Vitamin A (retinol), a fat-soluble lipid, is either derived directly from animal foods such as liver, egg yolks, cream, or butter or is derived from beta-carotene, a pigment that occurs in leafy green vegetables and in yellow fruits and vegetables. Vitamin A is essential to skeletal growth, normal reproductive function, and the health of the skin and mucous membranes. One form, retinal, is a component of visual purple, a photoreceptor pigment in the retina of the eye (see vision). In addition, beta-carotene, like other carotenoids, is now recognized as an important antioxidant.
A deficiency of vitamin A can cause retarded skeletal growth, night blindness, various abnormalities of the skin and linings of the genitourinary system and gastrointestinal tract, and, in children, susceptibility to serious infection. The eye disorders that result from a deficiency of vitamin A can lead to permanent blindness. Severe deficiency can cause death. As with the other fat-soluble vitamins, conditions that lead to an inability to absorb fats, such as obstruction of bile flow or excessive use of mineral oil, can produce a deficiency state. Overconsumption of vitamin A can cause irritability, painful joints, growth retardation, liver and spleen enlargement, hair loss, and birth defects. The National Research Council recommended daily dietary allowance for adults is 1,000 micrograms (retinol equivalents) for men and 800 micrograms for women.
Sections in this article:
More on vitamin Vitamin A from Infoplease:
See more Encyclopedia articles on: Biochemistry