Comparative Study of Body Composition by Dual-Energy X-ray Absorptiometry

Dual-energy x-ray absorptiometry (DEXA) is a readily available technique that has recently been applied to measurement of body composition. In order to validate this technique, results from DEXA were compared with fat-free mass (FFM) and fat mass (FM) measured by total body levels of potassium (TBK), nitrogen (TBN), water (TBW) and carbon (TBC). A healthy population of 127 white women and 38 black women with a body mass index of 18-30 were recruited. Compared with each of the other methods, DEXA overestimated FM and underestimated FFM. The slopes of the FM by DEXA versus the FM from each of the four methods were approximately the same, with a s.e.e. ranging from 2.4 to 3.0 units. The slopes of DEXA in comparison to FFM, however, ranged from 0.61 to 0.74 and were significantly less than unity, with a s.e.e. of 1.6 to 2.4 units. These findings suggest that at high values of FFM, DEXA is measuring something different from the FFM measured by TBW, TBN and TBK. The program for separating bone and soft tissue and for separating soft tissue into fat and lean at higher values in the DEXA method may need to be adjusted.