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

Calcium interferes with oxalate absorption in the gut. We studied stone formation in rats fed diets containing various amounts of oxalate and calcium.

In one experiment, male Wistar rats were fed one of five experimental diets: basal diet (292 mM calcium + 8 mM oxalate) or basal diet plus either 100, 300, 500, or 1000 mM oxalate. In the other experiments, 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. Urine specimens were collected every week up to week 4. The kidneys were examined for stone formation and used for determination of tissue oxalate concentration by ion chromatography. Calcium and magnesium were measured by atomic absorption spectrophotometry.

The higher the amount of oxalate in relation to calcium in the diet, the higher the urinary oxalate excretion. A low calcium level in the intestine enhanced the uptake of oxalate, leading to hyperoxaluria and calcium oxalate stone formation.

The bioavailability of dietary oxalate in rats depends mainly on the relative intestinal calcium level. Hyperoxaluria without hyperabsorption of calcium could be induced by oral administration of a relatively high-oxalate and low-calcium (oxalate:calcium >1 [mol/mol]) diet. Exaggerated hyperabsorption of oxalate persists for several weeks and leads to calcium oxalate urolithiasis.