VITAMIN E

Vitamin E is an essential fat-soluble vitamin that includes eight naturally occurring compounds in two classes designated as tocopherols and tocotrienols. Each of these compounds exhibits different biological activities. d-a-Tocopherol has the highest biological activity and is the most widely available form of vitamin E in food. The other isomers ( beta, delta, gamma), some of which are more abundant in a typical Western diet, are less biologically active than d-a-tocopherol. The commercially available synthetic forms of vitamin E are comprised of approximately an equal mixture of eight stereoisomeric forms of a-tocopherol, usually in the esterified form such as acetate or succinate. For practical purposes, 1 International Unit (IU) of vitamin E represents 1 mg of the synthetic form, racemic a-tocopherol acetate, while the natural form of d-a-tocopherol has a biopotency of vitamin E equal to at least 1.49 IU, if not more. The most widely accepted biological function of vitamin E is its antioxidant properties. Vitamin E is the most effective chain-breaking, lipid-soluble antioxidant in biological membranes, where it contributes to membrane stability. It protects critical cellular structures against damage from oxygen free radicals and reactive products of lipid peroxidation.

Absorption of vitamin E is dependent upon the digestion and absorption of fat. Free tocopherols are absorbed by a non-saturable, passive process into the lymphatic circulation along with fat. About 45% of an ordinary dose is absorbed into the lymph.

Deficiencies: The main signs of severe deficiency in animals are reproductive failure, nutritional "muscular dystrophy," hemolytic anemia, and neurological and immunological abnormalities. The last three processes also have been identified in humans. However, vitamin E deficiency occurs rarely in humans, having been reported in only two situations: premature infants with very low birth weight and patients who fail to absorb fat.

Diet recommendations: The Recommended Dietary Allowance (RDA) for vitamin E is based primarily on customary intakes from US food sources. The current RDA for males is 10 mg and 8 mg for females. However, the requirement for vitamin E increases with higher intakes of polyunsaturated fatty acids (PUFA). The recommended ratio of E/PUFA is 0.4 mg d-a-tocopherol per gram of PUFA. In defining the ideal intake, factors to consider are intake of other antioxidants, age, environmental pollutants, and physical activity.

Food sources: Vegetables and seed oils including soybean, safflower, and corn; sunflower seeds; nuts; whole grains; and wheat germ are the main sources of the tocopherols. Leafy vegetables also supply an appreciable amount of this nutrient. However, animal products and most fruits and vegetables are generally poor sources.

Toxicity: Vitamin E is relatively safe compared to the fat-soluble vitamins. Few side effects from high intakes of this vitamin have been reported, even at doses as high as 3200 mg daily. However, high vitamin E supplementation may be contraindicated when a coagulation defect is present due to vitamin K deficiency or in individuals receiving anticoagulant drugs.

Recent research: Vitamin E has been shown to influence signal transduction pathways. This effect, however, may not be mediated through its antioxidant properties. Evidence from in vitro studies shows that vitamin E influences expression of adhesion molecules on endothelial cells and monocyte adhesion to endothelial cells. Vitamin E supplementation at a dose of 200 IU/day significantly improved immune response in healthy elderly. High intake (³ 200 IU/ day) and high serum vitamin E levels have been associated with reduced risk for coronary heart disease in men and women, reduced risk of prostate cancer and may slow progression of Alzheimer's disease.

For further information:

Meydani, M. (1995) Vitamin E. Lancet 345: 170-175.

Miller, R. D. & Hayes, K. C. (1982) Vitamin excess and toxicity. In: Nutritional Toxicology (Hathcock, J. N., ed.) vol. 1, pp. 81-133. Academic Press, New York, NY.

Meydani, S. N., Meydani, M., Blumberg, J. B., Leka, L. S., Siber, G., Loszewski, R., Thompson, C., Pedrosa, M. C., Diamond, R. D. & Stollar, B. D. (1997) Vitamin E supplementation enhances in vivo immune response in healthy elderly: A dose-response study. JAMA. 277: 1380-1386.

Meydani, S. N., Wu, D., Santos, M. S. & Hayek, M. G. (1995) Antioxidants and immune response in aged persons: Overview of present evidence. Am. J. Clin. Nutr.; 62: 1463S-1462S.

Prepared By:

Mohsen Meydani, D.V.M., Ph.D.
Professor of Nutrition
Vascular Biology Laboratory
USDA Human Nutrition Research Center on Aging at Tufts University
Boston, MA 02111-1525
Phone: 617-556-3126
FAX: 617-556-3224
Email: Meydani_VBL@HNRC.Tufts.edu

K.C. Hayes, D.V.M., Ph.D.
Professor of Biology
Foster Biomedical Research Laboratory
Brandeis University
Waltham, MA 02254
Phone: 781-736-2051
FAX: 781-736-2054
Email: kchayes@brandeis.edu