Body composition by doubly labelled water (dlw) and dual energy x-ray absorptiometry (dxa) in tiny prematuse infants (PI)

To determine if changes in soft tissue composition can be detected in tiny PI, we conducted serial measures of total body water (TBW) and fat free mass (FFM) by DLW in 37 PI with bronchopulmonary dysplasia (BPD) (birth weight 867 ±195 g; post-conceptional age (PCA) 26±1.4 wk) in hospital at 34±1 and 38±1 wk PCA, and by DLW and DXA (Hologic QDR 1000/W) at 0.9±0.5 mo (1 mo) and 3.2±0.4 mo (3 mo) corrected age. The table shows data (mean±SD) for %weight as TBW converted into %FFM and %fat using lean tissue hydration (LTH) factors for the reference fetus/infant (RF/I) (Zicglo, 1976; Fomon. 1982) compared to DXA. Like symbols within rows and columns are different, p < 0.05. DLW DXA BEä Age n Weightfkg) %TBW %FFM FAT %FFM %FAT %FFM 34 wk 27 1.51±0.2 78±8"" 91±7 9±7 - - 92 38 wk 18 2.20±0.3 73±8" 87±9" 13±9 - - 91 1 mo 21 3.24±0.4 67±T 84±9' 16±9 76±7- 24±6 84 3 mo 21 4.95±0.7 64±7" 80±9" 20±9 73±8b 27±8 76 %TBW declined from 34 wk to 1 mo. %FFM and fat changed gradually; DLWderived values were similar to RF/I values but differed from DXA values. %FFM by DLW was > 100% for 19% of PI at 34 wk and 11 % of PI at 38 wk, indicating limits of the method. Excess extracellular water due to BPD may have caused an over-estimation of FFM, or body fat was extremely low. Discrepancy in %FFM between DLW and DXA was partially attributed to different LTH factors. Using the factor 0.833 adopted by DXA, XFFM by DLW was 81 ±8.5% at 1 mo and 77±8.4% at 3 mo. Values of %FFM by DLW and DXA were correlated at 1 mo (r=0.67, p<0.001) and 3 mo (r=0.45, p<0.05). DLW allows detection of changes in soft tissue in most infants too small to be measured accurately by DXA (Bmntoo et al, AJCN 1993;58:839). Comparison between studies of body composition io tiny infants by DLW and DXA should be done with caution. (Supported by MOH).