Vitamin A is a fat-soluble vitamin with four important functions in the body:

• It helps cells reproduce normally; a process called differentiation (cells that have not properly differentiated are more likely to undergo dangerous changes).

• It is required for vision; vitamin A maintains healthy cells in various structures in the eye and is required for the transduction of light into nerve signals in the retina.

• It is required for normal growth and development of the embryo and fetus, influencing genes that determine the sequential development of organs in embryonic development.

• It may be required for normal reproductive function, with influences on the function and development of sperm, ovaries, and placenta.

People who limit their intake of liver, dairy products and vegetables high in beta-carotene, can develop vitamin A deficiency. The earliest deficiency sign is poor night vision. Deficiency symptoms can also include dry skin, and other conditions. Severe deficiencies causing blindness are rare in the United States.

Less severe deficiencies are more likely to occur with a variety of conditions causing malabsorption. A high incidence of vitamin A deficiency in people infected with HIV has also been reported. People with hypothyroidism have an impaired ability to convert beta-carotene to vitamin A. Very old people with type 2 diabetes have shown a significant age-related decline in blood levels of vitamin A, regardless of their dietary intake.