Vitamin A is a fat-soluble vitamin that is derived from two sources: preformed retinoid and provitamin carotenoids. Retinoid such as retinal and retinoic acids found in animal sources like liver, kidneys, eggs and dairy products. Carotenoids such as beta carotene (which has vitamin A activity highest) as found in plants or dark yellow vegetables and carrots.Natural retinoid are present in all living organisms, either as preformed vitamin A or as carotenoids and are required for a vast number of biological processes such as vision and cell growth. An important biological function of vitamin A (as the metabolite retinal) is in the visual cycle. Research also suggests that vitamin A can reduce mortality from measles, prevent some cancers, to assist the growth and development and improve immune function.
The Institute for Medicine of the National Academy of Sciences USA has established levels of recommended daily allowance (RDA, in English) for oral intake of vitamin A in order to prevent vitamin A deficiency at recommended doses; it is considered that vitamin A is not toxic in general terms. An excessive dose can cause acute or chronic toxicity.
Vitamin A deficiency is rare in industrialized nations but remains a concern in developing countries, particularly in areas where malnutrition is common. Prolonged deficiency can lead to exophthalmia (dry eye) and, ultimately, to night blindness or total blindness as well as skin disorders, infections (like measles), diarrhea and respiratory disorders.