Effect of Potential Renal Acid Load of Foods on Calcium Metabolism of Renal Calcium Stone Formers

Abstract

Objectives: Diet has been proposed as a causative factor of hypercalciuria in patients with calcium stones. The aim of this study was to investigate the influence of diet on calcium metabolism of renal stone formers. Methods: Thirty-five renal calcium stone formers were entered in this study. A 2-day recall of dietary intake was obtained from each subject. The food records were coded and computer analyzed for total energy, protein, fat, carbohydrate, sodium, potassium, calcium, magnesium, phosphate, oxalate, vitamin C and fiber. Daily potential renal acid load (PRAL) of the diet was calculated considering the mineral and protein composition of foods, the mean intestinal absorption rate for each nutrient and the metabolism of sulfur-containing amino acids. A fasting blood sample was drawn and a 24-hour urine collection were obtained for analyses of calcium, phosphate and creatinine. Serum osteocalcin was also analyzed. A fasting 2-hour urine sample was collected in the morning for hydroxyproline, pyridinium cross-links and creatinine. Results: The mean daily dietary PRAL of renal stone formers was 22.4 ± 15.7 (range 4.2–65.8) mEq/day. Regression analysis demonstrated that urinary calcium excretion is dependent on daily protein intake and dietary PRAL, whereas the urinary pyridinium cross-links/creatinine ratio is inversely dependent on daily calcium intake. The urinary pyridinium cross-links/creatinine ratio was significantly lower in patients on a low calcium diet (<600 mg/day) than in other patients (19.5 ± 7.8 vs. 27.3 ± 7.5 nM/mM, p = 0.008). No significant difference was observed between the 2 groups for daily urinary calcium (254 ± 109 vs. 258 ± 140 mg/day), serum osteocalcin (8.2 ± 3.3 vs. 6.2 ± 2.4 ng/ml) and urinary hydroxyproline/creatinine (14.1 ± 7.4 vs. 10.3 ± 4 mg/g). Conclusions: The urinary calcium excretion of renal stone formers seems to be dependent on dietary acid load rather than dietary calcium intake. In patients consuming an acidifying diet a restriction of calcium intake could increase bone resorption leading to a progressive bone loss.