DIFFERENCES IN MINERAL HOMEOSTASIS, VOLUMETRIC BONE MASS AND FEMORAL-NECK AXIS LENGTH IN BLACK-AND-WHITE SOUTH-AFRICAN WOMEN

In South Africa, appendicular and lumbar spine bone mineral density (B MD) have been found to be similar in black and white women. However, f emoral BMD has been found to be higher in black than in white women. T wo different techniques were used to recalculate BMD to eliminate the possible confounding influence of ethnic differences in height on area l BMD measurements, Volumetric bone mineral apparent density (BMAD) va lues were calculated and bone mineral content (BMC) was corrected for body and bone size. This report analyses differences in BMD (corrected for height and weight), BMAD, BMC (corrected for body and bone size), femoral neck axis length (FNAL), mineral homeostasis and bone turnove r (BT) in a group of 20 to 49-year-old premenopausal (105 whites and 7 4 blacks) and 45 to 64-year-old postmenopausal (50 whites and 65 black s) female South African nurses. The corrected BMD and BMC findings wer e congruous, showing that both pre- and postmenopausal blacks and whit es have similar distal radius and lumbar spine bone mass but that whit es have lower femoral neck bone mass than blacks. In contrast, BMAD fi ndings suggest that pre- and postmenopausal whites have lower bone mas s at the lumbar spine and femoral neck than blacks but similar bone ma ss at the distal radius to blacks. There is a greater rate of decline in BMD in postmenopausal whites than in blacks. BMD at the femoral nec k was 12.1% lower in premenopausal whites and 16.5% lower in postmenop ausal whites than in blacks. There was a positive association between femoral neck BMD and weight in premenopausal blacks (R-2 = 0.5, p = 0. 0001) but not in whites. Blacks had shorter FNAL than whites in both t he pre- and postmenopausal groups. Blacks had lower serum 25-hydroxyvi tamin D (25-(OH)D) and higher 1,25-dihydroxyvitamin D (1,25-(OH)(2)D) levels than whites. There were no ethnic differences in biochemical ma rkers of bone formation (serum alkaline phosphatase and osteocalcin) o r bone resorption (urine hydroxyproline and pyridinoline), or in dieta ry calcium intake in either the pre- or postmenopausal groups. In the postmenopausal group, whites had higher ionized serum calcium (p = 0.0 03), similar serum albumin, lower serum parathyroid hormone (p = 0.003 ) and higher urinary calcium excretion (p = 0.0001) than blacks. These results suggest that the higher peak femoral neck BMD in South Africa n blacks than in whites might be determined by greater weightbearing i n blacks and that the significantly lower femoral neck BMD in postmeno pausal whites than in blacks is determined by lower peak femoral neck BMD and a faster postmenopausal decline in BMD in whites. The higher i ncidence of femoral neck fractures in South African whites than in bla cks is probably determined by the lower femoral neck BMD and longer FN AL in whites, The greater rate of decline in BMD in postmenopausal whi tes than in blacks is associated with an increase in urinary calcium e xcretion in whites. Measurement of biochemical markers of BT has not c ontributed to the understanding of ethnic differences in BMD and skele tal metabolism in our subjects.