Vitamin A was the first fat-soluble vitamin to be discovered. A light yellow crystalline compound, vitamin A is also known as retinol, a name given in reference to the participation of this compound in the functions of the retina of the eye. Vitamin A has also been called the "anti-infective" vitamin due to its role in supporting the activities of the immune system. It 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. It also helps lymphocytes (a type of white blood cell) fight infections more effectively. While retinol, or preformed Vitamin A, occurs only in foods of animal origin, fruits and vegetables that contain certain carotenoids also provide Vitamin A activity. The body can convert certain members of the carotenoid family, including beta-carotene, alpha-carotene, and gamma-carotene, into Vitamin A. These carotenoids are sometimes referred to as "provitamin A," and retinol as "preformed vitamin A."
Deficiency state of Vitamin A will manifest as irregular eye function, and processes concerning epithelial tissue. Signs of severe deficiency include; rough skin, diarrhea, malabsorption (key nutrients, vitamins, and minerals) kidney stones, and night blindness. The human retina contains four kinds of photopigments that store Vitamin A compounds. One of these pigments, called rhodopsin, is located in the rod cells of the retina. Rhodopsin allows the rod cells to detect small amounts of light, and, thus, plays a fundamental role in the adaptation of the eye to low-light conditions and night vision. Vitamin A stimulates several immune system activities, possibly by promoting the growth, and preventing the stress-induced shrinkage, of the thymus gland. It is known to enhance the function of white blood cells, increase the response of antibodies to antigens, and to have anti-viral activity. Vitamin A is also necessary for normal cell growth and development. Although the mechanisms by which Vitamin A promotes cell growth and development are not yet fully understood, it is known that retinoic acid is necessary for the synthesis of many glycoproteins, which control cellular adhesion (the ability of cells to attach to one another), cell growth and cell differentiation.
Vitamin A is commonly known as the anti-infective vitamin, because it is required for normal functioning of the immune system. The skin and mucosal cells (cells that line the airways, digestive tract, and urinary tract) function as a barrier and form the body's first line of defense against infection. Both vitamin A excess and deficiency are known to cause birth defects. Vitamin A helps form and maintain healthy teeth, skeletal and soft tissue, mucous membranes, and skin. It is also known as retinol because it produces the pigments in the retina of the eye. It promotes good vision, especially in low light. It may also be need for reproduction and breast feeding. Vitamin A comes from animal sources, such as eggs, meat, milk, cheese, cream, liver, kidney, cod, and halibut fish oil. Some good sources include cheese, eggs, oily fish (such as mackerel), milk, fortified margarine and yoghurt.
References:
Office of Dietary Supplements. http://ods.od.nih.gov/factsheets/vitamina.asp
Oregon State University. http://lpi.oregonstate.edu/infocenter/vitamins/vitaminA/
George Mateljan Foundation. http://www.whfoods.org/genpage.php?tname=nutrient&dbid=106
Supplementnews.Com. http://supplementnews.org/vitamin-a/
