Estimation of fat-free mass in Asian neonates using bioelectrical impedance analysis

Research in developmental origins of health and Disease (DOHaD) has pointed to the importance of body size and composition at birth which reflect the adaptation of the fetus to its intrauterine environment, and is linked to the risk of subsequent metabolic diseases (1). Neonatal body composition has been a focus for many research studies as it may be a modifiable factor for health and diseases later in life (2). Body composition can be measured by a variety of methods, which differ in accuracy, feasibility, cost and complexity. However, some methods are not suitable for neonates including under-water weighing, stable isotope dilution and dual-energy X-ray absorptiometry (DXA), while others such as magnetic resonance imaging (MRI) are limited by their cost, especially for large epidemiological studies. The measurement of body composition in neonates or infants is challenging, as the distribution of fluid in the body changes rapidly in first few days of life, with 5-10% reduction in body weight attributable to the change in total body water and body composition (3; 4). In addition, highly accurate techniques (e.g., MRI, stable isotope dilution) in neonates and infants generally require high compliance from the subject. This can be difficult to achieve in neonates and infants. Air displacement plethysmography (ADP) for healthy infants has been validated in several studies using the deuterium dilution method for body water and a four-compartment body composition model as the reference methods (5; 6; 7). ADP does not require the infant to be restrained during the 2 minute measurement. However, the infant ADP device is expensive, large and not readily portable and generally not available outside of major hospitals and research facilities. Therefore, an inexpensive, portable and reliable method to measure body composition is clearly required at this age. Bioelectrical impedance analysis (BIA) is a simple, non-invasive and useful method for estimating body composition in epidemiological and clinical research (8). BIA measures impedance of the body to an imperceptible, harmless electric current transmitted through electrodes placed on the hands and the feet. Impedance (Z) of a conductor is proportional to L2/Z. Thus impedance of the whole body can be used to estimate the volume of total body water (TBW) in the aqueous tissue compartments. Fat-free mass (FFM) can be calculated from TBW by using an hydration fraction for FFM, usually assumed to be 0.732 (9). Fat mass (FM) can be estimated by subtracting FFM from body weight. Since BIA was first introduced as a method for estimating body composition, many studies have demonstrated its validity to measure body water in healthy populations of older children and adults with normal fluid distribution (8; 10; 11; 12; 13; 14; 15; 16; 17). However, research on its usefulness for predicting body composition in neonates is limited especially in Asians. Although a few small studies have reported the use of BIA in infants, most did not compare the performance of equations developed from BIA neither with that of simple anthropometry nor did they use an independent validation group (18; 19; 20; 21). In addition, the studies were on infants requiring intensive care or low birth weight infants in whom the body composition profile may be different from that in healthy infants. Lingwood et al developed prediction equations for FFM based on BIA in a cohort of 77 healthy infants in the first few months of life and reported that the contribution of the BIA parameter in their model was not statistically significant and did not improve the prediction over that using weight alone until their study infants reached 3 months of age (22). In a recent study on predicting FFM using BIA reference to that of ADP also suggested the potential use of BIA in infants the subjects used to develop the prediction equation were of mixed age range from 0-6 months (23). These studies on predicting FFM in infants using impedance from BIA were conducted in Western population thus may not be necessarily applicable to the Asian population. A prediction equation for FFM using BIA in Asian infants would be a valuable tool especially in large cohorts or longitudinal cohort studies. Our aims were to develop a prediction equation for FFM during the early neonatal period based on BIA and to validate that prediction using the ADP device, PEAPOD infant body composition system in Asian neonates. The applicability of this equation will also be tested in the independent validation group in this cohort as well as in an independent Western cohort of children from a study conducted at the University of Queensland.