To determine the physiological effects following administration of the sulfur-containing amino acid L-methionine on urinary risk factors for calcium oxalate and phosphate stone formation under controlled, standardized conditions.

Twelve healthy subjects were studied while consuming a standardized diet for 6 days. Day 5 was considered as control day. On day 6 (test day), participants received 1500 mg L-methionine at 9 AM. On the control and test days, 24-hour fractional urine collections were obtained.

After single-dose administration of L-methionine, 24-hour fractional urinary pH decreased to values between 5.98 and 6.32. Urinary sulfate excretion, a direct marker for the metabolism of L-methionine, and ammonium excretion, which reflects the increase in net acid production, increased significantly after L-methionine administration. Urinary calcium excretion did not differ between control and test day (5.55 vs 5.45 mmol/24 hours; P = .754). The relative supersaturation of brushite and struvite decreased significantly by 25% (P = .010) and 34% (P = .049), respectively, whereas the risk of calcium oxalate stone formation was not affected by application of L-methionine (P = .477).

The present findings show that L-methionine effectively decreases urinary pH and the risk of struvite and calcium phosphate stone formation in healthy subjects. Hypercalciuria is not expected to occur at physiological doses of L-methionine.