Vitamins are essential micronutrients necessary for specific metabolic functions in the body. In 1941, the National Academy of Sciences published the Recommended Daily Allowance, or RDA, for vitamins. The RDA is a determination of the healthy amount of each nutrient according to age, gender and overall health. The RDA should be used as a general guide to ensure adequate nutrient intake.
Primary Function
Vitamin A, also known as retinol, plays a major role in vision, tissue strength and growth. As the name suggests, retinol is an essential part of rhodopsin, the pigment in the retina responsible for the eye's ability to adapt to different amounts of light.
Secondary Function
Retinol is also responsible for the integrity of epithelial tissue, which is the protective tissue of the body. This is the skin and inner membranes of the nose, throat, eyes and intestinal tract. Retinol is also important in the growth of skeletal tissue and plays a role in protein synthesis.
Food Sources
Vitamin A comes in two food forms: retinol and carotene. Retinol is the preformed source found in liver, egg yolks, fish-liver oils, butter and cream. Carotene is a pigment found in yellow and green plants that the body converts to vitamin A. The most common form of carotene consumption is beta-carotene. Sources of carotene include green leafy vegetables, carrots, sweet potatoes, yellow squash, apricots and peaches.
Deficiencies
The RDA for vitamin A is 800 micrograms for adult females and 100 micrograms for adult males. In mild vitamin A deficiencies, night blindness and slow adaptation to darkness can result. In extreme deficiency, xerosis -- which causes itchy, burning eyes and red, inflamed eyelids -- as well as complete blindness can occur.
References
- "Basic Nutrition and Diet Therapy"; Sue Rodwell Williams; 2001
- "Advanced Nutrition and Human Metabolism (third edition)"; James Groff and Sareen Groper; 2000
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