EFFECT OF DIETARY CALCIUM ON URINARY OXALATE EXCRETION AFTER OXALATE LOADS

An experimental model that allowed differentiation between endogenousl y and exogenously derived urinary oxalate was used to assess the effec t of different forms and doses of ingested calcium on oxalate absorpti on and excretion. In replication 1 (R-1), subjects participated in thr ee oxalate load (OL) tests: baseline (OL alone), calcium carbonate (OL with concomitant calcium carbonate ingestion), and calcium citrate ma late (CCM) (OL with concomitant CCM ingestion). The calcium salts each provided 300 mg elemental Ca. OLs consisted of 180 mg unlabeled and 1 8 mg 1,2[C-13(2)]oxalic acid. In R-2, subjects participated in four OL tests: baseline (OL alone) and OLs administered concomitantly with 10 0, 200, or 300 mg Ca. Timed urine samples after the OL were collected at 2-h intervals for the initial 6 h and samples were pooled into 9-h aliquots for the remaining 18 h of the 24-h period. In R-1, 24-h mean exogenous oxalate decreased (P < 0.05) after the OL from 36.2 mg (base line) to 16.1 mg (after calcium carbonate) and to 14.3 mg (after CCM) whereas endogenous oxalate remained relatively constant. Mean 24-h oxa late absorption decreased significantly from that at the time of the b aseline treatment (18.3%) after both calcium carbonate (8.1%) and CCM (7.2%) treatments. In R-2, mean 24-h oxalate absorption was significan tly lower after 200 (5.9%) and 300 (7.6%) mg Ca than after 100 mg Ca ( 9.1%) and the OL alone (11.3%). Concomitant meal ingestion significant ly decreased oxalate absorption in the absence of dietary calcium but not in association with the 300-mg Ca treatment. The overall data prov ide definitive evidence that dietary calcium can reduce oxalate absorp tion and excretion. Calcium carbonate and CCM were equally effective i n this regard and a minimum of 200 mg elemental Ca maximized this effe ct in conjunction with an oxalic acid intake of 198 mg.