As the amount of calcium intake decreases, the amount of oxalate available for absorption into the bloodstream increases; this oxalate is then excreted in greater amounts into the urine by the kidneys.In the urine, oxalate is a very strong promoter of calcium oxalate precipitation—about 15 times stronger than calcium.In the early 1990s, a study conducted for the Women's Health Initiative in the US found that postmenopausal women who consumed 1000 mg of supplemental calcium and 400 international units of vitamin D per day for seven years had a 17% higher risk of developing kidney stones than subjects taking a placebo.
The link between vitamin D intake and kidney stones is also tenuous.
For example, at a p H of 7.0, the solubility of uric acid in urine is 158 mg/100 ml.
Reducing the p H to 5.0 decreases the solubility of uric acid to less than 8 mg/100 ml.
Depending on the chemical composition of the crystal, the stone-forming process may proceed more rapidly when the urine p H is unusually high or low.
Supersaturation of the urine with respect to a calculogenic compound is p H-dependent.