Vitamin A

Vitamin A is found in two forms in food, in the body and in supplements; preformed vitamin A (retinol) and beta-carotene. Beta-carotene is converted in the liver into vitamin A by a process requiring several other nutrients including zinc. Since beta-carotene has been discussed elsewhere on this site, we will discuss what is normally called vitamin A - retinol.
Vitamin A and its metabolites play diverse roles in the body.
Retinol is the immediate precursor to two important active metabolites: retinal, which plays a critical role in vision, and retinoic acid, which serves as an intracellular messenger that affects transcription of a number of genes.
Vitamin A and its metabolites retinal and retinoic acid appear to serve a number of critical roles in physiology, as evidenced by the myriad of disorders that accompany deficiency or excess states. Some of the well-characterized effects of vitamin A include:

Retinal is a necessary structural component of rhodopsin or visual purple, the light sensitive pigment within rod and cone cells of the retina. If inadequate quantities of vitamin A are present, vision is impaired.
Vitamin A deficiency has been repeatedly shown to increase the frequency and severity of disease.
Skin and membrane "integrity": Many epithelial cells appear to require vitamin A. Lack of vitamin A leads to dysfunction - the skin becomes keratinized and scaly, and mucus secretion is suppressed. (Epithelial cells are the cells at the body's surfaces - in the skin, and also in the mucous membranes inside the body.)
Normal bone growth and maintenance is dependent upon vitamin A.
Normal levels of vitamin A are required for fertility in both men and women.
Vitamin A is present in many animal tissues, and is readily absorbed from such dietary sources in the small intestine. Liver is clearly the richest dietary source of vitamin A.
Vitamin A is stored in the liver as and, when needed, exported into blood, where it is carried by retinol binding protein for delivery to other tissues.
Both too much and too little vitamin A are well known causes of disease in man and animals.
Vitamin A deficiency usually results from malnutrition, but can also be due to abnormalities in intestinal absorption. Deficiency is prevalent in people, especially children, in certain underdeveloped countries. Because the liver stores rather large amounts of vitamin A, deficiency states typically take several months to develop. Some of the more serious manifestations of vitamin A deficiency include:

Blindness due to inability to synthesize adequate quantities of rhodopsin. Moderate deficiency leads to poor vision in low light ("night blindness"), while severe deficiency can result in severe dryness and opacity of the cornea (xeropthalmia).
Increased risk of mortality from infectious disease has been best studied in malnourished children. In such cases, supplementation with vitamin A has been shown to substantially reduce mortality from diseases such as measles and gastrointestinal infections.
Abnormal function of many epithelial cells, manifest by such diverse conditions as dry, scaly skin, inadequate secretion from mucosal surfaces, infertility, decreased synthesis of thyroid hormones.
Vitamin A is highly toxic when taken in large amounts, and the most common cause of this disorder in both man and animals is excessive supplementation.
Both hypovitaminosis A and hypervitaminosis A are known to cause congenital defects in animals and likely to have deleterious effects in humans. Pregnant women are advised not to take excessive vitamin A supplements, and consumption of large amounts of liver is probably not a good idea. For a pregnant woman or one likely to become so, the safest course may well be to take a multivitamin specifically designed for the job.