Body composition following hemodialysis: Studies using dual-energy X-ray absorptiometry and bioelectrical impedance analysis

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Abstract
The detection, prevention and treatment of disease is greatly facilitated by the availability of accurate and non-invasive techniques for measuring the amount and regional distribution of fat mass and fat-free mass. As differing degrees of hydration may influence these measurements, we used dual-energy X-ray absorptiometry (DXA) and bioelectrical impedance analysis (BIA) to detect changes in hydration following hemodialysis, and to determine whether fat mass, fat-free mass and bone density measurements were affected by these fluid changes. Ten subjects (7 men, 3 women) mean age 46.2 years (range 25–68 years), with renal failure had bone density, fat-free mass and fat mass measured by DXA, and total body water and fat-free mass measured by BIA, before and after hemodialysis. Thirty-two subjects had fat-free mass measured by DXA and BIA in an attempt to derive new equations (using fat-free mass measured by DXA as the reference standard) to improve the predictive value of BIA. The new equations were then used to derive the changes in fat-free mass following hemodialysis measured using BIA. In absolute terms, total tissue measured by DXA (r=0.99,p=0.01) and total body water measured by BIA (r=0.91,p=0.01) correlated with gravimetric weight. Following hemodialysis, fat mass and bone density measured by DXA were unaffacted by the fluid changes. The change in gravimetric weight was 1.8±0.3 kg,p=0.01 (mean±SEM). This change was measured as 1.9±0.3 kg by DXA, −0.9±1.0 kg by BIA using the published equation for fat-free mass, and 3.2±0.4 kg using the new equation for fat-free mass. The change in fat-free mass measured by DXA (r=0.75,p=0.01), and the change in total body water measured by BIA (r=0.70,p=0.02), correlated with the change in gravimetric weight. The change in fat-free mass measured by BIA did not correlate with the change in gravimetric weight. With the new equation, the change in fat-free mass measured by BIA correlated with the change in gravimetric weight (r=0.74,p=0.02). We conclude that absolute values, and changes in fluid in the range observed in this study, can be detected reliably by DXA. They are reflected in the fat-free mass measurement and do not confound measurements of fat mass and bone density. Although absolute values are reliably measured by BIA, the technique may have limited usefulness in detecting fluid changes. DXA thus provides a practical, non-invasive, precise and accurate method of measuring body composition which can be used as a ‘gold standard’ for validating other techniques.