This indirect method for measuring body composition is based on the conduction of an alternating electrical current applied to the human body. Current passes easily through tissues containing a lot of water and electrolytes like blood and muscles. Fat mass, air and bone are harder to pass. So, the larger the fat free mass, the bigger the capacity of the body to lead the current. By using a BIA measurement, the water compartments of the body can be determined. The Intracellular Water (ICW) and Extracellular Water (ECW) together form the Total Body Water (TBW).
About the method
During the measurement, electrodes are placed on hands and feet. By these electrodes an alternating current with different frequencies will be sent through the body. The method is not invasive, cheap and easily applicable at the patient’s bed. Changes in ICW can be monitored, which are a measure for body cell mass and a measure for malnutrition. A disadvantage of this method is that it is not validated in sick or elderly people. Probably there are some disturbing factors like changes in membrane qualities and hydration. The measurements can be unreliable.There are different ways of calculating bodycomposition by using bio-impedance analysis.
Single-frequency BIA (SF-BIA)
With this method the resistance (R) can be measured at one frequency (50 kHz). With a prediction equation based on the resistance index (L2/R50) total body water (TBW) or fat free mass (FFM) can be calculated. For healthy people this measurement is adequate. For sick people, however, especially when there is a imbalance in fluid compartments, this measurement is not advised. With a SF-BIA measurement it is not possible to measure intracellular (ICW) and extracellular water (ECW) separately. Clinical depletion is characterised by a decrease of ICW, often in combination with ECW expansion. Only with the data from TBW, it is not possible to determine the loss of body cell mass. Nutritional Assessment in a clinical setting is required for quantification of both fluid compartments.
Multi-frequency BIA (MF-BIA)
This method measures the impedance at different frequencies. At low frequencies the current will only go through ECW because the cell membranes and contact area between the tissues act like condensators. At high frequencies this quality does not work and the current will go through ECW and ICW. Resistance measured at low and high frequencies are used to calculate ECW and TBW based on the resistance index at specific frequencies.
Bioelectrical impedance spectroscopy (BIS)
This is a more theoretical approach. It uses extrapolated resistance values at zero and infinite frequency (Rinf) by use of the Cole-Cole model. The model assumes a parallel arrangement of the ECW and ICW. This method provides resistance values for intracellular water (Ricw) and extracellular water (Recw). These values can be used to calculate fluid compartments based on regression equations. It is even possible to use equations based on physical laws, the mixture equations. Theoretically these mixture equations are specifically less popular than the regression equations. For clinical use this method is very attractive. At this moment it’s not clear whether the mixture equations is universally applicable. Dieticians of the Maastricht University Medical Centre (MUMC+) use the BIS method.
How bio-impedance works, is explained on this website