Calcium and Kidney Stone

Kidney stones occur frequently. As many as 10 percent of men and 3 percent of women have a stone during their adult lives. About 80 percent of all stones are composed of calcium oxalate, alone or with a nucleus of calcium phosphate (apatite). The first step in the formation of calcium stones is the formation of microscopic crystals in the lumen of renal tubules as a result of supersaturation of the luminal fluid with calcium oxalate or calcium phosphate. Crystal growth, and therefore stone formation, is enhanced by the attachment of crystals to the surface of cells in the papillae and is reduced by the inhibitors of crystal growth and aggregation that are normally present in renal tubular fluid. Currently, however, attempts to prevent calcium stones remain focused on efforts to reduce urinary concentrations of calcium and oxalate by reducing the rate of urinary excretion of these substances and by increasing urine volume. In addition, efforts may be directed toward increasing urinary citrate excretion in order to increase the formation of calcium citrate, a highly soluble calcium salt whose formation decreases urinary concentrations of free calcium.

Among healthy subjects, an increase in dietary calcium intake results in an increase in urinary calcium excretion equal to about 8 percent of the amount by which the dietary calcium intake is increased. Among the approximately 30 to 50 percent of people with calcium stones who have hypercalciuria (urinary calcium excretion greater than 300 mg [7.5 mmol] per day in men and 250 mg [6.2 mmol] per day in women), urinary calcium excretion increases by about 20 percent of increased dietary calcium intake, because of the increased efficiency of intestinal calcium absorption that may be either dependent on calcitriol (1,25-dihydroxyvitamin D₃) or independent of it. It is standard practice to recommend dietary calcium restriction to prevent stone recurrence in patients with hypercalciuria, even though prospective trials have not established the efficacy of such restriction. In addition, advice to restrict dietary calcium intake is often extended, without a clear rationale, to patients with calcium stones who do not have hypercalciuria.

Lemann, J., Composition of the Diet and Calcium Kidney Stones, The New England Journal of Medicine, Volume 328:880-882