Vitamin A has several metabolic roles in supporting normal functions including vision, immune function, bone metabolism and cellular health. Vitamin A is a group of unsaturated organic compounds including retinol, retinal and beta-carotene. Rhodopsin is a light-absorbing molecule required for colour vision, and it is produced when retinal from Vitamin A combines with protein opsin.
Retinol is a fat-soluble form of vitamin A obtained from animal food sources. It is also commmercially manufactured as retinyl acetate or palmitate. From vegetable food sources, the carotenes including beta-carotene function as provitamin A in humans. The beta-carotene is cleaved in the intestines and converted to retinol.
Vitamin A regulates gene transcription. As vitamin A enters the cell in the form of retinol, it undergoes a series of biochemical steps resulting the biologically active retinoic acid which binds receptors in the nucleus, thereby regulating expression of genes. Retinoic acid derived from vitamin A maintains healthy skin by inducing gene expression in skin cells. This induces differentiation of keratinocytes into mature epidermal skin cells. In addition, vitamin A also has crucial functions in maintenance of the immune system, including T cell differentiation and proliferation.
Vitamin A is found in many foods including turkey, beef, pork, fish and chicken. Vegetables that have vitamin A include carrots, broccoli, kale, squash and spinach.
If you are not getting enough vitamin A in your diet, you can also take a vitamin A supplement. If you decide to take the supplement, recommended dosage requirements vary with age, so be sure to read the manufacturer’s label carefully.
Harvard Health Publishing, Harvard Medical School. “Listing of vitamins“.
Vitamin and mineral requirements in human nutrition 2nd Edition. World Health Organization and Food and Agriculture Organization of the United Nations. 2004. p.340–341. ISBN 9241546123.
Canadian Public Health Association. Food fortification with vitamins and minerals.