Vitamin E Functions Occurrence and Requirements

In animal cells, a-tocopherol is a component of all biological membranes. According to the current state of knowledge, its most important function is to protect membrane lipids and stored lip-ids from being degraded by lipid peroxidation. The food industry has long used the common knowledge about this antioxidant effect by enriching fats and oils with vitamin E to prevent peroxidation.

Due to exposure to light, heat, and/or chemicals, as well as through many metabolic processes, free radicals (A) form in the body. If a polyunsaturated fatty acid is attacked by a radical X, one of the two H atoms is removed from the methylene group between the two double bonds, resulting in a highly reactive lipid radical (missing electron). When the latter bonds with O2, a highly reactive lipid peroxide radical is formed. The latter will either react with another fatty acid to form a stable, nonphysio-logical but cytotoxic lipid peroxide or "fuse with" another peroxide molecule. During the transfer of the H atom, an additional lipid radical is formed, triggering an autocatalytic chain reaction. Uninterrupted, this process can quickly destroy the affected biological membrane's function. Vitamin E has a very strong affinity for lipid peroxide radicals: transfer of an H atom from vitamin E to the lipid peroxide radical results in a stable lipid hydroperoxide and a vitamin E radical. Since the latter is resonance-stabilized and therefore extremely inert, this interrupts the chain reaction. The tocopherol radical, which is anchored in the cell membrane, is probably reconverted to vitamin E by ascorbic acid (vitamin C) inside the watery cytosol (see p. 206).

Plant germs and seeds, as well as their oils, and products derived from them are the best sources of vitamin E (B). In wheat germ, sunflower seeds, cottonseed, and olive oil, RRR-a-tocopherol makes up most (50-100%) of the vitamin E, while y-tocopherol, which has only 10 % of the biological activity, dominates in soy and corn oil. Most oils sold as "vegetable oil" contain various amounts of soy, corn and cottonseed oil, margarines mostly soy, corn, and sunflower oil. Their natural vitamin E activity depends on their sunflower or cottonseed components, otherwise, synthetic vitamin E may be added as a stabilizer. Cold-pressed wheat germ oil has the highest natural vitamin E content.

Estimates of adequate vitamin E intakes (C) depend on the intake of polyunsaturated fatty acids: 0.5 mg RRR-a-tocopherol should be supplied for every gram of diene fatty acids; even many plant oils do not contain these amounts (e.g., soy oil, approximately 0.3 mg). An intake of 24 g of diene equivalents (18 g linoleic acid, 3 g lino-lenic acid) would, therefore, mean a calculated requirement of 12 mg a-TE per day. People in the U.S. generally achieve these approximate intakes, but higher intakes may well be desirable for optimal protection from peroxidation. People on low fat diets may be at risk of vitamin E deficiency due to low vegetable oil intakes.

In preemies, patients with fat malabsorption, various diseases like cystic fibrosis or coronary artery disease, vitamin E supplements have proven beneficial. The UL is 1000 mg of supplemental a-tocopherol (1500 IU vitamin E).

A. Protective Effects of Vitamin E -

• upid-H + x-radical" -Lipid radical- + x-h)
• Vitamin E-OH

Lipid peroxide radical—O—O" -► Vitamin E-O" + Lipid peroxide—O—O—H^

• Chain break
• Lipid radical" + Lipid peroxide—O—O—H^

• 3 Chain reaction
• B. Occurrence and Daily Requirement-

The daily requirement of 12 mg vitamin E is contained in:

1.2 kg cheese -70g margarine -

(depending on season)

30 eggs -20 g enriched ■ margarine 5 ml wheat germ oil* 20 - 30 ml safflower*, grapeseed*, olive oil*

800 - 1000 g smoked Chinook salmon, tuna

"Vitamin E balance depends on prevalence of unsaturated fatty acids

100 g sprouts 500 g whole grain

1. 5 - 3 kg meat
2. 2 kg fruit 200 g walnuts 50 - 80 g filberts, pine nuts 120 g peanuts 500 g asparagus, parsnips, spinach, dandelion leaves

2 kg vegetables

100 g sprouts 500 g whole grain

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