Different Dietary Calcium Intake and Relative Supersaturation of Calcium Oxalate in the Urine of Patients Forming Renal Stones

1. Dietary calcium restriction, an efficient practice in reducing urinary calcium excretion, has been reported to induce either an increase or no change in oxalate excretion, questioning its use in hypercalciuric stone-forming patients. In addition, calcium restriction has been previously demonstrated to induce other urinary changes which might influence the relative supersaturation of calcium oxalate. So the overall effect of calcium deprivation on the relative supersaturation of calcium oxalate is unpredictable. 2. The aim of the study was to evaluate the effect of dietary calcium restriction on the relative supersaturation of calcium oxalate in the urine of stone-forming patients utilizing a computer methodology which takes into account the main soluble complex species of oxalate. 3. We studied 34 stone-forming patients on both a free-choice diet, whose Ca and oxalate content (24 and 1.2 mmol respectively) was assessed by dietary inquiry, and after 30 days on a prescribed low-calcium and normal oxalate diet (11 and 1.1 mmol respectively). Under both conditions, the excretion of the main urinary parameters related to dietary composition, electrolytes, oxalate and daily citrate urinary excretion, were measured. The relative supersaturation of calcium oxalate was calculated by means of an iterative computer method which takes into account the main soluble complex species on which the solubility of calcium oxalate is dependent. In addition, intact parathyroid hormone and 1,25-dihydroxyvitamin D blood levels were also evaluated. In 13 of the patients intestinal calcium absorption was evaluated during both a free- and a low-calcium diet, utilizing kinetics methodology. 4. The low-calcium diet induced, together with an expected reduction of calcium excretion, a marked increase in oxalate urinary output. This finding was independent of the presence or otherwise of hypercalciuria and of the serum levels of parathyroid hormone and vitamin D. Intestinal calcium absorption was also stimulated by calcium deprivation and its levels were well correlated with oxalate excretion. Minor changes in magnesium and citrate excretion were also observed. The overall effect on the relative supersaturation of calcium oxalate consisted in a substantial increase in this parameter during the low-calcium diet. 5. In conclusion, our data reinforce the concept that dietary calcium restriction has potentially deleterious effects on lithogenesis, by increasing the relative supersaturation of calcium oxalate.