Vitamin A was the first vitamin identified as an essential micronutrient needed by humans. Although it has been recognized as a chemical entity, foods rich in this vitamin have long been prescribed as treatment for night blindness. Ancient Egyptian physicians recommended the consumption of ox or chicken liver for people unable to see at night. In nature, vitamin A and carotene are bound to proteins, particularly the lipoproteins. Indeed, the vitamin is stored as an ester in those tissues which synthesize lipoprotein. Usually, retinol is covalently bound to palmitic acid as well as being associated with a protein. The vitamin also complexes with other proteins as part of its mechanism of action at the subcellular level. The varying potencies of the different vitamin A compounds have, in the past, contributed confusion to the literature. This has been resolved by the establishment of a standard definition for vitamin A.