Are soft tissue composition of bone and non-bone pixels in spinal bone mineral measurements by DXA similar? Impact of weight loss

Weight loss seems associated with a decrease in bone mineral density (BMD) as measured by absorptiometry, which may be the result of accuracy errors caused by differences in soft tissue between non‐bone and bone pixels. The aim was to study the abdominal fat% and thickness in regions corresponding to non‐bone, soft tissue‐only and bone pixels for spinal BMD measurements by dual energy X‐ray absorptiometry (DXA), and to calculate the theoretical errors in measurement of changes in BMD by DXA as a result of changes in soft tissue heterogeneity with weight loss. Abdominal computed tomography (CT) and DXA scans were performed in 34 obese subjects (42·1 ± 10·1 years (mean ± SD), wt: 102·1 ± 12·8 kg and BMI: 36·6 ± 3·8 kg m–2) before and after weight loss (11·3 ± 6·9 kg after 1 year). There were some significant differences in fat% and thickness of soft tissue between abdominal regions corresponding to non‐bone and bone pixels, respectively, for spinal BMD measurements by DXA, both before and after weight loss. With weight loss there were some changes in the soft tissue heterogeneity, which caused a minor theoretical error (apparent, but false decrease of 1–2%) of borderline significance for the anterior–posterior (AP) spinal BMD by DXA.