Vitamin A 
Fat-soluble vitamin originally isolated from fish-liver oils. It was first synthesized in 1947 as a complex molecule made of 20 carbon atoms and containing an alcohol group. The chemical was named retinol.
Vitamin A is found in two sources: plants and animals. Vitamin A that comes from animals is called preformed vitamin A. It is absorbed in the body in the form of retinol. Sources include liver, whole milk, and fortified foods. Vitamin A from plants is called provitamin A carotenoid, that also can be made into retinol in the body. The most common provitamin A cartenoids found in foods is beta-carotene, found in carrots. The Recommended Dietary Allowance for males older than 19 is 900 mcg/day. Females of the same age should consume higher than 700 mcg/day.
Dietary sources 
Retinol, together with some of its derivatives, is naturally provided by foods of animal origin, is readymade and ready for use as an active vitamin by the body.
Natural sources : a) Retinol is found in liver, fish liver oils (cod, flounder, halibut), egg yolk, fat from dairy products ; b) b-carotene and other carotenoids with pro-vitamin A function (only a few carotenoids have such property) are present in: tomatoes, carrots, cabbage, green vegetables (spinach, broccoli, lettuce and other leafy vegetables for salads) ; apricots, mangoes, melons, pumpkins. - Toxicity : No toxicity is known for b-carotene (it can occasionally give a yellowish colour to the skin).
On the other hand, vegetable sources including fruits and proper vegetables, can provide precursors of vitamin A in the form of longer molecular chains of potential pro-vitamins belonging to the carotenoid pigments, that can be enzymatically split and converted into active vitamin A in the linings of the small intestine. The best known form of pro-vitamin A is β-carotene, a deep-coloured pigment present in yellow, orange or red fruits and other vegetable structures. Each molecule of β-carotene can be converted into two molecules of active vitamin. Any excess of vitamin A obtained from food is stored in the liver. Vitamin A used for therapeutic purposes is usually made up of 14 different isomers all of which should be present in the preparation.
Vitamin A is necessary for vision, in particular vision in dim light and night vision, where isomers of retinol are directly involved. Moreover, it protects the cells from free-radical damage ; helps bone and teeth formation (the making of dentin requires vitamin A) ; promotes the health of skin, gums, all body linings and mucosae and stimulates them to fight infections. This vitamin is also necessary to reproduction, in particular to the production of sperm and to the development of the foetus. A considerable body of research has stressed the importance of vitamin A as modulator of the immune function, where an important acid from retinol (retinoic acid) is specially involved. Actually, a correlation has been found in humans between the level of this vitamin and the immune response at a cellular level. Serious vitamin A deficiency is still the main cause of children death in the developing countries.
Recommended amounts 
(adults) : 900 to 1400 μg (micrograms) per day. Modern scientific unit is RE - Retinol Equivalent : 1 RE = 1μg of retinol = 6 μg of b-carotene. As to the still widely used International Unit (IU), stands the following equivalence: 1 RE = 3.3 IU As to retinol, it is recommended not to exceed 25.000 RE per day over prolonged periods (3.000 RE in children). Never exceed 500.000 RE in a single dose (20.000 RE in children). In pregnancy : do not exceed 3.000 RE per day.
Vitamin A Deficiency 
Vitamin A deficiency is a common problem around the world but not very problematic here in the US. This deficiency is a result of a deficiency of zinc in the body because zinc is what makes the body have the ability to move its Vitamin A stores from the liver to the body tissues. Blindness is the result of a severe lack of Vitamin A in one’s diet. Vitamin A deficiency also diminishes the body’s ability to fight infections.