Oxalate Absorption and Postprandial Urine Supersaturation in An Experimental Human Model of Absorptive Hypercalciuria

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Abstract
The effect of 1,25-dihydroxyvitamin D [1,25-(OH)2D] on dietary oxalate absorption and postprandial urine supersaturation with calcium oxalate was determined in 11 normal subjects. 1,25-(OH)2D increased the urinary excretion of orally administered [14C]oxalate in the 8 h period after a liquid meal containing 1.875 mmol of Ca and 0.83 mmol of oxalate (P < 0.01) and during a 48 h period when the subjects ingested a diet containing 25 mmol of Ca and 3.3 mol of oxalate/day (P < 0.01); 1,25-(OH)2D administration had no effect on [14C]oxalate excretion when Ca was removed from the liquid meal. 1,25-(OH)2D increased 24 h urinary oxalate excretion from 28.7 .+-. 2.1 mmol/mol of creatinine to 36.8 .+-. 2.6 mmol/mol of creatinine (P < 0.05) on the 10 mmol/day Ca diet and from 26.4 .+-. 2.9 to 33.2 .+-. 2.2 mmol/mol of creatinine (P < 0.1) on the 25 mmol/day Ca diet. A linear correlation (r = 0.72) was found between plasma 1,25-(OH)2D levels and urinary [14C]oxalate excretion after the liquid meal. 1,25-(OH)2D adminstration produced postprandial supersaturation of urine with calcium oxalate and calcium oxalate crystalluria. 1,25-(OH)2D evidently increases oxalate absorption (and urinary excretion) by increasing Ca absorption, which results in less binding of Ca to oxalate in the intestine; therefore more oxalate is available for absorption. The combined effect of increased Ca and oxalate absorption results in postprandial supersaturation of urine with Ca oxalate, with resultant crystalluria.