Evaluation of air-displacement plethysmography and bioelectrical impedance analysis vs dual-energy X-ray absorptiometry for the assessment of fat-free mass in elderly subjects

Objective: To evaluate air-displacement plethysmography (ADP) and bioelectrical impedance analysis (BIA) vs dual-energy X-ray absorptiometry (DXA) for the assessment of fat-free mass (FFM) in healthy elderly subjects. Subjects: Forty-two women and twenty-six men aged 60-84 years. Methods: FFM was measured by DXA and ADP. Body impedance (Z) was measured by four-polar BIA and the impedance index (ZI) was calculated as stature2/Z. Selection of predictors (gender, age, weight and ZI at 5, 50 and 100 kHz) for BIA algorithms was carried out using bootstrapped stepwise linear regression on 1000 samples of 68 subjects. Limits of agreement were used as measures of interchangeability of ADP and BIA with DXA. Results: The limits of agreement of ADP vs DXA were -11.0 to 2.4 kg in males and -4.8 to 2.2 kg in females. Gender, weight and ZI100 were selected as predictors of FFM by bootstrapped stepwise linear regression. In males, ZI100 (-12.2 to 12.2 kg) was much less accurate than weight (-6.0 to 6.0 kg) at predicting FFM and their combination did not improve the estimate (-6.0 to 6.0 kg). In females, ZI100 (-6.8 to 6.8 kg) was less accurate than weight (-5.6 to 5.6kg) at predicting FFM and their combination improved the estimate only slightly (-5.0 to 5.0 kg). Conclusions: In healthy elderly subjects, (1) ADP and DXA are not interchangeable for the assessment of FFM, especially in males; and (2) ZI100 is not superior to weight for the prediction of FFM and their combination is of little advantage and only in females