The importance of renal net acid excretion as a determinant of fasting urinary calcium excretion.

To evaluate the effects of changing rates of fixed acid production on fasting urine Ca/creatinine, we studied five healthy men fed constant diets during control conditions (serum HCO3 27.3 +/- 2.6 SD mEq/liter and blood H+ 40.4 +/- 1.5 microEq/liter) and then during the administration of NH4Cl 3.0 mEq/kg/day (serum HCO3 22.5 +/- 4.9 mEq/liter; P less than 0.025, and H+ 46.8 +/- 2.3 mEq/liter; P less than 0.005). In addition to the expected increase in daily urinary Ca excretion from 5.2 +/- 2.0 to 12.5 +/- 3.0 mmole/day; P less than 0.001 as daily urinary net acid excretion was increased from 48 +/- 32 to 257 +/- 33 mEq/day; P less than 0.001 we observed that fasting urinary net acid/creatinine excretion also increased from 2.9 +/- 1.2 to 11.1 +/- 1.2 mEq/mmole creatinine; P less than 0.001 and fasting urine Ca/creatinine increased from 0.158 +/- 0.111 to 0.456 +/- 0.109 mmole/mmole creatinine; P less than 0.005. The additional Ca appearing in the urine during acidosis ultimately reflected augmented net bone resorption since daily urinary hydroxyproline excretion was increased from 0.232 +/- 0.062 to 0.377 +/- 0.108 mmole/day; P less than 0.01. Since variations in diet composition can cause fixed acid production and thus renal net acid excretion to vary from about zero to 200 mEq/day, such a range could cause fasting Ca/creatinine to vary from 0.09 to 0.37 mmole/mmole (0.03 to 0.13 mg/mg) and should be taken into account in the evaluation of fasting Ca/creatinine.