Retinol (Vitamin A1) is one of the animal forms of vitamin A. It is a diterpenoid and an alcohol. It is convertible to other forms of vitamin A, and the retinyl ester derivative of the alcohol serves as the storage form of the vitamin in animals. When converted to the retinal (retinaldehyde) form, vitamin A is essential for vision, and when converted to retinoic acid is essential for skin health, teeth remineralization and bone growth. These chemical compounds are collectively known as retinoids, and possess the structural motif of all-trans retinol as a common feature in their structure. Structurally, all retinoids also possess a β-ionone ring and a polyunsaturated side chain, with either an alcohol, aldehyde, a carboxylic acid group or an ester group. The side chain is composed of four isoprenoid units, with a series of conjugated double bonds which may exist in trans- or cis-configuration. Retinol is produced in the body from the hydrolysis of retinyl esters, and from the reduction of retinal. Retinol in turn is ingested in a precursor form; animal sources (liver and eggs) contain retinyl esters, whereas plants (carrots, spinach) contain provitamin A carotenoids (these may also be considered simply vitamin A). Hydrolysis of retinyl esters results in retinol, while provitamin A carotenoids can be cleaved to produce retinal by carotene dioxygenase in the intestinal mucosa. Retinal, also known as retinaldehyde, can be reversibly reduced to produce retinol or it can be irreversibly oxidized to produce retinoic acid, which then cannot function as the vitamin in the eye. Commercial production of retinol typically requires retinal synthesis through reduction of a pentadiene derivative and subsequent acidification/hydrolysis of the resulting isomer to produce retinol. Pure retinol is extremely sensitive to oxidization and is prepared and transported at low temperatures and oxygen free atmospheres. When prepared as a dietary supplement, retinol is stabilized as the ester derivatives retinyl acetate or retinyl palmitate.