Urinary urea excretion in idiopathic hypercalciuria of children

In adults, it is well known that high protein intake may increase the risk of nephrolithiasis through increased urinary calcium excretion and other mechanisms. We aimed to assess the role of protein intake on urinary calcium excretion in children with idiopathic hypercalciuria (HC).

The 24-h urinary urea excretion as a reflection of protein intake was evaluated in 65 children with HC and in 76 normocalciuric control children. All of them were on a free diet and did not receive any specific drug.

Urinary urea excretion was 18.9+/-6.6 (SD) mmol/kg/day in the HC group and 14.2+/-4.9 in controls (P<0.0001). Urinary urea excretion significantly decreased with age, as well as with body weight and height gain in both groups. Calcium excretion significantly increased with increasing sodium and urea excretion in the HC but not the control group. There was a significant (P=0.005) interaction between urinary sodium and urea excretion in the rising calciuria of the HC group. Multiple regression analysis showed that, in the HC group, variations in urinary urea explained 11.4% of the entire variability of urinary calcium excretion, whereas adding the urinary sodium effect increased this relation only to 16%.

Children with HC have a higher dietary protein intake than normocalciuric children. The decrease in urea excretion with increasing age and body mass may reflect the relatively higher protein intake of young growing individuals. Salt and protein have a cumulative effect on rising calcium excretion in HC children, but the role of protein intake is outstanding.