Vitamin A was the first vitamin officially named and given the letter A. Retinol, another name for vitamin A, relates to its importance in vision. Several carotene pigments found in foods, mainly yellow and orange vegetables and fruits, can be converted to vitamin A. Beta-carotene is the most available and also the one that yields the highest amount of A.

Vitamin A is absorbed primarily from the small intestine. As a fat-soluble vitamin, vitamin A can be stored in the body and used when there is decreased intake. About 90 percent of the storable vitamin A is in the liver; it is also stored in the kidneys, lungs, eyes, and the fat tissue.

Vitamin A is required for night vision, and for a healthy skin. It assists the immune system, and because of its antioxidant properties is great to protect against pollution and cancer formation and other diseases. It also assists your sense of taste as well as helping the digestive and urinary tract and many believe that it helps slow aging.

It is required for development and maintenance of the epithelial cells, in the mucus membranes, and your skin, and is important in the formation of bone and teeth, storage of fat and the synthesis of protein and glycogen.

A deficiency of vitamin A may lead to eye problems with dryness of the conjunctiva and cornea, dry skin and hair, night blindness as well as poor growth. Dry itchy eyes that tire easily are normally a warning of too little vitamin A. If the deficiency becomes severe, the cornea can ulcerate and permanent blindness can follow.

Abscesses forming in the ear, sinusitis, frequent cold and respiratory infections as well as skin disorders, such as acne, boils and a bumpy skin, as well as weight loss might be indicative of the vitamin being in short supply. Insomnia, fatigue and reproductive difficulties may also be indicative of the shortage of vitamin. Your hair and scalp can also become dry with a deficiency, especially if protein is also lacking.

Vitamin A is essential for vision, adequate growth, and tissue differentiation. Vitamin A has excellent antioxidant properties that help neutralize free-radicals. Vitamin A stimulates the production of mucous, and is absorbed by the body 3-5 hours after ingestion. The conversion and absorption of carotene takes 6-7 hours after ingestion; approximately 1/3 of carotene in food is converted into provitamin A. Taking vitamin A and iron together helps overcome iron deficiency more effectively than iron supplements alone. Vitamin A is essential in the conversion of cholesterol into female estrogens and male androgens. Thyroxine, a thyroid hormone, stimulates the conversion of carotene into a usable nutrient, fat-splitting enzymes and bile salts convert carotene. An adequate supply of zinc is needed so that the liver can mobilize vitamin A out of its storage depots.

Dosages exceeding 15,000 IU per day must be taken under medical supervision. Toxicity can appear in some individuals at relatively low dosages and the symptoms may include nausea, dizziness, menstrual problems, skin changes and dryness, itchiness, irritability, vomiting, headaches. Long term use can cause hair loss, bone and muscle pain, headaches, liver damage, and an increase in blood lipid concentrations. Pregnant women must be careful as a high intake of this vitamin can cause birth defects.

Pro-vitamin A - beta-carotene does not cause toxicity. Be careful if you in the unlikely event run across polar bear on a menu — 500 gram (about ½ a pound) of polar bear liver will deliver about 9,000,000 IU to your diet — a very lethal dose. Headaches, blurred vision, loss of hair, drowsiness and diarrhea, enlargement of the spleen and liver can all be indications when your intake is too high.

More of this vitamin is required when you consume alcohol, on a low-fat diet, or a diet high in polyunsaturated fatty acids, if you smoke or live in a polluted area. It may also be indicated if you suffer from diabetes or have an under-active thyroid gland. Be careful of vitamin A in pregnancy.

Liver, milk, eggyolk, carrots, dark green leafy vegetables and yellow fruits are high in vitamin A or beta-carotene.