bidity of kidney stones is not limited to the pain of stone passage; stones increase the risk of renal and urinary tract infections as well as renal insufficiency. A contributing factor in stone formation is hypercalciuria from any cause; another is hyperabsorption of calcium from the gut. Hypercalciuria increases the risk for nephrolithiasis (Pak and Holt, 1976). Hypercalciuria can be present in the absence of hypercalcemia and may reflect routine excretion of excess calcium intake.

Incidence rates for kidney stones vary by age and gender. The rates are highest in men, rising after age 20, peaking between 40 and 60 years, and then beginning to decline (Johnson et al., 1979; Hiatt et al., 1982; Curhan et al., 1993). For women, incidence rates seem to be higher in the late 20s, decreasing by age 50, and then remaining relatively constant (Johnson et al., 1979; Hiatt et al., 1982; Curhan et al., 1997, 2004).

Although calcium is present in approximately 80 percent of kidney stones (Coe et al., 1992), the role of calcium and other nutrients, acting alone or in concert as risk factors, is not completely understood and may be a function of physiological context. Various dietary and non-dietary factors are associated with stone formation, making data difficult to interpret. Rodent models that have been used to explore the effect of dietary factors on the propensity to form calcium oxalate and calcium phosphate stones suggest that the role of supplemental calcium in determining risk for nephrolithiasis varies by interaction with a given dietary component. One study in rats compared renal oxalate crystallization relative to the consumption of calcium-supplemented or oxalate-rich diets as well as control diets. The study found that rats fed the calcium-supplemented diet had enhanced calcium and oxalate accumulation as well as crystallization in renal tissues, even though urinary oxalate and citrate excretion was not significantly different in rats fed the control diet (Mourad et al., 2006). In this study, measures of renal function, including glomerular filtration rate, fractional excretion of urea, and fractional reabsorption of water and magnesium were not affected by the calcium-supplemented diet, and calciuria was only slightly increased.

Recently, a study using data from the Women’s Health Initiative (WHI) trial, which recruited more than 36,000 post-menopausal women ages 50 to 79 years (mean age 62 years), reported findings on the incidence of kidney stones (Jackson et al., 2006). Participants were randomly assigned to receive a placebo or 1,000 mg of elemental calcium (calcium carbonate) per day with 400 International Units (IU) of vitamin D₃. The primary outcome focus was fractures and measures of bone density. Mean baseline intake of calcium was approximately 1,100 mg/day and the supplement added another 1,000 mg/day, for a total average calcium intake of about