Schedule of taking calcium supplement and the risk of nephrolithiasis.

BACKGROUND: Variation in the timing of calcium supplement may affect gastrointestinal absorption of both calcium and oxalate differently and may associate with variable risk of calcium oxalate nephrolithiasis. There are few human studies addressing specifically the appropriate time for taking calcium supplement. Therefore, this study was performed to compare calcium bioavailability and the risk of calcium oxalate stone formation for calcium supplement taken with meal vs. taken at bedtime. METHODS: Thirty-two healthy male navy privates, 22.7 +/- 1.9 years old (mean +/- SD), who had normal renal function (serum creatinine less than 150 umol/L) and no history of renal stone, participated in the study. The subjects were randomly allocated into two groups of 16 each. Group A took 1 g of calcium carbonate with meal, 3 times/day; and group B took 3 g/day of calcium carbonate at bedtime. After taking the regimens for 1 week, followed by 1 month of washout period, crossover between both groups was done. The diet was controlled throughout the study. Twenty-four-hour urine collections for the determination of urinary constituents were obtained at baseline and after taking both regimens of calcium supplement. Activity product for calcium oxalate was determined to assess the risk of calcium oxalate stone formation. RESULTS: Urinary excretions of calcium were significantly elevated above the baseline values when taking calcium supplement both with meal (3.48 +/- 2.13 mmol/day vs. 5.17 +/- 2.61 mmol/day, P < 0.05) and at bedtime (3.09 +/- 1.70 mmol/day vs. 5.08 +/- 2.34 mmol/day, P < 0.05). There was no difference between the two regimens in the urinary calcium excretions. The urinary oxalate was decreased significantly when the subjects took calcium supplement with meal compared with the corresponding baseline value (0.13 +/- 0.05 vs. 0.17 +/- 0.07 mmol/d, P= 0.01). In contrast, there was no alteration in urinary oxalate when calcium supplement was taken at bedtime compared to the baseline values (0.15 +/- 0.05 mmol/day vs. 0.15 +/- 0.06 mmol/day, P= 0.9). Compared with the corresponding baseline values, there was no significant increase in the activity product for calcium oxalate when taking calcium with meal (0.54 +/- 0.25 vs. 0.57 +/- 0.22, P= 0.54), but it was increased significantly when calcium supplement was taken at bedtime (0.47 +/- 0.21 vs. 0.72 +/- 0.27, P < 0.01). CONCLUSION: Calcium supplement should be taken with meal in order to avoid increasing the risk of calcium oxalate nephrolithiasis.

Effects of calcium supplements on the risk of renal stone formation in a population with low oxalate intake.

It has been speculated that calcium supplement in subjects with low oxalate intake might increase the risk of calcium stone formation due to an increase in calcium absorption without a significant reduction in oxalate absorption. There have been no human studies addressing specifically the effects of taking calcium supplements in populations whose dietary oxalate is low. This study was conducted to determine the effects of calcium supplements on the risk of calcium stone formation in a population with low oxalate intake. Thirty-two healthy male navy privates, 22.7 +/- 1.9 (mean +/- SD) years old, who had oxalate intake of less than 1 mmol/day, a serum creatinine of less than 150 micromol/l, and no history of renal stones, participated in the study. Dietary oxalate was controlled to be under 1 mmol/day throughout the study. Twenty-four hour urine collections for the determination of urinary constituents were obtained at baseline and after taking calcium supplements. Detection of calcium oxalate was performed to assess the risk of calcium oxalate stone formation. The urinary excretion of calcium was significantly elevated above baseline values while taking the calcium supplements (3.48 +/- 2.13 vs 5.17 +/- 2.61 mmol/d, p < 0.05) and urinary oxalate was significantly decreased when the subjects took calcium supplements compared to the corresponding baseline value (0.13 +/- 0.05 vs 0.17 +/- 0.07 mmol/d, p = 0.01). Urinary citrate was significantly elevated when the subjects took calcium supplements compared to the baseline (0.83 +/- 0.57 vs 0.64 +/- 0.39 mmol/d, p = 0.03). There was no significant alteration in the activity products of calcium oxalate while taking the calcium supplements (0.54 +/- 0.25 vs 0.57 +/- 0.22, p = 0.54). The effect of calcium supplements with meals, for the reduction of the risk of calcium stone formation, was unchanged, even in a population whose oxalate intake is rather low. Taking calcium supplements resulted in a reduction in urinary oxalates and an elevation in urinary citrates. Both alterations in urinary constituents counterbalanced the elevation in urinary calcium which resulted from the calcium supplements.