Calcium, Phosphorus and Citrate Interactions in Oxalate Urolithiasis Produced with α Low-Phosphorus Diet in Rats

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Abstract
A low-phosphorus (0.16%), normal calcium (0.56%) diet containing casein which produced a high incidence (80%) of mild urolithiasis in weanling male rats was described. This is the first experimental low-phosphorus diet that has been reported to produce calcium oxalate rather than calcium citrate stones. Calculi were produced in young rats by 9 weeks. Mature rats, receiving the diets for a 40-week period, did not develop stones. Both the low phosphorus level and the high calcium: phosphorus ratio were essential for stone formation in weanling rats. Mineral metabolism and urine composition data indicated that urolithiasis was not a direct result of increased urinary calcium or oxalate excretion. This, combined with the frequent occurrence of large PAS-positive casts, led to the postulation that the low phosphorus level and high calcium:phosphorus ratio in the basal diet caused an increase in the production of an organic matrix material, which, combined with the high urine calcium level, resulted in severe urolithiasis. Doubling the vitamin D level of the basal diet reduced the incidence and severity of urolithiasis. This effect was not the direct result of increased urine citrate excretion.