Estimation of neonatal body composition: Isotope dilution versus total-body electrical conductivity

The objective of the study is to evaluate neonatal body composition determined by the isotope dilution method compared with the total-body electrical conductivity (TOBEC) method. An oral dose of O-18- and H-2-labeled water was given to 40 healthy term newborns, and 26 infants successfully completed the protocol. The isotope concentrations in urine samples were measured by gas-isotope ratio mass spectrometry. Fat and percent body fat (%F) estimated by isotope dilution were correlated with fat and %F estimated. Total body water measured by isotope dilution of O-18 and H-2 was 2.44 +/- 0.36 and 2.49 +/- 0.35 kg, respectively (p < 0.001). Fat and %F estimated by O-18, H-2, and TOBEC were 0.48 +/- 0.20 kg and 13.7 +/- 5.1%, 0.48 +/- 0.19 kg and 13.7 +/- 4.6%, and 0.38 +/- 0.21 kg and 10.9 +/- 5.0%, respectively. Neither fat nor %F estimates by isotope dilution showed good correlation with the TOBEC estimates (r = 0.22-0.57). Because of the wide variation of water content in individuals, the isotope dilution method may not be the optimal way of assessing body composition, and specifically body fat, in newborns.