Vitamin A is a fat-soluble vitamin that is an essential nutrient. The term "vitamin A" encompasses a group of chemically related organic compounds that includes retinol, retinyl esters, and several provitamin carotenoids, most notably β-carotene. Technically, retinol is the primary alcohol form that can be esterified to retinyl esters for hepatic storage or oxidized to retinal, the aldehyde required for the visual cycle, and to retinoic acid, the bioactive acid that regulates gene transcription via retinoic acid receptors RAR and RXR. Absorption from the intestine requires dietary lipids and bile salts, with intestinal beta-carotene cleavage catalyzed by beta-carotene 15,15-prime-monooxygenase to yield retinal. In circulation, retinol is transported bound to plasma retinol binding protein RBP4 and often complexed with transthyretin. Chemically, vitamin A compounds are polyene molecules with conjugated double bonds that confer light sensitivity and susceptibility to oxidative degradation; common analytical characterization uses HPLC with UV detection and mass spectrometry to resolve isomers and oxidation products.
Pharmaceutical and food-grade materials are monitored for related compounds and degradation products including retinal, retinoic acid, cis geometric isomers (9-cis, 13-cis), retinyl esters such as palmitate and acetate, epoxides, peroxides and polymeric oxidation products. Typical specification ranges used in industry and pharmacopoeial monographs aim to keep individual specified related compounds generally below 0.2 to 1.0 percent w/w depending on the compound and matrix, with total related compounds commonly limited to under 2.0 percent w/w for active ingredient preparations. Geometric isomers are often reported in the 0.1 to 0.5 percent w/w range each. Oxidative impurities and low-molecular-weight breakdown products are controlled to trace levels and are frequently reported in parts per million, for example commonly targeted below 500 ppm for peroxide and epoxide species. Stability testing under ICH conditions is used to define allowable impurity limits for specific formulations.
Many fruits supply provitamin A carotenoids rather than preformed retinol; notable examples are mango, cantaloupe, papaya, apricot and guava, which provide beta-carotene and other carotenoids that the body can convert to vitamin A.
Vitamin A is essential for the visual cycle as retinal combines with opsin to form rhodopsin, for maintenance and differentiation of epithelial tissues, for immune function, and for regulation of gene expression via retinoic acid acting on nuclear receptors.
The common name most often used for the active alcohol form is retinol; retinyl esters such as retinyl palmitate or retinyl acetate are common storage or supplement forms and beta-carotene is a common provitamin A source.
Vitamin A deficiency first impairs dark adaptation and can progress to night blindness, xerosis and xerophthalmia, increased susceptibility to infections, epithelial keratinization and, in severe cases, irreversible corneal damage and growth impairment in children.
