Validation of air-displacement plethysmography for estimation of body fat mass in healthy elderly subjects

Background Air-displacement plethysmography (ADP) is a non-invasive method for body composition analysis that divides the body into fat-free mass (FFM) and fat mass (FM) (= 2 compartment model, 2C). It places low demands on subject performance and is therefore most convenient in the elderly. Objective To validate ADP against dual energy X-ray absorptiometry (DEXA) and to compare it to a four-compartment model of body composition (4C; fat mass, total body water, bone mineral content and residual mass) in the elderly. Methods Body composition was assessed by ADP, DEXA and bioelectrical impedance analysis (BIA) in 26 healthy elderly subjects (15 women, 11 men) aged 60-82 years. Results Despite a high correlation of %FM assessed by ADP and DEXA we observed significant differences between the results of these methods for both sexes (2.5 +/- 3.4 %; bias SD). Deviations of %FMADP from %FMDEXA were dependent on bone mineral content (BMCDEXA) fraction of FFM. A low BMCDEXA was related to an overestimation of DEXA-derived %FM by ADP There was a systematic bias between results from ADP and the 4C model. 76 % of its variance was explained by the assumption of a fixed density of FFM. 96 % of the variance in the density of FFM was explained by water content and only 4 % by BMCDEXA of FFM. When compared to a 4C model, overestimation of %FMADP increases with increasing water fraction of FFM. Conclusion Although there is a tendency for overestimation of %FMADP, ADP is a valid method for body composition measurement in the elderly. The bias in %FMADP is mainly related to water content of FFM and indicates that a correction factor for TBW may improve the accuracy of the ADP measurements in the elderly.