Vitamin A

A fat-soluble vitamin essential for normal growth, for the formation of bones and teeth, for cell structure, for night vision, and for protecting the linings of the respiratory, digestive, and urinary tracts against infection. Vitamin A is absorbed by the body in the form of retinol. This is found in liver, fish-liver oils, egg yolk, dairy produce, and is added to margarines. Carotene, which the body converts into retinol, is found in various vegetables and fruits. Vitamin A deficiency is rare in developed countries. In most cases, it is due to malabsorption. Vitamin A deficiency may also result from long-term treatment with certain lipid-lowering drugs. Deficiency is common in some developing countries due to poor diet. The first symptom of deficiency is night blindness, followed by dryness and inflammation of the eyes (see xerophthalmia), keratomalacia, and eventually blindness. Deficiency also causes reduced resistance to infection, dry skin, and, in children, stunted growth. Prolonged excessive intake of vitamin A can cause headache, nausea, loss of appetite, skin peeling, hair loss, and irregular menstruation. In severe cases, the liver and spleen become enlarged. Excessive intake during pregnancy may cause birth defects. In infants, excessive intake may cause skull deformities, which disappear if the diet is corrected. The drug tretinoin (a derivative of vitamin A) is used to treat severe acne.