Dietary calcium and serum 1,25-(OH)2-vitamin D concentrations as determinants of calcium balance in healthy men.

We previously reported that experimental elevations of serum 1,25-(OH)2-vitamin D [1,25-(OH)2-D] concentrations produced by the chronic oral administration of calcitriol, 0.75 micrograms every 6 hr, to healthy human males eating diets providing only 4 mmoles Ca/day stimulate net bone resorption as evidenced by more negative Ca balances and higher rates of urinary hydroxyproline excretion. To determine whether increased dietary Ca intake modifies this response we have compared serum 1,25-(OH)2-D and iPTH concentrations, Ca and PO4 balances, and urinary hydroxyproline excretion in three healthy human males adapted to diets providing 22.3 +/- 1.3 mmoles Ca/day and three healthy human males adapted to diets providing 9.3 +/- 0.7 mmoles Ca/day before and during the continuous oral administration of calcitriol 0.5 micrograms every 6 hr. For all six subjects, serum 1,25-(OH)2-D levels averaged 89 +/- 25 pM during control and 143 +/- 26 pM during calcitriol. Net intestinal Ca absorption and urinary Ca excretion rose during calcitriol but Ca balances did not change, averaging +2.2 +/- 2.2 mmoles/day during control and +4.3 +/- 2.2 mmoles/day during calcitriol for the subjects fed 22 mmoles Ca/day and -1.6 +/- 1.5 mmoles Ca/day during control and -1.7 +/- 2.0 mmoles Ca/day during calcitriol for the subjects fed 9 mmoles Ca/day. Urinary hydroxyproline excretion also did not change. Thus, when serum 1,25-(OH)2-D levels are elevated, the availability of dietary Ca appears to prevent more negative Ca balances and increased urinary hydroxyproline excretion suggesting that net bone resorption is not stimulated.