The Validity of Bioelectrical Impedance Analysis to Measure Body Composition in Phenylketonuria

To compare the measurement of total body water (TBW) and fat-free mass (FFM) using the criterion method of deuterium dilution space (²H₂O) with bioelectrical impedance analysis (BIA) using a portable QuadScan 4000, Bodystat^® in children and adolescents with phenylketonuria (PKU).

Sixteen patients with PKU, median age is 12.5 (range 5-20.6) years, were recruited into this cross-sectional study. TBW was measured by both deuterium dilution and BIA on the same occasion as per a standard protocol. FFM was estimated from predictive equations.

There was no significant difference between TBW_Deut and TBW_BIA (p = 0.344) or FFM_Deut and FFM_BIA (p = 0.111). TBW_Deut and TBW_BIA were highly correlated (r = 0.990, p < 0.0001), as were FFM_Deut and FFM_BIA (r = 0.984, p < 0.0001). Bland-Altman plots demonstrated that there was no proportional bias between the criterion method, TBW_Deut, and the test method TBW_BIA, in estimating TBW (β = -0.056, adjusted r ² = 0.069, p = 0.169) or FFM (β = -0.089, adjusted r ² = 0.142, p = 0.083).

Our results suggest that when compared with the criterion method, the QuadScan 4000, Bodystat^® can reliably be used to predict TBW and FFM in patients with PKU. We suggest that due to the portability and non-invasive approach, this method can reliably be used to monitor body composition in the outpatient clinic setting, to further improve the monitoring and assessment of nutritional status in PKU.