Risk factors for kidney stones in older women in the southern United States

Background: The occurrence of kidney stones is disproportionate in the southern region of the United States. Risk factors for the occurrence of kidney stones in this geographic area have not been reported previously. Methods: The Women's Health initiative (WHI) is an ongoing multicenter clinical investigation of strategies for the prevention of common causes of morbidity and mortality among postmenopausal women. A case-control ancillary study was conducted on 27,410 (white or black) women enrolled in the 9 southern WHI clinical centers. There were 1,179 cases (4.3%) of kidney stones at the baseline evaluation. Risk factors for stone formation were assessed in cases versus age- and race-matched control subjects. Results: Risk factors (univariate) included low dietary potassium (2,404 versus 2,500 mg/day, P = 0.006), magnesium (243 versus 253 mg/day, P = 0.003) and oxalate (330 versus 345 mg/ day, P = 0.02) intake, as well as increased body mass index (28.5 versus 27.7 kg/m(2), P = 0.001) and a history of hypertension (42% versus 34%, P = 0.001). A slightly lower dietary calcium intake (683 versus 711 mg/day, P = 0.04) was noted in case subjects versus control subjects, but interpretation was confounded by the study of prevalent rather than incident cases. Supplemental calcium intake >500 mg/day was inversely associated with stone occurrence. Conclusion: Multivariate risk factors for the occurrence of kidney stones in postmenopausal women include a history of hypertension, a low dietary intake of magnesium, and low use of calcium supplements.