Calcium absorption responses to calcitriol in black and white premenopausal women.

Abstract

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.