Vitamin A

Posted by Blog Thursday, March 3, 2011

 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 Awas 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.

 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 .
Forms of Vitamin A
Carotenes such as beta-carotene are from plant foods and are antioxidants.

Retinyl esters come from animal foods.

Retinol supports healthy skin.

Retinal is needed for vision.

Retinoic acid supports skin and epithelial tissue.

Retinyl palmitate is the storage form inside the human liver.

Inside the body vitamin A is found in five forms: retinol, retinal, retinoic acid, retinyl palmitate, and beta-carotene, as seen in Figure 3-1. 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.

Figure 3-1 The forms 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, as previously seen in Figure 3-1.

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, as seen in Figure 3-2.

Figure 3-2 Chemical structures of vitamin A forms.

More about A -Vitamin:
Vitamin A
Vitamin A and Night Vision
Infections and Vitamin A
Cell Formation and Vitamin A
Sources of Vitamin A
Toxicity of Vitamin A


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