Measuring body composition in preterm neonates near term: bioelectrical impedance provides only few additional information compared to anthropometry alone

Background: Detailed knowledge of body composition in preterm neonates is important in nutritional intervention studies. However, little attention has been paid to test whether bioelectrical impedance analysis (BIA) is a useful bedside method that allow to predict fat-free mass (FFM) for this population.

Objective: To develop prediction equations using BIA and anthropometric measurements for determining FFM in preterm neonates.

Design: FFM of 118 white subjects (51 males and 67 females), mean gestational age of 30.1±3.1 weeks and birth weight of 1.26±0.47kg were measured at a gestational age of 38.6±3.8 weeks and an actual body weight of 2.6±0.54kg using dual-energy X-ray absorptiometry (FFMDXA). Body weight (W), height (Ht) and bioelectric impedance (I) measurements were collected. Multiple regression analysis was performed to develop prediction equations to estimate FFM with impedance index (Ht2/I) (cm2/Ohm), W (kg) as predictor variables. Bootstrap analysis was performed for validating the derived prediction equations.

Results: Correlations between FFMDXA and weight were 0.96, 0.98 and 0.97 in boys, girls and both sexes, respectively. Those between FFMDXA and Ht2/I were: 0.73, 0.81, 0.79. Equations used to predict FFM (kg) were for boys: FFM=0.05 Ht2/I + 0.68 W + 0.40 (R2=0.919), for girls: FFM=0.04 Ht2/I + 0.71 W + 0.29 (R2=0.957) and for both sexes: FFM=0.05 Ht2/I + 0.69 W + 0.31 (P<0.0005).

Conclusions: In preterm neonates, body weight is a stronger predictor of FFM than impedance index. The present study provides a bedside procedure for estimating FFM in white preterm neonates.