Effects of calcium and magnesium on urinary oxalate excretion after oxalate loads

Purpose: Urinary oxalate is a primary determinant of the level of calcium o xalate saturation and the formation of calcium oxalate crystals, a key even t in kidney stone formation. The primary objective of this study was to com pare the effects of calcium carbonate and magnesium oxide on oxalate absorp tion. Materials and Methods: An experimental model was used that allowed differen tiation between endogenously and oxalate load-derived urinary oxalate. Twen ty-four healthy subjects (10 males, 14 females) participated in three oxala te load (OL) tests: control (OL alone), calcium carbonate (OL with concomit ant calcium carbonate ingestion), and magnesium oxide (OL with concomitant magnesium oxide ingestion). Oxalate loads consisted of 180 mg. unlabeled an d 18 mg. 1,2[C-13(2)] oxalic acid. Timed urine samples were collected after the OL for analysis of oxalate, calcium, magnesium, and creatinine. Results: Both the calcium carbonate and magnesium oxide treatments were ass ociated with significantly lower load-derived oxalate levels at all time po ints within the initial 24-hour post-oxalate ingestion period compared with levels observed for the control treatment. There were no treatment effects on endogenous oxalate levels. The efficiency of oxalate absorption for the calcium carbonate (5.1%) and magnesium oxide (7.6%) treatments was signifi cantly lower than that for the control treatment (13.5%). Conclusions: The results suggested that magnesium was nearly as effective a s calcium in reducing oxalate absorption and urinary excretion. Higher leve ls of urinary oxalate, calcium, and magnesium in males appeared to be large ly a function of body size since gender differences either disappeared or w ere reversed when a correction was made for urinary creatinine excretion.