Comparison between the air displacement method and dual energy x-ray absorptiometry for estimation of body fat

Air displacement plethysmography (ADP) is a method for the determining percent body fat (%BF) using the two-compartment model, in which the body is partitioned into body-fat mass and fat-free mass (FFM). Although this model assumes a constant density of FFM as 1.10 g/ml, its density may depend upon the bone mineral content (BMC) and total body water (TBW) which vary with age, gender, and race/ethnicity. This study compared %BF determined from ADP (ADP%BF) with %BF obtained by dual-energy x-ray absorptiometry (DXA%BF), and also investigated the effects of BMC, TBW, and other factors on its value. The subjects were 721 female and male Japanese aged 40 to 79 years. Body density was measured by ADP and %BF was calculated using Brozek et al's equation. BMC and body-fat volume were measured using DXA, and TBW was measured by multifrequency bioelectrical impedance. A series of anthropometric measurements was taken. Although ADP%BF was highly correlated with DXA%BF (female: r = 0.89, male: r = 0.90) (p < 0.001), ADP%BF differed significantly from DXA%BF (female: -1.30 +/- 0.14% (mean +/- s.e.m.), male: 1.22 +/- 0.13%) (p < 0.001). The difference in %BF (ADP%BF-DXA%BF) was negatively associated with BMC/FFM but not with TBW/FFM in both genders. The difference in %BF was also positively correlated with waist circumference. Considering previous studies, this result may be explained by the underestimation of DXA%BF, rather than by the overestimation of ADP%BF. In conclusion, ADP may be a useful method to measure %BF. However, BMC should be taken into consideration. Furthermore, DXA%BF may be underestimated in people with large waists.