Hypo- and hypervitaminosis

Serious vitamin A deficiency is not a major public health problem in North America however, where poverty and poor nutrition are common many persons suffer from vitamin A deficiency with active corneal involvement. 

This condition leads to permanent blindness in about half of cases, and many of those affected are preschool children. Night blindness occurs early in vitamin A deficiency. There is a reduction in rhodopsin concentration, followed by retinal degeneration and loss of photoreceptor cells. Degenerative changes may be due to instability of free opsin or may indicate an additional nutritive function for retinaldehyde (or retinoic acid) in retinal cells. The degenerative changes of retinitis pigmentosa, a relatively common, inherited cause of blindness, closely resemble those of vitamin A deficiency. 

Ingestion of vitamin A in large excess of the RDA can cause toxicity. Daily intake of more than 7500 retinol equivalents (25,000 lU) is not recommended, and doses in excess of 3000 retinol equivalents (10,000 lU) should only be used with medical supervision. Acute toxicity after 

Structures of 13-cis-retinoic acid (isotretinoin) (a) and etretinate (b), synthetic retinoids used as pharmacological agents, 

Schematic representations of mammalian retinal rod and cone cells. There are about 500-1500 disks in the outer segment of each rod cell. The deep membrane invaginations in the plasma membrane of the cone cell outer segment are thought to be functionally equivalent to the rod cell disks. 

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