What is Vitamin A?

Vitamin A plays a role in the immune system by protecting cells from free radicals, but its most critical function is supporting eyesight.

How it works in the body

Rod cells, a type of photoreceptor cell in the eye, utilize a protein called rhodopsin to detect light. Rhodopsin[2] is itself composed of the protein opsin coupled with a derivative of vitamin A known as an 11-cis-retinal.[3] When rhodopsin absorbs light, the 11-cis-retinal molecule is transformed to a different type of retinoid, known as all-trans-retinal. The transformation triggers the signaling of visual neuron cells, which transmit the corresponding visual stimulus to the brain.[4]

Vitamin A plays roles in a variety of other processes in the body, including immune function, apoptosis (apoptosis is a programmed removal of unneeded cells) and cell-to-cell communication. Vitamin A's primary connection to these various processes lies in its ability to regulate which genetic instructions (which parts of a cell's DNA) are acted upon by the cell.[5],[6] 

Vitamin A compounds are absorbed in the small intestine and transported, like other fat-soluble vitamins, to the liver where 80 percent of vitamin A is found.[7]

Citations

[1] Carotenoids, a class of pigments commonly found in carrots, are a type of provitamin A compound.

[2] Rhodopsin is a  member of the G-Protein Receptor Family.

[3] 11-cis-retinal is a cofactor (a non-protein compound that is critical to a protein's function) of rhodopsin.

[4]  "Vitamin A." In Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc, 85. Washington, D.C.: National Academies Press, 2001. http://plk.tn/1HMaEkp

[5] This is referred to as gene expression.

[6] Stipanuk, Martha H., and Marie A. Caudill. "Vitamin A." In Biochemical, Physiological, and Molecular Aspects of Human Nutrition, 686. Third Edition ed. St. Louis: Elsevier, 2013.

[7] Erdman, John W., Ian MacDonald, and Steven H. Zeisel. "Vitamin A." In Present Knowledge in Nutrition, 154. Washington, DC: International Life Sciences Institute, 2012.