High-calcium intake abolishes hyperoxaluria and reduces urinary crystallization during a 20-fold normal oxalate load in humans

Background. The aim of the study was to test whether increasing dietary calcium intake lowers intestinal oxalate absorption and thereby prevents hyperoxaluria and urinary crystallization during a 20-fold normal oxalate load in healthy subjects. Methods. Fourteen healthy male volunteers (age 23-44 years, BMI 21.5-27.7kg/m(2)) collected 24-h urines while on free-choice diet as well as on two standardized diets. The latter contained 2545 kcal, 2500 mi of mineral water, 102 g of protein, 13.6 g of sodium chloride and 2220 mg of oxalate (similar to 20-fold content of an average diet). Subjects were studied twice while on the standardized diet, once while eating a normal amount of calcium (1211 mg/day, oxalate-rich diet), and once while eating 3858 mg of calcium/day (calcium and oxalate-rich diet). Results. Compared with the free-choice diet (322 +/- 36 mu mol/d), U-Ox x V increased to 780 +/- 72 mu mol/d on the oxalate-rich diet (P = 0.001) and fell again to 326 +/- 31 mu mol/d on calcium and oxalate-rich diet (P = 0.001 vs oxalate-rich diet). Urinary glycolate (a metabolic precursor of Ox) always remained below the upper limit of the normal range and did not change between different diets, indicating that changes in U-Ox x V reflect respective variations in intestinal absorption of Ox. U-Ca x V was 4.60 +/- 0.45 mmol/d on the free-choice diet and 3.20 +/- 0.32 mmol/d on the oxalate-rich diet (P = 0.011 vs free-choice diet); it increased to 7.28 +/- 0.74 mmol/d on the calcium- and oxalate-rich diet (P = 0.001 vs free-choice and oxalate-rich diets). As indicated by the AP (CaOx) index (Tiselius), urinary supersaturation did not vary significantly between the three diets. In freshly voided morning urines (studied in 8/14 subjects) on the oxalate-rich diet, CaOx crystals or crystal aggregates of up to 80 mu m diameter were found in 5/8 urines, whereas this never occurred on the free-choice diet and only once on the calcium- and oxalate-rich diet. Conclusion. Increasing calcium intake while eating Ox-rich food prevents dietary hyperoxaluria and reduces CaOx crystallization in healthy subjects. This further illustrates that dietary counseling to idiopathic calcium-stone formers should ensure sufficient calcium intake, especially during oxalate-rich meals.