BODY-COMPOSITION DURING WEIGHT-LOSS IN OBESE PATIENTS ESTIMATED BY DUAL-ENERGY X-RAY ABSORPTIOMETRY AND BY TOTAL-BODY POTASSIUM

OBJECTIVE: To validate the ability of DXA to measure total body compos ition before and after weight loss and the composition of the lost bod y mass. DESIGN: Cross sectional and follow-up study of body compositio n before and after a weight loss of 10.6 +/- 6.8 kg. SUBJECTS: 31 obes e subjects with a mean body weight of 105.2 +/- 15.2 kg at baseline, a nd a mean body weight of 94.6 +/- 16.5 kg at follow-up. MEASUREMENTS: Body composition was measured by dual X-ray absorptiometry, total body potassium counting, and high precision scales before and after a weig ht loss. RESULTS: DXA and the scales showed a strong linear relation ( r=1). At baseline, however, DXA underestimated the body weight by a ma ximum of 3.2 kg because the subjects were too large for the scan table . After weight loss body weight measurements were accurate. Before and after weight loss the linear relations between DXA and TBK for FFM we re strong (r=0.92 and 0.93). Bland and Altman plots showed limits of a greement of +/-9 kg before and after weight loss; DXA underestimated F FM in women and overestimated FFM in men. DXA accounted for 80% of the lost body weight. The composition of the lost body mass did not diffe r from that estimated by TBK (7.6% FFM and 92.4% FM by TBK; 11% FFM an d 89% FM by DXA). CONCLUSION: DXA estimates accurately the body compos ition and the composition of weight loss in groups of obese subjects. However, the scan table may be too small for patients weighing more th an 95 kg.