Group of fat soluble compounds (retinoids) which exist in several isomeric forms and occur preformed only in foods of animal origin. The two vitamin A forms are: retinols, which predominate in mammals and marine fish; and dehydroretinols, which predominate in freshwater fish. Vitamin A is present in yellow and green leafy plants as provitamin A, of which there are several forms. The most important ones in human nutrition are the carotenoids, α- and β-carotene and cryptoxanthin. These are converted to the active vitamin in the intestinal wall and liver. Richest sources of preformed retinols are fish liver oils, egg yolks and fortified milk. Biologically active carotenoids are found in dark green leafy vegetables and yellow fruits and vegetables, such as squashes and carrots. In humans, common signs of vitamin A deficiency are poor growth, lowered resistance to infection, night blindness and rough scaly skin. Severe deficiency leads to keratomalacia and xerophthalmia.