Bioelectrical impedance analysis during acute changes of extracellular osmolality in man

Bioelectrical impedance analysis (BIA) is widely used as an inexpensive and noninvasive method to provide estimates of body compartments such as total body water, lean body mass and fat mass. The present study was performed to test the reliability of this method during acute changes of extracellular osmolality in eight young health men.

Hyperosmolal isohydration was achieved by overnight infusions of hypertonic saline solutions (2 and 5% NaCl) and thirsting, and hypoosmolal hyperhydration by drinking of free water and overnight application of desmopressin. The control study (isoosmolality) consisted of oral water ad libitum.

When plasma osmolality and sodium concentrations increased (from 285 +/- 1 to 296 +/- 1 mmol/kg (P<0.001) and from 141.9 +/- 0.7 to 148.3 +/- 0.6 mmol/l (P<0.0001)) and total body water remained unchanged, body impedance decreased and calculated total body water increased from 42.7 +/- 2.7 to 45.6 +/- 2.3 liters (P<0.03). In contrast, during hypoosmolal hyperhydration total body water increased by 1.56 +/- 0.17 kg and plasma osmolality decreased from 285 +/- 1 to 272 +/- 1 mmol/kg (P<0.001) and plasma sodium concentrations from 142 +/- 0.5 to 134.8 +/- 0.4 mmol/l (P<0.0001). In spite of these changes of body water, impedance measurements and calculated total body water remained unchanged. During conditions of isoosmolal isohydration (as demonstrated by unchanged plasma sodium concentrations and osmolality) the measurements by BIA also remained unchanged.

Measurements of total body water using BIA under conditions of unknown hydration status (hyper-, hypo- or isohydration) and unknown osmolality (hyper-, hypo- or isoosmolality) may not be reliable. Therefore bioelectrical impedance analysis is not a suitable bedside method to assess changes of body compartments under unstable hydration status.