Vitamin E is the collective term for four fat-soluble, natural compounds with similar chemical structure: alpha-, beta-, gamma- and delta-tocopherol. Of these, alpha-tocopherol is believed to have the highest nutritional activity. Eight structural isomers of alpha-tocopherol are possible, but only the d-alpha-tocopherol occurs in nature. The other seven isomers have less nutritional activity, and are present only in synthetic vitamin E (dl-alpha tocopherol). Vitamin E is thought to be one of the body’s most important antioxidant nutrients. Antioxidants have the potential to protect healthy cells from oxidative and free radical damage. Free radicals are unstable chemicals formed in the body during metabolism and from exposure to environmental sources, such as pollution and cigarette smoke. Free radicals are necessary for energy metabolism and immune function, but when an excessive number of free radicals is formed, they can attack healthy cells, especially cell membrane lipids and proteins. Vitamin E can be an especially valuable antioxidant in the cell membranes, where it is thought to help prevent oxidation of unsaturated fatty acids by trapping free radicals. This has the potential to help stabilize and protect cell membranes, especially red blood cells and tissues sensitive to oxidation, such as the lungs, eyes, and arteries. Vitamin E is also believed to protect the liver and other tissues from the free-radical damage of toxicants, such as mercury, lead, ozone, nitrous oxide, carbon tetrachloride, benzene, cresols, and various drugs.
Vitamin E is believed to be important for normal immune function, and many studies show that it has the potential to prevent lipid peroxidation of blood lipoproteins, such as LDL-cholesterol. Intestinal absorption of vitamin E is associated with fat absorption: some dietary fat must be present for efficient vitamin E absorption to occur. Studies show that natural d-alpha-tocopherol is more efficiently absorbed and has significantly higher bioavailability than synthetic dl-alpha-tocopherol. Once absorbed, vitamin E is transported into the circulatory system via chylomicrons, and then transferred to VLDL, LDL, and HDL particles. Tissues slowly accumulate vitamin E from these plasma lipoproteins. Liver and adipose tissues have the highest concentrations, but muscle also accounts for a large proportion of vitamin E storage in the body.