When
necessar
y
(Technical
notes)
When
necessar
y
(Technical
notes)
GB
GB
49
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.
- What is the DXA method?
- What is the visceral fat?
48
- Principles of body composition measurement
Technical notes
Body composition measurement by the BIA method.
Introduction
This equipment provides estimated values for each measured value of body fat percentage, fat mass,
fatfree 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.
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 (50kHz, 90µA). 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.
Visceral adipose tissue (VAT) is fat that accumulates in the abdominal cavity and around internal organs.
VAT is said to be more likely to cause lifestyle-related diseases than subcutaneous adipose tissue (SCAT).
Accordingly, knowing and periodically checking the VAT accumulation risk serves as an important guide
in the prevention of lifestyle-related diseases.
Tanita has developed the technology for measuring the VAT accumulation risk through bioelectrical
impedance analysis (BIA) in comparison with image analysis applied to magnetic resonance imaging
(MRI), in addition to the established technology for measuring the percent of body fat. The VAT
accumulation risk is calculated by estimating the VAT area by the BIA method on the basis of MRI image
processing. This method has a higher correlation than the estimation of the VAT accumulation risk based
on BMI or abdominal circumference (waist circumference), allowing estimation that corresponds more
precisely to individuals.
* The VAT area by MRI is calculated by carrying out an image processing of the cross section of the lumber
vertebra L4-L5 regions.
(Fig. 1 - Fig. 3: Research results by N. Y. Columbia University and Jikei University Published by the North
American Association for the Study of Obesity [NAASO] in 2004.)
- 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 (D
2
O) 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)
