Elevated glomerular filtration rate in early diabetes may be explained by increased sodium reabsorption secondary to an impairment in renal tubular handling of phosphate.

Glomerular filtration rate, the renal threshold concentration for phosphate and the tubular reabsorption rates of glucose, phosphate and sodium were determined in the fasting state of 26 diabetic children without clinical signs of microangiography and in 28 healthy school children. The renal threshold concentration for phosphate was significantly decreased in the juvenile diabetics (p less thn 0.001) and was negatively correlated to the rate of glucose (r = 0.59, p less than 0.01). The tubular sodium reabsorption was markedly increased as compared to the healthy children (p less than 0.001) and for all children the threshold concentration for phosphate was negatively correlated to the tubular sodium reabsorption (p less than 0.001). In the diabetic children the tubular sodium reabsorption was closely correlated to the absolute phosphate reabsorption (p less than 0.001) and to GFR (p less than 0.0001). Therefore, in spite of the defective renal threshold for phosphate induced by the increased glucose reabsorption, the absolute phosphate reabsorption became normalized in the juvenile diabetics by an increase in sodium reabsorption and thus by an increase in GFR.