Technical Notes
20
Body composition measurement by the BIA method.
- Principles of body composition measurement
BIA is a means of measuring body composition – fat mass, predicted muscle mass, etc. – by
measuring bioelectrical impedance in the body. Fat within the body allows almost no electricity to
pass through, while electricity passes rather easily through water, much of which is found in
muscles. The degree of difficulty with which electricity passes through a substance is known as the
electrical resistance, and the percentage of fat and other body constituents can be inferred from
measurements of this resistance.
The TANITA Body Composition Analyzer measures body composition using a constant current
source with a high frequency current (50 kHz, 90 µA). The 8 electrodes are positioned so that electric
current is supplied from the electrodes on the tips of the toes of both feet, and voltage is measured
on the heel of both feet. The current flows into the upper limbs or lower limbs, depending on the
body part(s) to be measured.
- What is dilution method?
In the dilution method, a labeled substance for a known amount is given and the concentration in
equilibrium diffusing evenly is measured to obtain the total amount of the solvent that dilutes the
labeled substance.
To measure the total body water (TBW), deuterium oxide (D2O) is generally used as the labeled
substance. Deuterium oxide uses the overall total body water as dilution space so the total body
water can be obtained. To obtain the extracellular fluid amount, sodium bromide (NaBr) is used as a
labeled substance. Bromine (Br) is said to not enter the inside of cells, and uses extracellular fluid as
the dilution space.
Body composition measurement
results obtained by DXA
(Lunar Co., Ltd; DPX-L)
- What is the DXA method?
DXA was originally designed to measure bone mineral content, but
in the full-body scan mode the body fat percentage, fat mass, and
fat free mass of individual body parts (arms, legs, trunk) can also
be measured. The image below shows one example of body
composition measurement results obtained by DXA.
- Introduction
This equipment provides estimated values for each measured value of body fat percentage, fat mass,
fat-free mass, muscle mass and bone mass by the DXA method, estimated value for the total body
water measured value by the dilution method and estimated value for the visceral fat rating by MRI
method using the Bioelectrical Impedance Analysis (BIA method).
For measurement, a mode must be selected based on body type.
1) Standard (for 5
–
99 years of age)
2) Athletic (for Athletic persons who exercise considerably more than non-athlete)
Making a distinction by body type in the measurement mode produces more reliable body
composition measurements for athletic persons, whose body compositions differ from those of
average persons.
