The measurement of whole body water by H2 18O dilution in newborn pigs

Whole body water was measured in newborn piglets by isotope dilution with H2(18)O. The results were compared with those obtained by direct analysis of whole body water made by freeze-drying. When the tracer dose of H2(18)O was given to 30 piglets by intragastric gavage, post-equilibration values measured in urine were highly variable (average equilibration error equalled 9%) and the random error of the technique in predicting body water was 13%. When the dose was given to 49 animals intravenously, the post-equilibration values in urine were less variable (equilibration error equalled 4%), the random error of the technique was 7%, and body water was overestimated by 2%. The random error was reduced to 6% if data from animals in which equilibration errors exceeded 10% were excluded. When 18O enrichment in plasma instead of urine was measured, the technique was less precise (random error 11%). The estimation of body water from H2(18)O overestimates body water by 2%. Isotope dilution with 18O is a safe, accurate and repeatable method for the estimation of whole body water, and is suitable for use in adult and newborn humans.