In addition to the general limiting effect on growth common to all nutrients, vitamin A seems to affect almost exclusively the mechanism of the adaptation of vision to dim light (dark adaptation) and the health and integrity of the epithelium. The functional and structural changes resulting from a deficiency of this vitamin constitute the manifestations of the deficiency disease.
The adaptation of vision to dim light, principally a function of the retinal rods, requires an adequate supply of vitamin A (vitamin A1). The process depends on the presence of an adequate amount of visual purple (rhodopsin). Vitamin A with a protein forms visual purple. Visual purple, or rhodopsin, is reversibly broken down in bright light to retinene, which in turn is reconverted to vitamin A1 by a reaction involving reduced cozymase as a co-enzyme. Recent studies indicate a complicated process possibly involving two
YOUMANS JB. DEFICIENCIES OF THE FAT-SOLUBLE VITAMINS. JAMA. 1950;144(1):34–45. doi:10.1001/jama.1950.62920010016008
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