All natural sources of vitamin A in the diet are derived ultimately from provitamin A carotenoids. For much of the world's human population, and particularly in parts of the developing world, vegetables and fruits provide the main dietary sources of vitamin A in the form of P-carotene and other provitamin carotenoids. In other parts of the world, milk, butter, cheese and eggs are important dietary sources of vitamin A. The liver of meat animals is a particularly rich source as this organ stores the vitamin for the body's use. Preformed vitamin A is present in animal tissues and in milk as a consequence of the enzymatic conversion of ingested provitamin carotenoids in the intestinal wall of the animal. Dietary preformed vitamin A consists mainly of retinol esterified with long chain fatty acids, particularly palmitic acid. Retinyl esters are also found in processed foods supplemented with vitamin A.
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