The intracellular fluid compartment is smaller than commonly believed when measured by whole-body bioimpedance

To report our data on the total body water (TBW), intracellular volume (ICV), extracellular volume (ECV), and fat-free mass (FFM) from studies using whole-body bioimpedance (BIA) with the aim of contrasting them to commonly cited reference values.

Data were retrospectively retrieved from three single-center studies of adult healthy male volunteers and one study of women scheduled for abdominal hysterectomy where multifrequency BIA had been applied to obtain measurements of TBW, ICV, ECV, and FFM.

Based on measurements performed in 44 males, the TBW, ICV, ECV, and FFM represented 49.1 (4.9)%, 23.32 (3.1)%, 25.8 (2.2)%, and 67.4 (7.4)% of the BW, respectively (mean, SD). In 15 females, these volumes were 40.4 (4.5)%, 18.0 (2.1)%, 22.4 (2.6)%, and 55.6 (6.1)% per kg BW, respectively. The deviation of these measurements from the reference values increased linearly with body weight and age.

Body fluid volumes indicated by BIA showed that TBW amounted to 80% of the reference volume, which is 60% per kg BW in adult males. The ratio between the ICV and the ECV was approximately 1:1, while this ratio is traditionally reported to be 2:1.