Estimation of total body water in harbor seals: How useful is bioelectrical impedance analysis?

We evaluated bioelectrical impedance analysis (BIA) as a means of rapidly and inexpensively estimating total body water (TBW) of harbor seals (Phoca vitulina). Deuterium oxide dilution was used to estimate TBW in 17 adult females and 16 of their pups between birth and late lactation. Isotope dilution was also used to determine TBW in 12 adult males early and 10 of these males late in the breeding season. At the same time, resistance (Rs) and reactance (Xc) measurements were taken using a tetrapolar, impedance plethysmograph (Model 101A, RJL Systems). Seals were sedated with diazepam prior to taking BIA measurements. Within-day duplicate Rs measurements on pups and adults, taken 2-240 min apart, differed by an average of 3.0% ± 1.4% (n = 42, CV = 102%). Movement of the seal during BIA measurements caused variability in both Rs and Xc values. BIA measurements were generally poor predictors of TBW. Rs was significantly correlated with TBW in pups only (Rs = 0.93, P = 0.001, n = 11). Bioelectrical conductor volume (length2/Rs) was significantly correlated with TBW only in adult females (Rs = 0.63, P = 0.02, n = 14). We conclude that BIA is not a reliable method of estimating TBW in wild harbor seals.