Effect of dietary anion composition on acid-induced hypercalciuria in the adult rat.

The effects of replacing dietary carbonate (0.3 meq/g diet) by sulfate (SO4) or chloride (Cl) on urinary calcium (Ca) excretion were investigated in the adult male rat. When anion substitutions were made by manipulating Ca and Mg salts, urinary Ca excretion was significantly higher in the SO4 and Cl groups, the Cl-induced calciuria being twice that of the SO4 group. Replacement of carbonate by chloride was associated with higher net acid excretion. Replacement by SO4 promoted an equivalent excretion of acid, if apparent absorption of anion is taken into account. When substitutions were made by manipulating magnesium, sodium and potassium but not Ca salts, urinary Ca excretion and net acid excretion were higher in both fixed anion groups. In this second study, absorption of anion in the Cl and SO4 groups was not different (54 and 59%, respectively). Feeding either Cl or SO4 raises urinary Ca to a similar extent when equivalent changes in acid-base balance are induced. The excessive calciuria, ascribed by others to the formation of CaSO4 complexes in the urine, is not manifest when net acid excretion is significantly increased.