BIA is based on the underlying principle that resistance or impedance to the flow of an electrical current through the body is dependent on three variables: the length of the conductive path, the volume of the conductive material, and
the resistivity of the conductive material. In the human system, only body water, with its dissolved electrolytes, will conduct a current. Hence, using the assumption that the resistivity of the conductive material is constant, and estimating the length of the conductive path from an individual’s height, total body water can be estimated by measuring impedance to the flow of a small current. Assuming that total body water constitutes a fixed percentage of lean mass (usually 73%), body composition can be estimated. Specific prediction equations have been developed to evaluate body composition from height, weight, and impedance
Impedance, resistance and reactance
Impedance is a measure of how a current is slowed or stopped as it passes through a material. The impedance is affected by two factors, resistance and reactance.
Resistance is a measure of the amount of electoral current a substance will stop. Adipose tissue is about 80% fat. Fat is an excellent resistor. Currents flow much better through any body compartment high in water content, including blood, extracellular fluid and muscle tissue.
Reactance is the measure of a material's ability to slow a current. Cell membranes, for example, can store a charge for a short period of time, thus slowing the current. The slowed current is said to "lag" behind the rest of the current. Cell membranes are able to act as capacitors.
Whether a cell membrane acts as a capacitor or a resistor is dependent upon the frequency of the current applied. A low frequency current is stopped by cell membranes. At this point, the membranes are resistors, as no current is conducted through them.
Therefore, at low frequencies below 50 kHz, any current conducted through a body is passing through the extracellular fluids only. This allows the measurement of only extra cellular fluid.
Current with a frequency higher than 50kHz is able to pass through cell membranes. This allows the measurement of impedance of substances inside of cells, as well as outside of cells. Such currents are essential in effectively assessing body composition.
Several factors can influence the impedance:
- Hydration (overhydration increase conductivity and reduce impedance, giving a low percent body fat reading by BIA). Dehydration will elevate the reading for body fat percentage.
- Distribution of water: After a subject has lying down for more than a few minus, their fluids tend to "settle." Settling for more than 5-10 may produce false readings.
- Orientation of tissues: tissues arranged perpendicular to the current will slow the flow as more membranes will have to be crossed. Parallel tissue and current reduce the impedance. Run BIA testing at least 8-12 hours after exercise or drinking alcohol at least two hours after drinking or eating, within 5 minutes of lying down.
Consultation with naturopath and herbalist, Erin Collins or Michael Thomsen, including the Body Composition Analysis
Or contact reception by email or telephone 03 6223 4842