CALCIUM RETENTION AND HORMONE LEVELS IN BLACK-AND-WHITE WOMEN ON HIGH-CALCIUM AND LOW-CALCIUM DIETS

Black-white differences in calcium metabolism are sought because they may reveal why blacks have higher bone mass than whites. Comparative s tudies of calcium absorption in blacks and whites are not available. I n this study, we compare fractional calcium retention, an index of cal cium absorption, and calcium regulating hormone levels in black and wh ite women on a high-calcium diet and after adaptation to a low-calcium diet. A total of 30 healthy women (15 black and 15 white) had measure ments of fractional Ca-47 retention and calcium regulating hormone lev els after 8 weeks on a 2000 mg calcium diet and, subsequently, after 1 , 2, 4, and 8 weeks on a 300 mg calcium diet. By 2 weeks after the die t change, fractional Ca-47 retention, parathyroid hormone (PTH), and 1 ,25-dihydroxyvitamin D [1,25-(OH)2D] had reached a plateau at higher l evels, urine calcium at a low level, and serum calcium at the same lev el (repeated-measures ANOVA). Fractional Ca-47 retention, serum calciu m, and PTH were similar on both diets in blacks and whites. Blacks had higher levels of 1,25-(OH)2D on both diets (e.g., 125.1 +/- 53.5 SD v ersus 73.4 +/- 19.0 pmol/liter, P = 0.003 on low-calcium diet) and a g reater increase in 1,25-(OH)2D after the diet change (33.9 +/- 30.1 SD versus 11.8 +/- 17.9 pmol/liter, P = 0.021). Serum phosphorus was low er in blacks throughout. For hormone levels and fractional calcium ret ention to stabilize after a diet change, 2 weeks was needed. Similar l evels of fractional retention in black and white women despite higher levels of 1,25-(OH)2D in blacks suggest that blacks may have a gut res istance to the action of 1,25-(OH)2D.