High-calcium diet may raise risk of stone formation

The trial enrolled five men and five women, all healthy and without hypercalciuria or a history of nephrolithiasis, and used a two-phase, randomized, crossover design to compare the effects on stone risk factors of low-calcium (400 mg/day) and high-calcium (1,000 mg/day) diets with liberal oxalate intake (200 mg/day). The investigators hypothesized that, with sufficient dietary oxalate intake, urinary oxalate would decline during a high-calcium diet and would attenuate a rise in urinary saturation of calcium oxalate.

"Our study results verify that urinary oxalate increases with dietary calcium restriction when oxalate consumption is unrestricted, but show that the decline in urinary oxalate excretion that occurs with increased calcium intake is insufficient to overcome the effect of the concomitant increase in urinary calcium on urinary saturation for calcium oxalate. We now will be repeating this study in persons with hypercalciuria, but it appears that altering the risk of calcium oxalate stone formation through dietary manipulation may require achieving the correct balance between dietary calcium and oxalate intake."

Dr. Matsumoto explained that dietary calcium binds oxalate in the bowel to attenuate oxalate absorption. However, researchers in a previous two-phase, randomized, crossover study reported that among normal subjects with limited oxalate intake, low- and high-calcium diets were associated with similar urinary oxalate excretion rates and calcium oxalate relative saturation ratios (J Urol 2003; 169:470-4). The latter finding was due to differences between the two diets in ingested fluid, potassium, and phosphate levels. After adjusting for those and other confounding factors, the relative saturation ratio of calcium oxalate was increased significantly by the high-calcium diet.

"That study suggested dietary oxalate restriction could abolish the calcium-oxalate interaction in the bowel and prevent the decline in urinary oxalate typically seen with a high calcium intake. Our trial was designed to further elucidate that mechanism," Dr. Matsumoto said.

In the current study, each diet was followed for 1 week and the two phases were separated by a 4-week washout period. During each diet phase, participants followed an instructed diet at home for 3 days and then for the next 4 days, they received a controlled metabolic diet.

In addition to the difference in calcium content, the high-calcium diet also featured slightly higher amounts of magnesium and protein in order to achieve increased calcium delivery through dietary means. Serum and 24-hour urine specimens were collected during the last 2 days of the study for measurement of stone risk factors.