estimating the total vitamin E activity of mixed diets from limited information, the Food and Nutrition Board has recommended the follwoing procedure. If only a-tocopherol is reported (in mg), the value should be multiplied by 1.2 to account for the estimated 20% higher total activity due to the other tocopherols present. When separate values are available, they can be converted to a-tocopherol equivalents by multiplying mg f3 tocopherol by 0.5, mg a-tocopherol by 0.1, and mg atocotrienol by 0.3. Added to the reported a-tocopherol value, the total vitamin E activity is obtained as milligrams a-tocopherol equivalents.
Absorption, transport, storage, and excretion
The intestinal absorption of vitamin E follows the pattern for fat and other fat-soluble nutrients, reaching the blood through the lymph chylomicrons. The percentage of dietary vitamin E absorbed appears to be relatively constant over a narrow range of normal dietary intakes 46 and to decrease with increasing dose from high intakes of a-tocophero1.47 However, there are great differences in the actual efficiency of vitamin E absorption observed in different studies, the reported values ranging from less than 30% to over 90%.46-48
Transported by the lipoproteins, the total plasma tocopherol concentration shows a positive correlation vvith plasma total lipids over a wide range of lipid concentrations, in both children 49,50 and adults.51 Normal plasma tocopherol values in adults range from 0.5 mg to 1.6 mg per d1. Values below 0.5 mg per dl have been considered deficient for all ages, as they appear to be associated with increased peroxide-induced erythrocyte hemolysis in vitro, another measure of vitamin E adequacy.53 An alternate and, according to some investigators, preferred measure of vitamin E adequacy is the ratio of plasma tocopherols (mg) to total lipids (g); a ratio of 0.8 or greater is considered norma1.Whereas plasma tocopherol levels of infants and children may be lower than in adults,49,50,54 their normal tocopherol to total lipid ratios appear to be above 0.8.,50
Newborn infants tend to have low plasma levels of vitamin E (approximately one-third the adult level) due not only to lower concentrations of blood lipids in the newborn but also to limited transfer of the vitamin across the placenta. Plasma levels begin to rise after birth, more rapidly in the breast-fed infant, and reach normal concentrations by about one month of age. Low birth weight (LBW) infants have even lower levels of vitamin E at birth. However, recent findings suggest that an appreciable number of children between 4 months and 13 years of age have plasma tocopherol levels below 0.5 mg per dl, with normal erythrocyte hemolysis test results.55 It is not known at present below wha t plasma tocopherol level increased erythrocyte hemolysis becomes evident in this age group.
Vitamin E is readily taken up from plasma lipoproteins by the erythrocytes and other body tissues. Their vitamin E levels reflect the average intake of the vitamin over a period of time. 56 It appears to be concentrated in the phospholipids of the cellular and subcellular membranes in all tissues, and the excess is stored in the adipose tissue fat as free tocopherolY The major route of atocopherol excretion is through the bile into feces as unaltered vitamin; its oxidation products are atocoquinone and a-tocohydroquinone, the latter mainly in the form of glucuronides.Some oxidation products are also found in the urine.
