The measurement of whole body water by H₂¹⁸O dilution in newborn pigs

Whole body water was measured in newborn piglets by isotope dilution with H₂¹⁸O. The results were compared with those obtained by direct analysis of whole body water made by freeze-drying. When the tracer dose of H₂¹⁸O 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 intravenouly, 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 16% 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 H₂¹⁸O 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.