The possible protective role of -carotene, retinol, and retinoids against some epithelial cancers is under investigation. A deficiency is usually caused by prolonged dietary deprivation. It is endemic in areas, such as southern and eastern Asia, where rice, devoid of carotene, is the staple. Secondary vitamin A deficiency may be due to inadequate conversion of carotene to vitamin A or to interference with absorption, storage, or transport of vitamin A. Interference with absorption or storage is likely in celiac disease, sprue, cystic fibrosis, pancreatic disease, duodenal bypass, congenital partial obstruction of the jejunum, obstruction of the bile ducts, giardiasis, and cirrhosis. The severity of the effects of vitamin A deficiency is inversely related to age.
Growth retardation is a common sign in children.
Follicular hyperkeratosis of the skin is common. Dysfunction of the retina is followed by changes in the structure and function of epithelial cells. A depletion is unobtainable in the preclinical stage, except for a history of inadequate intake. Plasma retinol levels fall after liver stores are exhausted. Secondary infection may complicate the corneal changes.