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Vitamin A Vitamers and Units of Activity

The fat-soluble vitamins comprise vitamins A, D, E, and K, whose biological activities are attributed to a number of structurally related compounds known as vitamers. Also included are those carotenoids that are precursors of vitamin A. Recommended dietary allowances (RDAs) based on human epidemiological and experimental animal studies have been published in the United States for vitamins A, D, E, and K (1). Other countries and international bodies have compiled similar recommendations. In the United States and Canada, fluid milk is supplemented by law with vitamin D to a level of 400 international units per quart (10 /zg/0.95 L) to meet the RDA of 10 p%. Other commodities, such as margarine, milk products, ready-to-eat breakfast cereals, and dietetic foods, are commonly supplemented with vitamins A, D, and E. Except for infant formulas, vitamin K is not added to foods. The addition of vitamins to a particular processed food is intended to provide a specific proportion of the RDA. [Pg.321]

Research Council defined 1 mg of a-tocopherol as 1 unit of a-TE (mg x 1). The activities as a-TE of other vitamers were (3-tocopherol, mg x 0.5 y-tocopherol, mg xO. 1 8-tocopherol, mg x 0.03 a-tocotrienol, mg x 0.3 and (3-to-cotrienol, mg x 0.05. The activities of y- and 8-tocotrienol were undetectable. The Recommended Dietary Allowances (RDAs) are only based on intake of the 2R-stereoisomeric forms of a-tocopherol (RRR-, RSR-, RRS-, and RSS-tocopherol) from food, fortified food, and vitamin supplements (Food and Nutrition Board, 2000). The 2S-stereoisomeric forms of a-tocopherol and the other tocopherols ((3-, y-, and 5-tocopherol) and tocotrienols are not used to estimate the RDAs. [Pg.489]

Biological samples may contain any or all of three forms of vitamin K vitamin Ki(20) (phylloquinone) which is synthesized by green plants and found in chloroplasts of photosynthetic plants and is the major form of vitamin K in plant and most animal foods vitamin K2, a group of vitamers (menaqui-nones) synthesized by bacteria and ranging from MK-4 to MK-13 according to the number of isoprene units in the side chain, and finally synthetic vitamin K3 (menadione), which exhibits vitamin K activity by virtue of its in vivo conversion to menaquinones, chiefly MK-4. The latter is a pharmaceutical and water-soluble form which does not occur in food and unlike the other forms is toxic at high intakes. Such water-soluble derivatives of menadione are used as animal feed supplement but owing to toxicity to humans menadione cannot be used in human medicine or as a food supplement. [Pg.4913]

Based on biological assay in vitamin E-deficient rats, the vitamers have widely varying biological activity. The original international unit (iu) of vitamin E potency was equated with the activity of 1 mg of (synthetic) dl- -tocopherol acetate on this basis, pure D-a-tocopherol (i i i -a-tocopherol, the most potent vitamer) is 1.49 iu per mg. The precise mixture of stereoisomers in this original standard is unknown, and the different stereoisomers have very different biological activities, so that different preparations may differ considerably. [Pg.111]


See other pages where Vitamin A Vitamers and Units of Activity is mentioned: [Pg.31]    [Pg.31]    [Pg.33]    [Pg.31]    [Pg.31]    [Pg.33]    [Pg.31]    [Pg.31]    [Pg.33]    [Pg.31]    [Pg.31]    [Pg.33]    [Pg.31]    [Pg.31]    [Pg.33]    [Pg.31]    [Pg.31]    [Pg.33]    [Pg.7]    [Pg.7]    [Pg.79]   


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Vitamin vitamers

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