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

Summary.

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 %FM_ADP from %FM_DEXA were dependent on bone mineral content (BMC_DEXA) fraction of FFM. A low BMC_DEXA 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 BMC_DEXA of FFM. When compared to a 4C model, overestimation of %FM_ADP increases with increasing water fraction of FFM.

Conclusion:

Although there is a tendency for overestimation of %FM_ADP,ADP is a valid method for body composition measurement in the elderly. The bias in %FM_ADP 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.