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Vitamin A includes A1 and A2, and A1 is retinol. Vitamin A2 is 3-dehydroepianthol, and its physiological activity is 40% of that of vitamin A1.
Vitamin A can promote growth and reproduction, maintain the normal secretion of bone, epithelial tissue, vision and mucous epithelium and other physiological functions. Vitamin A and its analogues can prevent precancerous lesions. In the absence of it, growth retardation and dark adaptability decrease, resulting in night blindness. Dry eye disease occurs due to the dryness, desquamation, hyperkeratosis and decreased lacrimal gland secretion of epithelial cells of epidermis and mucosa. The cornea in severe cases is softened, perforated and blind. The epithelial cells of respiratory tract keratinized and lost cilia, which made the resistance reduced and easy to infect. Foods with more vitamin A include liver, egg yolk and milk powder of poultry and livestock. Carotene can be changed into vitamin A in intestinal mucosa, red yellow and dark green vegetables, and fruits contain more carotene.