Total body water measurement by tritiated water in growing animals.

Data on 177 beagles were used to examine the hypothesis that the volume distribution of tritiated water (HTO) is significantly larger in some species of young growing animals than the volume of total body water (TBW) measured by desiccation in more mature animals. The beagles were divided into three groups. Group 1 consisted of 169 animals which ranged in age from 0 to 365 days and in weight from 0.25 to 13.5 kg. Group 2 consisted of 42 animals randomly selected from Group 1 over the entire age and weight range. Group 3 consisted of eight beagles which ranged in age from 2 to 5 years and in weight from 10.1 to 16.3 kg. Total body water was measured in Group 1 by HTO, in Group 2 by HTO and desiccation, and in Group 3 by HTO. The mean percentage difference between the measurements of TBW in Groups 1 and 2 increased between body weights of 0.2 to 3.4 kg (2 months old): in Group 1 from 14 to 28% and in Group 2 from 18 to 32%. From body weights of 4 to 13 kg, there was a progressive decrease in the mean difference between the two measures of TBW: in Group 1 from 28 to 4% and in Group 2 from 32 to 5%. Group 3 animals were a separate population statistically from other groups of beagles. At body weights of 12 and 13 kg, which overlapped with those of Group 2 dogs, there was no significant difference between the two measures of TBW. The present data support the proposed hypothesis.