Vitamin A

Fat-soluble vitamin, occurring either as the preformed vitamin (retinol) found in animal foods or as a precursor (carotenes) found in plant foods. Required for control of growth, cell turnover and fetal development, maintenance of fertility and maintenance of the normal moist condition of epithelial tissues lining the mouth and respiratory and urinary tracts; essential in vision. The main active metabolites in the body are retinaldehyde, all-trans- and 9-cis-retinoic acids. Deficiency leads to slow adaptation to see in dim light (poor dark adaptation), later to night blindness; then drying of the tear ducts (xerophthalmia) and ulceration of the cornea (keratomalacia) resulting in blindness.
The vitamin A content of foods is expressed as retinol equivalents, i.e. retinol plus carotene; 1μg retinol = 6μg β-carotene = 12μg other active carotenoids = 3.33 international units.