Increased dietary oxalate does not increase urinary calcium oxalate saturation in hypercalciuric rats

Background. Human calcium oxalate (CaOx) nephrolithiasis may occur if urine is supersaturated with respect to the solid-phase CaOx. In these patients, dietary oxalate is often restricted to reduce its absorption and subsequen t excretion in an effort to lower supersaturation and to decrease stone for mation. However, dietary oxalate also binds intestinal calcium, which lower s calcium absorption and excretion. The effect of increasing dietary oxalat e on urinary CaOx supersaturation is difficult to predict. Methods. To determine the effect of dietary oxalate intake on urinary super saturation with respect to CaOx and brushite (CaHPO4), we fed 36(th) and 37 (th) generation genetic hypercalciuric rats a normal Ca diet (1.2% Ca) alon e or with sodium oxalate added at 0.5%, 1.0%, or 2.0% for a total of 18 wee ks. We measured urinary ion excretion and calculated supersaturation with r espect to the CaOx and CaHPO4 solid phases and determined the type of stone s formed. Results. Increasing dietary oxalate from 0% to 2.0% significantly increased urinary oxalate and decreased urinary calcium excretion, the latter presum ably due to increased dietary oxalate-binding intestinal calcium. Increasin g dietary oxalate from 0% to 2.0% decreased CaOx supersaturation due to the decrease in urinary calcium offsetting the increase in urinary oxalate and the decreased CaHPO4 supersaturation. Each rat in each group formed stones . Scanning electron microscopy revealed discrete stones and not nephrocalci nosis. X-ray and electron diffraction and x-ray microanalysis revealed that the stones were composed of calcium and phosphate; there were no CaOx ston es. Conclusion. Thus, increasing dietary oxalate led to a decrease in CaOx and CaHPO4 supersaturation and did not alter the universal stone formation foun d in these rats, nor the type of stones formed. These results suggest the n ecessity for human studies aimed at determining the role, if any, of Limiti ng oxalate intake to prevent recurrence of CaOx nephrolithiasis.