Calcium absorption responses to calcitriol in black and white premenopausal women

On the basis of recent findings that adult black women had similar calcium absorption but higher levels of 1,25-dihydroxyvitamin D [1,25-(OH)2D] than white women, we hypothesized that blacks have a gut resistance to the action of calcitriol. To test this, we studied 11 black [age, 32.4 +/- 5.7 (+/- SD) yr] and 12 white women (28.4 +/- 5.5 yr). The women were maintained on a constant 500-mg calcium diet for 4 weeks, and each received calcitriol (0.25 microgram) four times daily for the last 2 weeks. After 2 and 4 weeks, each subject had measurements of fractional 45Ca absorption index and blood and urine tests. At 2 weeks, the black women had similar calcium absorption indexes [18.7 +/- 1.9% (+/- SEM)/L vs. 20.0 +/- 1.8%/L; age adjusted], borderline higher 1,25-(OH)2D levels [95.7 +/- 6.4 (+/- SEM) vs. 78.2 +/- 6.2 pmol/L; P = 0.071; age adjusted], higher serum PTH levels, and lower urinary calcium excretion. Calcitriol therapy induced similar increments in plasma 1,25-(OH)2D levels in the two groups, but a smaller increment in calcium absorption in the black women (18.4 +/- 8.6% vs. 44.6 +/- 7.8%; P = 0.043; means adjusted for age and initial absorption index). These findings support the hypothesis that, compared with whites, healthy premenopausal black women have gut resistance to the action of calcitriol.