Vitamin A is a major nutrient associated with healthy skin and eyes , growth , and immune system function. Vitamin A is a fat-soluble vitamin. Such vitamins are absorbed into the body dissolved in lipids (oily or fatty substances). The body stores fat-soluble vitamins in the liver and fat tissues.
Vitamin A functions in the growth and formation of tissues such as epithelial tissue, which covers the skin and lines the gastrointestinal (digestive) and respiratory tracts. Vitamin A keeps the skin and mucous membranes healthy and supports the immune system to resist infection. It is essential for reproduction, the development of babies, and the growth of children. Vitamin A forms part of the pigment (coloring matter) in specialized vision cells of the eye called rods and cones.
The United States National Academy of Medicine has established a recommended dietary allowance (RDA) for vitamin A. For most healthy adults, the RDA is from about 700 to 900 micrograms of retinol activity equivalents (RAE), a measure of vitamin A and related compounds that the body can convert into vitamin A. The RDA for children ages 1 through 13 is between 300 and 600 micrograms RAE.
Types.
Two different forms of vitamin A are found in foods: preformed vitamin A and provitamin A. Preformed vitamin A is found in fatty fish, liver, milk, eggs, and meat. Food manufacturers also fortify ready-to-eat breakfast cereals with this nutrient. Preformed vitamin A consists of a group of chemical compounds called retinoids. Retinoids include substances called retinol, retinal, retinoic acid, and retinyl esters .
Provitamin A consists primarily of beta-carotene. This substance is found in dark green leafy vegetables and in yellow, orange, and red vegetables and fruits, such as carrot and pumpkin. Beta-carotene consists of two molecules of retinol bound together. The body makes the active form of vitamin A by splitting the two retinol molecules apart. Inside the body, beta-carotene functions as an antioxidant , a substance may help prevent certain types of cell damage.
Deficiency.
Over time, continued lack of a vitamin can result in a vitamin deficiency disease. Because the body stores vitamin A in fat tissue, a deficiency of vitamin A in the diet may take one to two years to cause symptoms. Night blindness (difficulty seeing in reduced light) is an early symptom of vitamin A deficiency. A person may also develop xerophthalmia << `zeer` of THAL mee uh >> , a condition in which the surface of the eye becomes dry and likely to develop infection. Other symptoms of vitamin A deficiency include problems of the skin and mucous membranes, poor growth in children, skeletal abnormalities, and increased risk of infection due to impairment (weakening) of the immune system.
Toxicity.
Vitamin A is stored in body fat and is not readily excreted (removed from the body). Consuming too much preformed vitamin A can be harmful, as the vitamin can build up in the body to toxic amounts. The National Academy of Medicine has set a tolerable upper intake level (UL) of vitamin A at 3000 micrograms RAE per day. Regular consumption of vitamin A above this level may increase the risk of negative health effects.
In the past, vitamin A poisoning was rare. It most often occurred among Arctic explorers, who sometimes ate dog or polar bear liver, which contain a high concentration of preformed vitamin A. Today, people who take nutritional supplements may consume doses many times larger than the RDA. Excessive consumption of preformed vitamin A can cause nausea, vomiting, headache, and changes in mucous membranes. A long-term excess of vitamin A may cause skin problems, liver disease, and fragile bones. Women who may become pregnant should avoid taking excessive vitamin A from supplements, as toxic levels can harm a developing fetus.
Consuming large amounts of beta-carotene is not toxic, because the body converts this substance to vitamin A only as needed. However, excess consumption of beta-carotene over a long period can cause a buildup of the substance in the skin and mucous membranes. This buildup may give the skin a yellow or orange coloration, especially on the palms and the soles of the feet.
History.
Scientists have studied essential nutrients in animals and humans since the early 1800’s. In 1913, the American biochemists Elmer McCollum and Marguerite Davis demonstrated that butter and egg yolks contained substances necessary for the healthy growth in laboratory rats. This growth-supporting accessory factor became known as fat-soluble A in 1918. The substance was renamed vitamin A in 1920. The Swiss chemist Paul Karrer determined the chemical structure of vitamin A in 1932.
