Vitamin A influence on calcium metabolism and calcification.

Vitamin A deficiency has been shown to affect vision, reproduction, cellular differentiation, and maintenance of epithelial and skeletal tissues. Different vitamin A compounds, i.e., retinol and retinoic acid, vary in their ability to fulfill these biologic functions. Aside from vision, the exact molecular mechanisms of action for retinoids are largely unexplained. High doses of vitamin A are known to increase bone resorption through stimulation of lysosomal proteolytic activity, which also leads to increased secretion of PTH. Deficiency of retinol affects bone morphology by increasing bone thickness. Using a standardized guinea pig model, bone formed entirely during a retinol-deficient (A --) period contained less calcium and took up more 35S into glycosaminoglycan (GAG) fractions in vitro than did control (A+) samples. Using rat calvaria, a pulse-chase experiment indicated that the rate of 35S loss from calvaria cultured with A -- serum was lower than in controls. Thus, retinol deficiency seems to increase the amount of sulfated GAG in bone through a defect in the degradation of GAG.