Vitamin A, also known as retinol, has many essential functions in the body. It is insoluble in water, but is very soluble in fatty substances. Because of this, vitamin A can pass unimpeded through the lipid plasma membranes of target cells to directly enter the cytoplasm. Once inside the cytoplasm, vitamin A migrates to the nucleus where it binds to specific intracellular receptors that interact directly with the cell's DNA to regulate the expression of genes. Here are some of the reasons why vitamin A is so important:
Importance of Vitamin A
Vitamin A plays important roles in vision and in the normal proliferation and maturation of cells in the skin. It is present in the diet as a retinyl ester, which is converted in the small intestine to retinol, the alcohol form of the vitamin. Retinol is the form in which vitamin A is absorbed from the gut and subsequently stored in the body. However, it is not the physiologically active form. The active forms are actually retinal, the aldehyde form, and retinoic acid, the acid form, which are heavily involved in bodily processes. Once inside the body, retinol spontaneously converts to retinal and retinoic acid.
Yellow and orange vegetables like carrots and pumpkins are rich in beta-carotene, a yellowish pigment made up of two connected retinyl groups. While beta-carotene is structurally similar to vitamin A, they are not the same. Despite the greater quantity of retinyl groups in beta-carotene, your body has a limited ability to separate the two retinyl groups from each other, and the biological activity of beta-carotene is only 1/6 that of retinol. Beta-carotene can contribute to adequate vitamin A levels in the body, but it is still necessary to have sufficient intake of pure vitamin A.
Retinal is the form of vitamin A that takes part in the visual cycle. 11-cis-retinal is bound to the pigment rhodopsin in the rod and cone cells in the eye's retina. The rods and cones are visual receptors that receive light entering the eye. When light hits the rods and cones, 11-cis-retinal is converted to all-trans-retinal, which induces an electrical signal that flows through the optic nerve. The brain perceives this electrical signal as vision.
On Skin Health
Retinoic acid is the form of vitamin A that maintains normal skin health. It enters skin cells and interacts with intracellular receptors to influence the expression of genes. Retinoic acid ensures the normal maturation and proliferation of cells in the epidermis.
In dermatology, 13-cis-retinoic acid (isotretinoin) is the most effective treatment for acne. Isotretinoin reduces the size and secretions of sebaceous glands in the skin, reduces the number of bacteria in the glands and diminishes inflammation. Combined, these effects lead to the speedy resolution of acne.
Vitamin A deficiency is a serious and prevalent problem worldwide, affecting up to a third of all children, especially in developing countries. Vitamin A deficiency can lead to eye dryness, destruction of the cornea and, eventually, blindness. Deficiency also leads to impaired immunity and increased susceptibility to infections, and thickening and drying of the skin.