Vitamin A is the name given to three lipid-soluble vitamin compounds of C20 (collectively called retinoids):
- Retinoic acid
Carotenoids (a C40 compound) are not Vitamin A, but vitamin A can be formed from C40 carotenoids, including β-carotene.
Vitamin A is essential for vision, growth and maintenance of epithelial tissues, and resistance to infection.
Retinal is a necessary structural component of rhodopsin. If there is a Vitamin A deficiency, vision is impaired.
Epithelial tissue maintenance
Deficiency of Vitamin A leads to dysfunction of many epithelia, which require vitamin A for proper differentiation and maintenance. This results in scaly, pellagra-like skin . Also, vitamin A is important in reproduction and bone remodelling.
Resistance to infection
In almost every infectious disease studied, vitamin A deficiency increases both the frequency and severity of disease. The anti-infective effect of Vitamin A is complex, but due in part to Vitamin A's involvement in immune cell differentiation.
Vitamin A is widely distributed in animal food. Although it does not occur in plants, β-carotene and other C40 carotenoids do. These can be converted into vitamin A within the intestine and other tissues. Liver is clearly the richest dietary source of vitamin A, which is absorbed readily in the terminal small intestine.
Vitamin A deficiency can result in blindness (due to inability to synthesize rhodopsin) or xeropthalmia. It is linked to increased risk of mortality from infectious disease in malnourished children, and also leads to abnormal function of many epithelial cells, manifest by dryness and the like.
While vitamin A can be toxic if taken in excess, β-carotene is not toxic, even in high doses. Usually, a vitamin A excess is the result of massive consumption of liver oils.