VOLUMETRIC BONE-DENSITY OF THE LUMBAR SPINE IS RELATED TO FAT MASS BUT NOT LEAN MASS IN NORMAL POSTMENOPAUSAL WOMEN

We have previously found that fat mass but not lean body mass is relat ed to bone mineral density (BMD) in women. In these and most other stu dies of the dependence of BMD on body composition, areal rather than v olumetric bone density was measured. It is possible that the dependenc e of this variable on body size introduced a scale artifact that contr ibuted to the previous findings. The present study addresses this issu e by measuring the volumetric density of the third lumbar vertebra fro m simultaneous anteroposterior (AP) and lateral scans using dual-energ y X-ray absorptiometry in 119 normal postmenopausal women. Whole body fat and lean body mass were also measured using this technique. In the AP projection, BMD was similarly related to body weight and to fat ma ss (r = 0.44, p < 0.0001 for both) but not to lean body mass (r = 0.17 , NS). BMD in the lateral projection was less closely related to body composition than was AP BMD, but the greater impact of fat (r = 0.25, p < 0.01) than lean body mass (r = 0.09, NS) was still evident. When A P or lateral BMDs were divided by height, arm span or the square root of the scan area to produce an index with the dimensions of volumetric density, the dependence of BMD on body weight and fat mass were not a ffected but the relationship to lean body mass was eliminated (-0.02 < r < 0.09). Similarly, the volumetric density of the third lumbar vert ebra was related to fat mass (r = 0.21, p = 0.02) but not to lean body mass (r = 0.01). It is concluded that BMD is related to fat mass and that previously reported associations between lean body mass and BMD a re probably contributed to by a scaling factor arising from failure to measure volumetric bone density.