CHANGE IN FAT-FREE MASS ASSESSED BY BIOELECTRICAL-IMPEDANCE, TOTAL-BODY POTASSIUM AND DUAL-ENERGY X-RAY ABSORPTIOMETRY DURING PROLONGED WEIGHT-LOSS

A total of 16 obese women (body mass index (BMI) 30-43 kg m(-2)) parti cipated in a weight reduction study. Before and after a weight loss of 11.7 +/- 7.4 kg (mean +/- SD), body composition was assessed by dual energy X-ray absorptiometry (DXA), and total body potassium counting ( TBK). These measurements were compared with bioimpedance analysis (BIA ) by applying 11 predictive BIA equations published in the literature. Predictive equations for the present study population were developed, with the use of fat-free mass (FFM) as assessed by TBK and DXA as ref erences in multiple regression analysis. The results of the BIA equati ons varied widely; FFM was generally overestimated by BIA as compared with DXA and TBK before and after weight loss. During weight loss, the FFM did not change, as estimated by DXA (1.3 +/- 2.3 kg, p > 0.05) an d TBK (0.9 +/- 2.9 kg, p > 0.05). The recorded change in impedance (R) was also insignificant. Three BIA equations from the literature, whic h were not specific for the degree of obesity in the present study gro up, predicted changes in FFM (from 0.5 +/- 3.6 to 2.4 +/- 4.4 kg, p > 0.05) that were comparable with those estimated by the reference metho ds. Eight equations from the literature, which included equations spec ific for the degree of obesity in the study group, and the group speci fic equations developed for the present population predicted significa nt changes in FFM during weight loss (from 2.3 +/- 3.0 to 5.0 +/- 3.0 kg, p < 0.05). We conclude that in obesity most predictive equations a re unable to predict static body composition and are not reproducible for individuals over time. However, a significant or insignificant cha nge in R (without accompanying predictive equations) may be used to in dicate whether FFM is lost or preserved in groups of obese subjects.