Anticystinuric effects of glutamine and of dietary sodium restriction

We studied four patients with cystinuria to assess the effects of glutamine and dietary sodium on the urinary excretion of dibasic amino acids. In Patient 1, at an ad libitum dietary sodium intake of about 300 mmol per day, oral administration of glutamine led to reproducible and marked anticystinuria and antiornithinuria, whereas the excretion of lysine and arginine was not significantly affected. In Patient 2, at an ad libitum dietary sodium intake of about 150 mmol per day, no effect of glutamine could be demonstrated in studies lasting up to three weeks. Since the principal difference between Patients 1 and 2 was their dietary intake of sodium, Patient 3 was studied during dietary sodium intakes of 150 and 300 mmol per day. His cystine excretion was found to be higher at 300 than at 150 mmol per day. Glutamine suppressed his cystine excretion at a sodium intake of 300 mmol per day but had no effect at 150 mmol per day. When the effect of a further reduction in sodium intake alone was studied in a fourth patient, a decrease of 150 to 50 mmol per day was found to reduce cystine excretion markedly within 17 days. The low-sodium diet alone also reduced the excretion of lysine, arginine, and ornithine. We conclude that glutamine may reduce the excretion of dibasic amino acids at a high sodium intake but not at an intake of about 150 mmol per day. However, since a sodium-dependent excretion of the dibasic amino acids occurs at an intake down to about 50 mmol of sodium per day, dietary restriction of sodium can provide a safe approach to the treatment of cystinuria.