Vitamin A is a group of compounds that play an important role in vision, bone growth, reproduction, cell division, and cell differentiation (in which a cell becomes part of the brain, muscle, lungs, blood, or other specialized tissue). Vitamin A helps regulate the immune system, which helps prevent or fight off infections by making white blood cells that destroy harmful bacteria and viruses. Vitamin A also may help lymphocytes (a type of white blood cell) fight infections more effectively.
In general, there are two categories of vitamin A, depending on whether the food source is an animal or a plant.
Vitamin A found in foods that come from animals is called preformed vitamin A. It is absorbed in the form of retinol, one of the most usable (active) forms of vitamin A. Sources include liver, whole milk, and some fortified food products. Retinol can be made into retinal and retinoic acid (other active forms of vitamin A) in the body.
Vitamin A that is found in colorful fruits and vegetables is called provitamin A carotenoid. They can be made into retinol in the body. In the United States, approximately 26% of vitamin A consumed by men and 34% of vitamin A consumed by women is in the form of provitamin A carotenoids. Common provitamin A carotenoids found in foods that come from plants are beta-carotene, alpha-carotene, and beta-cryptoxanthin. Among these, beta-carotene is most efficiently made into retinol. Alpha-carotene and beta-cryptoxanthin are also converted to vitamin A, but only half as efficiently as beta-carotene.
Of the 563 identified carotenoids, fewer than 10% can be made into vitamin A in the body. Lycopene, lutein, and zeaxanthin are carotenoids that do not have vitamin A activity but have other health promoting properties. The Institute of Medicine (IOM) encourages consumption of all carotenoid-rich fruits and vegetables for their health-promoting benefits.
The use of acne medicines (i.e. Acutane) has led to birth defects and even death in children born to mothers using these compounds. This has helped make the public more aware of the toxic properties of vitamin A. In adults a condition known as hypervitaminosis exhibits itself after chronic ingestion of the vitamin in doses that are ten times the RDA (10 mg RE). Symptoms of vitamin A toxicity include: anorexia, headache, bone and muscle pain, vomiting, alopecia, liver damage, and coma. These symptoms slowly reside as vitamin A intake levels are reduced. To date the only side effect of excess beta-carotene has been yellowing of the skin, most commonly in the fatty areas of the hands and palms. The yellowing disappears as beta-carotene intake decreases. This commonly ingested dietary precursor to vitamin A has yet to exhibit any signs of toxicity even at levels as high as 180 mg per day. Researchers believe that the presentation of unbound retinol to the cell is a major factor in toxicity. Excessive intakes of vitamin A saturate RBP and instead of retinol being transferred bound to RBP, it is transferred to the tissue via plasma lipoproteins. When retinol reaches the tissue by a carrier other than RBP it is hypothesized that the retinol is released and causes toxic side effects.Vitamin A is essential for numerous intrinsic processes. The most well-known and understood process is that of vision. The 11-cis retinal form of vitamin A is essential for the neural transmission of light into vision. Epithelial cells are highly dependent on retinoic acid and are commonly used to treat a variety of skin diseases. A developing fetus is also highly dependent on retinoic acid, as it is essential to the growth of the eyes, lungs, ears and heart. The retinoids are not only the most active form of vitamin A, but also a current area of interest to many scientists. The role of vitamin A as an antioxidant is debatable. Vitamin A has been shown to possibly have some antioxidant characteristics. However, the carotenoids such as beta-carotene have in recent years received more attention from the scientific community because of the harmful role they may play as pro-oxidants. A great deal more research is needed that addresses the role of vitamin A as an antioxidant to determine the exact role the vitamin and precursors play.
Retinol, the active form of vitamin A, is rarely found in foods. Instead, precursors to retinol, fatty acid retinyl esters, are found in the human diet. The esters are commonly found in foods of animal origin, such as egg yolks, liver, fish oil, whole milk and butter. Plants can synthesize the carotenoids, but cannot convert them to retinoids; this process occurs in the human body. The carotenoids are red, yellow, and orange in color and substantial in number (over 400 types). It is estimated that only 10% of the pigments have "vitamin A activity", with beta-carotene having the greatest activity, followed by the alpha and gamma forms. Fruits and vegetables that appear bright orange or yellow in color are usually high in carotenoids. All green vegetables also contain substantial amounts of carotenoids, but the orange or yellow color is masked by chlorophyll. The wide variety of vitamin A precursors allows for adequate amounts of the vitamin in all diet types.
Deficiency of vitamin A is very rare in the United States, unless confounding malabsorption conditions such as steatorrhea, or diseases of the liver, pancreas, or gallbladder are present. In contrast vitamin A deficiency is prominent in young children (<5 years old) living in third world countries. At birth many neonates experience low plasma vitamin A content, but the levels are corrected with a diet sufficient in vitamin A. Symptoms of vitamin A deficiency include metaplasia (changing of normal tissue into abnormal tissue), poor growth, xerophthalmia (dry corneas), and keratinization of epithelial cells resulting in a loss of differentiation. If vitamin A deficiency has not been chronic leading to permanent debilitation the symptoms can often be reversed through supplementation.
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