Duodenal calcium absorption and a vitamin D-dependent duodenal calcium- binding protein are depressed in rats with alloxan- or streptozotocin-induced diabetes. To test for possible abnormal vitamin D metabolism in diabetes we measured serum concentrations of25-hydroxyvitamin D and 1,25-dihydroxyvitamin D in control, streptozotocin diabetic, and insulin-treated diabetic rats. The serum concentration of 1,25-dihydroxyvitamin D was depressed in untreated diabetic rats to one eighth of the level in controls and was restored to control levels by insulin treatment. The serum concentration of 25-hydroxyvitamin D was the same in all three groups. Hence, effects of diabetes on duodenal calcium transport can be explained by reduced concentrations of 1,25-dihydroxyvitamin D resulting eitherfrom failure of renal la-hydroxylation of 25-hydroxyvitamin D or increased catabolism of 1,25-dihydroxyvitamin D.
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