(Scanned from the applicant's description): Increases in dietary salt, i.e. sodium chloride (NaCI), increase urinary calcium over the range of intakes commonly consumed. Both salt loading studies and reports of within-population correlations find that increased urinary calcium losses are approximately 1 mmol (40 mg) for each 100 mmol (2300 mg) increase in dietary NaC1. Individuals with hypercalciuria and/or a history of calcium kidney stones appear to have 2 times greater proportional increases in urinary calcium per 100 mmol increase in salt intake. Thirty-two subjects with a history of at least one calcium oxalate kidney stone will be recruited; 16 with hypercalciuria, 16 normocalciuria. This study consists of two, seven-day periods. For all seven days of both dietary treatments, each participant will eat only the foods provided by the investigators, a nutritionally adequate diet prepared from common foods containing 50 mmol/d salt. The first five days of each treatment (adaptation), the participant will be free-living, the sixth and seventh in the WSU metabolic unit. One of the two weeks, 150 mmol supplemental salt will be added as tablets taken with each meal; the total of 200 mmol is the average consumption measured in previous studies of stone formers. The major outcome measures are urinary oxalate, calcium, magnesium, phosphate and citrate, and two measures of calcium salt saturation as indicators of risk of calcium salt precipitation. The two measures of calcium salt precipitability will be the Tiselius risk index, a measure of calcium oxalate precipitability from solution which includes the effects of volume, magnesium and citrate concentrations as well as calcium and oxalate, and Ap (CaP), a measure of calcium phosphate precipitability from solution, which includes the effects of volume, citrate and pH as well as calcium and phosphate. Because increased urinary calcium loss from adding salt occurs, bone breakdown may be increased in compensation, so 4 markers of bone turnover will be measured, bone alkaline phosphatase, osteocalcin, deoxypyridinoline, and N-telopeptide. We will also compare the responsiveness of hypercalciuric vs normocalciuric participants for all the outcome variables, because the literature suggests that hypercalciuric stoneformers may be more sensitive to dietary salt effects on urinary calcium. Reduction of dietary NaC1 from typical intakes of 200 mmol/day to an intake of 50 mmol/day may decrease the risk of recurrence of calcium-containing kidney stones and slow rates of bone loss, thus reducing risk of osteoporosis as well.
