[Calcium decreases urinary oxalate]

The effects of calcium supplementation on urinary oxalate excretion was tested in 9 normal subjects, 4 males and 5 females between 23 and 49 years of age. In a crossover study 800 mg calcium was orally administered as active absorbable algal calcium (AAACa) (A) and calcium carbonate (B), and compared with non-calcium containing placebo (C). Calcium, oxalate, osmolality, creatinine and pH were measured in the first three morning urine samples and Ca/osmolality, Ca/osmolality/body weight, Ca/creatinine and oxalate/osmolality were calculated to correct for urine dilution. Ca x oxalate product was also calculated and Ca oxalate crystal in the sediment was microscopically examined and semiquantitatively estimated as -, +, ++, and +++ expressed as 0, 1, 2 and 3 respectively. Urinary Ca excretion was similar in A and B, but significantly larger than C, regardless of the method of correction for dilution. Urinary oxalate excretion tended to be lower in A than in B and C. Urine pH was similar among all three groups. Ca x oxalate product was higher in C than in A and B. AAACa, unlike calcium carbonate, appeared to decrease urinary oxalate excretion and Ca x oxalate product more efficiently than Ca carbonate, suggesting a possibility of inhibiting the formation of Ca x oxalate kidney stones. Formation of calcium oxalate was also tested in vitro by adding oxalate to urine samples and aqueous calcium solution.