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THE DISCREPANCY OF FAT-FREE MASS MEASURED BY DUAL ENERGY X-RAY ABSORPTIOMETRY AND AIR-DISPLACEMENT PLETHYSMOGRAPHY VARIES WITH AGE AND ADIPOSITY – IS IT RELATED TO FAT INFILTRATION?

T.W. Auyeung, J.S.W. Lee, T. Kwok, M. Li, S. Hui, J. Woo

J Aging Res Clin Practice 2013;2(1):7-10

Background: The discrepancy of fat-free mass (FFM) measured by dual energy x-ray absorptiometry (DXA) and air-displacement plethysmography (ADP) could be attributed to the erroneous inclusion of inter-muscular adipose tissue (IMAT) which increases with age and adiposity, as part of FFM measured by DXA. We, therefore, attempted to examine whether this disagreement was related to age and adiposity. Methods: One hundred and seventy two participants aged 20 to 76 years, were examined by ADP and DXA to measure their body density (BD) and FFM respectively. FFM was also derived from BD using the 2-compartment model where: (FM+FFM)/BD = FM/FM density + FFM/FFM density. The FFM and FM density was assumed to be 1.1000 and 0.9007 respectively. The difference between DXA-measured and ADP-derived FFM was calculated and the association between this difference with age, gender, body mass index (BMI), Waist-hip ratio (WHR) was examined by multiple linear regression. Results: The DXA-measured FFM was 2.2 kg (2.3 %FFM) higher than the ADP-derived FFM. In multivariate analysis, higher BMI, higher WHR and older age was significantly associated with greater difference between DXA-measured and ADP-derived FFM. Conclusion: DXA-measured FFM was higher than that derived by the 2-compartment model using ADP BD measurement. This variation was significantly associated with older age, general and central adiposity. Comparison of DXA-measured muscle mass across a wide range of age and adiposity should take this into consideration. Fat infiltration into skeletal muscles in older and more obese adults may contribute to the unexplained discrepancy between the two methods.

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