Vitamin A consists of three biologically active molecules, retinol, retinal and retinoic acid.
Each of these compounds are derived from the plant precursor molecule, b-carotene. Beta-carotene (provitamin form of vitamin A), consists of two molecules of retinal linked at their aldehyde ends.
Food Sources of Vitamin A
Liver, milk, egg-yolk, carrots, dark green leafy vegetables and yellow fruits are high in vitamin A or beta-carotene. The precursor form, beta-carotene, is found in plants. Sources of beta-carotene are carrots, pumpkin, sweet potatoes, winter squashes, cantaloupe, pink grapefruit, apricots, broccoli, spinach, and most dark green, leafy vegetables. The more intense the color of a fruit or vegetable, the higher the beta-carotene content. These vegetable sources of beta-carotene are free of fat and cholesterol.
Functions of Vitamin A
Vitamin A helps form and maintain healthy teeth, skeletal and soft tissue, mucous membranes, and skin. It is also known as retinol because it generates the pigments in the retina. Vitamin A promotes good vision, especially in dim light. It may also be required for reproduction and breast-feeding. Beta carotene, which has antioxidant properties, is a precursor to Vitamin A. Antioxidants quench free radicals, which are unstable substances that can react with and damage cells, tissues and organs.
Free radicals are believed to be associated with many of the degenerative changes seen with aging. However, it is not yet known whether antioxidants can prevent these changes and studies have been conflicting. For example, one study found increased cancer risk in smokers who took high doses of beta carotene. With Vitamin A, as with many other vitamins, a certain amount is necessary, but too much is toxic.
Vitamin A is found in two forms: performed vitamin A, known as retinol, and pro vitamin A, called beta carotene. Retinol is found only in foods of animal origin. Beta carotene, a carotenoid, is a pigment found in plants. Beta carotene has a slight nutritional edge, boasting antioxidant properties and the ability to help lower harmful cholesterol levels.
Retinol also functions in the synthesis of certain glycoproteins and mucopolysaccharides necessary for mucous production and normal growth regulation. This is accomplished by phosphorylation of retinol to retinyl phosphate which then functions similarly to dolichol phosphate.
The carotenoids are a class of compounds related to vitamin A. Other types of carotenoids that have been identified are the xanthophylls (including beta-cryptoxanthin, canthaxanthin, lutein, and zeaxanthin); the limonoids (including limonene); and the phytosterols (including perillyl alcohol). Science has not yet discovered all of the carotenoids, although one source documents 563 different carotenoids identified so far. Combinations of carotenoids have been shown to be more beneficial than individual carotenoids taken alone.
The current recommended intake of vitamin A is 5,000 IU for men and 4,000 IU for women. Many breakfast cereals, juices, dairy products, and other foods are fortified with vitamin A. Many fruits and vegetables, and some supplements, also contain beta-carotene and other vitamin A precursors, which the body can turn into vitamin A.
What are the deficiency symptoms of vitamin A?
May result in night blindness; increased susceptibility to infections; rough, dry, scaly skin; loss of smell & appetite; frequents fatigue; lack of tearing; defective teeth & gums' retarded growth.
The increased risk of cancer in vitamin deficiency is thought to be the result of a depletion in b -carotene. Beta-carotene is a very effective antioxidant and is suspected to reduce the risk of cancers known to be initiated by the production of free radicals.
Overdosage signs of vitamin A
Signs and symptoms are:hair loss, anorexia,nausea, fatigue,gingivitis,dry skin.
Acute toxicity occurs after accidental ingestion of a single large dose(100,000-300,000 UI).
Drugs or Nutrients that decrease its absorption or levels in the body:Neomycin,Mineral Oil,Orlistat,Corticosteroid medications,Bile acid sequestrants (Cholestyramin ,Colestipol).
How to Store the Vitamin A?
Heat and/or moisture may alter the vitamin. Refrigeration is recommended.
Vitamin A Deficiency