Vitamin A The Night Sight Vitamin

Vitamin A

The Night Sight Vitamin

 

 As long ago as 300 B.C., the Hippocratic School of Medicine recommended liver (rich in vitamin A) for children with night blindness or infections. Vitamin A was the first vitamin discovered. In 1907, the fat-soluble vitamin A was found necessary for growth. In 1930 it was learned that there were two related forms—beta-carotene and a fat-soluble Vitamin A. Vitamin A was first synthesized in 1947. Most of the vitamin A in the body is stored in the liver in the form of retinyl palmitate.

The Forms of Vitamin A

Vitamin A is a family of compounds with similar structures called  retinoids. In plant-based foods, vitamin A is found in the form of  provitamin A, principally beta-carotene. These plant-based carotenes are known as provitamin A because some of them can be sliced apart to become the other forms of vitamin A. Foods derived from animals or animal products contain a different form of vitamin A called retinyl esters. The retinyl esters can also be converted to the other forms of vitamin A in the body.

Inside the body vitamin A is found in five forms: retinol, retinal, retinoic acid, retinyl palmitate, and beta-carotene. Each of these forms of vitamin A performs functions that the others cannot. Retinol is the major form of vitamin A for transport in the body. Retinol is the alcohol form of vitamin A. Retinol is required to maintain the integrity and immune function of the skin and mucous membranes. Retinol can be converted to three of the other forms inside the body.

Retinal is the form of vitamin A famous for working in the rod cells of the eyes to enhance night vision. Retinal is the oxidized form of retinol. Retinal is a needed intermediary in the conversion of retinol to another active form of vitamin A, retinoic acid. Retinal can be oxidized to produce this third form of vitamin A, retinoic acid.

Once retinal is oxidized into retinoic acid, it cannot be changed back to retinal again, Retinoic acid acts as a hormone, affecting genes in the nucleus of the cell.

Retinoic acid influences gene transcription, the expression of genes, and the synthesis of proteins. Retinoic acid regulates the developing cells for specialized uses during growth and embryonic development.

All of the forms of vitamin A are called preformed except provitamin A, which is also known as beta-carotene. Beta-carotene can be split into two, and each half can become a molecule of retinal.

Antioxidant Activity

Vitamin A has several forms that are used for vital functions. Provitamin A, betacarotene, performs antioxidant functions that none of the other forms of vitamin A can achieve. In addition to its vital antioxidant functions, beta-carotene can be split apart into retinal and converted to all other forms of preformed vitamin A, as pre-
viously.

Beta-carotene is one of the most powerful antioxidants in food. Antioxidants neutralize free radicals to reduce the risk of macular degeneration, cancer, heart disease, and stroke. Some of the beta-carotene in foods and supplements can be converted into the retinal form of vitamin A. About 10 percent of the carotenoids (beta-carotene is one of the carotenoids) in plant foods can be converted into retinal. The remaining carotenoids may be used as antioxidants.