Impact of dietary calcium and oxalate ratio on urinary stone formation in rats

Background and Purpose: Calcium interferes with oxalate absorption in the g ut. We studied stone formation in rats fed diets containing various amounts of oxalate and calcium. Materials and Methods: In one experiment, male Wistar rats were fed one of five experimental diets: basal diet (292 mM calcium + 8 mM oxalate) or basa l diet plus either 100, 300, 500, or 1000 mM oxalate, In the other experime nts, rats were given one of five diets: calcium-free diet alone or calcium- free diet plus 300 mM oxalate and either 0, 100, 200, or 300 mM calcium. Ur ine specimens were collected every week up to week 4, The kidneys were exam ined for stone formation and used for determination of tissue oxalate conce ntration by ion chromatography, Calcium and magnesium were measured by atom ic absorption spectrophotometry, Results: The higher the amount of oxalate in relation to calcium in the die t, the higher the urinary oxalate excretion. A low calcium level in the int estine enhanced the uptake of oxalate, leading to hyperoxaluria and calcium oxalate stone formation. Conclusion: The bioavailability of dietary oxalate in rats depends mainly o n the relative intestinal calcium level. Hyperoxaluria without hyperabsorpt ion of calcium could be induced by oral administration of a relatively high -oxalate and low-calcium (oxalate:calcium >1 [mol/mol]) diet. Exaggerated h yperabsorption of oxalate persists for several weeks and leads to calcium o xalate urolithiasis.