Bioelectrical impedance analysis during acute changes of extracellular osmolality in man

Background & Aims: Bioelectrical impedance analysis (BIA) is widely used as an inexpensive and noninvasive method to provide estimates of body compart ments such as total body water, lean body mass and fat mass. The present st udy was performed to test the reliability of this method during acute chang es of extracellular osmolality in eight young health men. Methods: Hyperosmolal isohydration was achieved by overnight infusions of h ypertonic saline solutions (2 and 5% NaCl) and thirsting, and hypoosmolal h yperhydration by drinking of free water and overnight application of desmop ressin. The control study (isoosmolality) consisted of oral water ad libitu m. Results:When plasma osmolality and sodium concentrations increased (from 28 5 +/- 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 imp edance 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 hyperh ydration total body water increased by 1.56 +/- 0.17 kg and plasma osmolali ty decreased from 285 +/- 1 to 272 +/- 1mmol/kg (P < 0.001) and plasma sodi um 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 calculate d total body water remained unchanged. During conditions of isoosmolal isoh ydration las demonstrated by unchanged plasma sodium concentrations and osm olality) the measurements by BIA also remained unchanged. Conclusions: Measurements of total body water using BIA under conditions of unknown hydration status (hyper-, hypo- or isohydration) and unknown osmol ality (hyper-, hypo- or isoosmolality) may not be reliable. Therefore bioel ectrical impedance analysis is not a suitable bedside method to assess chan ges of body compartments under unstable hydration status. (C) 2000 Harcourt Publishers Ltd.