Effects of insulin on altered mineral and vitamin D metabolism in streptozotocin-induced diabetes

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Abstract
In order to ascertain whether or not abnormal mineral and vitamin D metabolism in diabetes can be reversed by insulin therapy, plasma Ca ionized Ca, P, parathyroid hormone (PTH) and vitamin D metabolites were measured in control, streptozotocin (STZ) diabetic and insulin-treated diabetic rats. Blood glucose levels in diabetic rats treated with insulin decreased to normal. The low plasma Ca and ionized Ca levels in diabetic rats were normal in insulin-treated diabetic rats. An elevated PTH level was observed in the diabetic group, but it was at normal levels in the insulin-treated diabetic group. Plasma 25-hydroxyvitamin D (25(OH)D) and 24,25-dihydroxyvitamin D (24,25(OH)2D) in the diabetic group were decreased compared to those in control rats, but these were also fully restored to control levels by insulin therapy. However, plasma 1,25-dihydroxyvitamin D (1,25(OH)2D) levels in the untreated diabetic group tended to be lower than in controls, and the values in insulin-treated rats were significantly decreased compared to the control group. The ratio of 1,25(OH)2D to 25(OH)D in diabetic rats was higher than in controls, but it was decreased after insulin therapy and was significantly lower than in the control group. It is suggested, therefore, that the negative Ca balance and decreased 25(OH)D and 24,25(OH)2D levels are derived from the metabolic derangement due to the insulin deficiency. Insulin seems to suppress the conversion of 25(OH)D to 1,25(OH)2D in experimental diabetes in vivo.