The influence of insulin on plasma and bone mineral homeostasis was studied in the BB rat model, which develops an autoimmune form of diabetes at the age of about 100 days. Untreated diabetes of short duration resulted in hypercalciuria and intestinal calcium malabsorption despite increased free concentrations of serum 1,25-dihydroxyvitamin D. The concentrations of two vitamin D-dependent calcium-binding proteins were also decreased: a low duodenal calbindin-D 9K concentration corresponding to the low intestinal active calcium absorption and a low serum osteocalcin concentration, corresponding to a low bone formation and highly correlated with serum IGF-I concentration. Indeed, on bone histology a very low number of osteoblasts and low osteoblast activity (osteoid formation and mineral apposition rate) were observed. Similar abnormalities persisted in rats with long-standing diabetes resulting in markedly decreased bone mass and increased brittleness of bone. Diabetes therefore resulted in low-turnover osteoporosis. Several hormones (testosterone, growth hormone and 1,25-dihydroxyvitamin D) and growth factors (IGF-I and its binding proteins) with known effects on bone were markedly decreased in diabetic rats. A continuous infusion of testosterone, GH or 1,25-(OH)2D3 for 14 d by miniosmotic pumps could not improve the biochemical or histomorphometric abnormalities. Insulin infusion for 2 weeks, however, rapidly increased and overcorrected the number of osteoblasts, normalized serum osteocalcin and IGF-I concentrations but could not yet normalize bone mineralization. Continuous infusion of IGF-I alone did not improve the osteoblast number of osteocalcin but markedly stimulated bone mineralization. From these data we can conclude that both insulin and IGF-I are potent bone growth factors but with different mode of action. In human type 1 diabetes, a similar decrease in serum osteocalcin and IGF-I was observed. A reduction of regional bone mass, both in long and trabecular bones, is frequently observed in human diabetes. Cumulative data from case control studies indicate that the life-time fracture risk is increased in diabetes.