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The kidney stone-forming risk of a high sodium diet was evaluated by assessing the effect of such a diet on the crystallization of stone-forming salts in urine. Fourteen normal subjects participated in 2 phases of study of 10 days duration each, comprising a low sodium phase (basal metabolic diet containing 50mmol. sodium per day) and a high sodium phase (basal diet plus 250mmol. sodium chloride per day). The high sodium intake significantly increased urinary sodium (34 ± 12 to 267 ± 56mmol, per day), calcium (2.73 ± 1.03 to 3.93 ± 1.51mmol. per day) and pH (5.79 ± 0.44 to 6.15 ± 0.25), and significantly decreased urinary citrate (3.14 ± 1.19 to 2.52 ± 0.83mmol. per day). Arterialized venous blood bicarbonate and total serum carbon dioxide concentrations decreased significantly during the high sodium diet, whereas serum chloride concentration increased. However, no change in arterialized venous pH was detected. Thus, a high sodium intake not only increased calcium excretion, but also increased urinary pH and decreased citrate excretion. The latter effects are probably due to sodium-induced bicarbonaturia and a significant decrease in serum bicarbonate concentration, respectively.

Commensurate with these changes, the urinary saturation of calcium phosphate (brushite) and monosodium urate increased, and the inhibitor activity against calcium oxalate crystallization (formation product) decreased. The net effect of a high sodium diet was an increased propensity for the crystallization of calcium salts in urine.

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