vitamin A

vitamin A Essential in the diet either as the preformed vitamin (retinol) found in animal foods or as a precursor, carotene, found in plant foods (usually both are present in the diet). Required for control of growth, cell turnover and fetal development, maintenance of fertility, and maintenance of the epithelial tissues lining the mouth and respiratory and urinary tracts; essential in vision.

Deficiency leads to slow adaptation to see in dim light (poor dark adaptation), an early sign of deficiency, and later to night blindness; then drying of the tear ducts (xerophthalmia) and ulceration of the cornea (keratomalacia) resulting in blindness.  Retinol occurs in animal products, especially liver, kidney, fish liver oils, milk, and butter. Carotene is found in green‐ and orange‐coloured vegetables and fruits; especially rich in red palm oil and carrots.

The vitamin A content of foods is expressed as retinol equivalents, i.e. retinol plus carotene; 1 μg retinol = 6 μg β‐carotene = 12 μg other active carotenoids = 3.33 international units.