Thursday, March 6, 2008

Understanding terms used to describe nutrient recommendations

Nutrient listings use the metric system. RDAs for protein are listed in grams. The RDA and AIs for vitamins and minerals are shown in milligrams (mg) and micrograms (mcg). A milligram is 1⁄1000 of a gram; a microgram is 1⁄1000 of a milligram. Vitamin A, vitamin D, and vitamin E are special cases.

For instance, one form of vitamin A is preformed vitamin A, a form of the nutrient that your body can use right away. Preformed vitamin A, known as retinol, is found in food from animals —liver, milk, and eggs. Carotenoids (red or yellow pigments in plants) also provide vitamin A. But to get vitamin A from carotenoids, your body has to convert the pigments to chemicals similar to retinol. Because retinol is a ready-made nutrient, the RDA for vitamin A is listed in units called retinol equivalents (RE). One mcg (microgram)

RE is approximately equal to 3.33 international units (IU, the former unit of measurement for vitamin A).
Vitamin D consists of three compounds: vitamin D1, vitamin D2, and vitamin D3. Cholecalciferol, the chemical name for vitamin D3, is the most active of the three, so the RDA for vitamin D is measured in equivalents of cholecalciferol. Your body gets vitamin E from two classes of chemicals in food: tocopherols and tocotrienols. The compound with the greatest vitamin E activity is a tocopherol: alpha-tocopherol. The RDA for vitamin E is measured in milligrams of alphatocopherol equivalents (a-TE).

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