Vitamin E is the generic term for all tocol and tocotrienol derivatives having a-tocopherol biological activity. There are eight naturally occurring forms of vitamin E: a-, b-, g-, and d- tocopherols and a-, b-, g-, and d-tocotrienols. Among these, D-a-tocopherol possesses the greatest biological activity. One international unit of vitamin E is the activity of 1 mg of DL-a-tocopheryl acetate. All racemic (i.e., DL-a-tocopherol) has about 70% of the activity of pure D-a-tocopherol. Bieri and McKenna consider b-tocopherol and g-tocopherol to have only 40 and 10% of the activity, respectively, of a-tocopherol (set at 100%). The only other natural form to possess activity is a-tocotrienol, which on the rating scale used above was estimated to contain a biopotency of 25%.
Vitamin E is subject to destruction by oxidation, and this process is accelerated by heat, moisture, unsaturated fat, and trace minerals. Losses of 50 70% have been observed to occur in alfalfa hay stored at 32°C for 12 wk; losses up to 30% have been known to occur during dehydration of alfalfa. Treatment of high-moisture grains with organic acids also greatly enhances vitamin E destruction. However, even mildly alkaline conditions of vitamin E storage are detrimental to vitamin E stability. Thus, finely ground limestone or MgO coming in direct contact with vitamin E can markedly reduce its bioavailability.
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