Comparison of urea space, deuterium oxide space and body composition in growing pigs

Urea and deuterium oxide (D2O) space were compared by simultaneous infusion into pigs weighing approximately 55 or 90 kg. Urea was cleared from the plasma pool more rapidly than D2O and appeared to equilibrate at a relative concentration which was lower than that of D2O. Consequently, urea and D2O space values were closest when extrapolated to zero time values. Correlations between urea space and D2O space were highest at 15 minutes post infusion (R2 = .75) or between urea space at 15 minutes and D2O at equilibrium (R2 = .86). Results of urea and D2O space measurements were compared with water, lipid and protein content of the carcass. Urea space at 15 minutes and D2O space at 35 minutes most closely approximated total body water while D2O space at 15 minutes was nearly equivalent to empty body water. Overall, D2O space at equilibrium had the highest correlations with carcass values of water, lipid and protein and appears to be preferable to urea space for estimating carcass composition in pigs.