Vitamin A is a generic term for a large number of related compounds. Retinol (an alcohol) and retinal (an aldehyde) are often referred to as preformed vitamin A. Vitamin A that is found in colorful fruits and vegetables is called provitamin A carotenoid. They can be made into retinol in the body. In the United States, approximately 26% of vitamin A consumed by men and 34% of vitamin A consumed by women is in the form of provitamin A carotenoids.

Vitamin A is commonly known as the anti-infective vitamin, because it is required for normal functioning of the immune system . The skin and mucosal cells (cells that line the airways, digestive tract, and urinary tract) function as a barrier and form the body's first line of defense against infection.
Both vitamin A excess and deficiency are known to cause birth defects.
Red blood cells, like all blood cells, are derived from precursor cells called stem cells. These stem cells are dependent on retinoids for normal differentiation into red blood cells. Additionally, vitamin A appears to facilitate the mobilization of iron.
Vitamin A promotes healthy surface linings of the eyes and the respiratory, urinary, and intestinal tracts.
Vitamin A, a fat-soluble vitamin, is involved in the formation and maintenance of healthy skin, hair, and mucous membranes.
Normal levels of vitamin A are required for sperm production, reflecting a requirement for vitamin A by spermatogenic epithelial (Sertoli) cells. Similarly, normal reproductive cycles in females require adequate availability of vitamin A.
Vitamin A helps us to see in dim light and is necessary for proper bone growth, tooth development, and reproduction.
In almost every infectious disease studied, vitamin A deficiency has been shown to increase the frequency and severity of disease.
Good food sources are fish oil, liver (pork, lamb, chicken, turkey or beef), eggs, butter and orange or yellow vebetables or fruits. Broccoli is a good source of Vitamin A, but the cholorphyll (green) camoflages the yellow carotene color. If it weren't for chlorophyll, broccoli would be yellow or orange.