Vitamin E; sources, significance

THE SIGNIFICANCE OF VITAMIN E

Vitamin E has a fundamental role in the normal metabolism of all cells. Therefore, its deficiency can affect several different organ systems. Its function is related to those of several other nutrients and endogenous factors that, collectively, comprise a multicomponent system that provides protection against the potentially damaging effects of reactive species of oxygen formed during metabolism or that are encountered in the environment. Both the need for vitamin E and the manifestations of its deficiency can be affected by such nutrients as selenium and vitamin C, and by exposure to such pro-oxidant factors as polyunsaturated fatty acids (PUFAs), air pollution, and ultraviolet (UV) light. Recent evidence indicates that vitamin E may also have non-antioxidant functions in regulating gene expression and cell signaling. Unlike other vitamins, vitamin E is not only essentially non-toxic, but also appears to be beneficial at dose levels appreciably greater than those required to prevent clinical signs of deficiency. Most notably, supranutritional levels of the vitamin have been useful in reducing the oxidation of low-density lipoproteins (LDLs), and thus reducing the risk of atherosclerosis. Although vitamin E is present in most plants, only plant oils are rich sources, and most people consume less than recommended levels.1 Its low regular intake, the ubiquitous and complex nature of its biological function, its demonstrated safety, and its apparent usefulness in combating a variety of oxidative stress disorders have generated enormous interest in this vitamin among the basic and clinical science communities as well as the lay public.




Vitamin E deficiency

SOURCES OF VITAMIN E 

Distribution in Foods

Vitamin E is synthesized only by photosynthetic organisms – plants, algae, and some cyanobacteria – where it is thought to function as a protective antioxidant in germination and cold adaptation. All higher plants appear to contain α-tocopherol in their leaves and other green parts. Because α-tocopherol is contained mainly in the chloroplasts of plant cells (whereas the β-, γ-, and δ-vitamers are usually found outside of these particles), green plants tend to contain more vitamin E than yellow plants. The richest food sources are plant oils. Wheatgerm, sunflower, and safflower oils are rich sources of α-(RRR)-tocopherol, whereas corn and soybean oils contain mostly γ-(RRR)- tocopherol. Some plant tissues, notably bran and germfractions,2 can also contain tocotrienols, often in esterified form – unlike the tocopherols, which exist only as free alcohols. Animal tissues tend to contain low amounts of α-tocopherol, the highest levels occurring in fatty tissues.

Dietary Sources of Vitamin E

The important sources of vitamin E in human diets and animal feeds are vegetable oils and, to lesser extents, seeds and cereal grains. The dominant dietary form (70% of tocopherols in American diets) is γ-tocopherol. Wheatgerm oil is the richest natural source, containing 0.9–1.3 mg of α-tocopherol per gram, i.e., about 60% of its total tocopherols. The seeds and grains from which these oils are derived also contain appreciable amounts of vitamin E. Plants also synthesize tocotrienols. The richest food sources are rice-bran oil, in which tocotrienols comprise most of the vitamers E, and palm oil, in which tocotrienols comprise 70% of total vitamers E. Cereals contain small amounts of tocotrienols. Accordingly, cereals in general and wheatgerm in particular are good sources of the vitamin. Foods that are formulated with vegetable oils (e.g., margarine, baked products) tend to vary greatly in vitamin E content owing to differences in the types of oils used, and to the thermal stabilities of the vitamers E present. α-Tocopherol is used in dietary supplements.  Regardless of the form consumed, α-tocopherol is the main form found in tissues.

REFERENCE

The Vitamins

Fourth Edition

Gerald F. Combs, Jr

Professor Emeritus

Cornell University

Ithaca, NY

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