Effects of calcitriol administration on calcium metabolism healthy men

To evaluate the relationship between daily and fasting urinary calcium excretion and serum 1,25-(OH)2-D concentrations, we studied six healthy men during control and during chronic oral calcitriol administration (0.6, 1.2, or 1.8 nmoles every 6 hours for 6 to 12 days) while they ate normal and low calcium diets (calcium content, 19.2 or 4.2 mmoles/day). Daily urinary calcium excretion was directly related to serum 1,25-(OH)2-D concentrations but increased more (P less than 0.025) while subjects ate the normal calcium diet (slope, 0.081 mmole/day/pmole/liter, r = 0.94) than when eating the low calcium diet (slope, 0.043 mmole/day/pmole/liter; r=0.83). During calcitriol and ingestion of the low calcium diet, daily urinary calcium excretion averaged 7.32 +/- 2.6 mmoles/day, exceeding the dietary calcium intake (P less than 0.02). Moreover, during calcitriol, when serum 1,25-(OH)2-D exceeded 140 pmoles/liter, fasting urinary calcium/creatinine exceeded 0.34 mmole/mmole (the upper limit of normal) on either diet. When serum 1,25-(OH)2-D concentrations are elevated, a high fasting urinary calcium/creatinine or high daily urinary calcium excretion, even on a low calcium diet, is insufficient criteria for the documentation of a renal calcium leak.