Introduction, Body impedance measurement methods

Technical Notes

Reliability of body composition measurements with an 8-electrode BIA

Introduction

This device calculates body fat percentage, fat mass, fat free mass, and predicted muscle mass on the basis of data obtained by Dual Energy X-ray Absorptiometry (DXA) using Bioelectrical Impedance Analysis (BIA). One of two settings must be selected when taking measurements:

1Standard (for persons 7 ~ 99 years of age)

2Athlete (sportsmen/women who exercise considerably more than non-athletes)

Making a distinction by body type in the measurement mode produces more reliable body composition measurements for athletes, whose body compositions differ from those of average persons.

Principles of body composition measurement via 8-electrode BIA

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, 500μA). The 8 electrodes are positioned so that electric current is supplied from the electrodes on the tips of the toes of both feet and the fingertips of both hands, and voltage is measured on the heel of both feet and the thenar side of both hands. The current flows into the upper limbs or lower limbs, depending on the body part(s) to be measured.

Body impedance measurement methods

In the conventional 4-electrode method, current is supplied from the electrodes at the tips of the toes on both feet, with the voltage measured on the heel. This current flows from one lower limb through the lower abdomen and then into the other lower limb, and the bioelectrical impedance (hereinafter, “impedance,”) thus measured is the impedance between the two feet. In the newly developed 8- electrode method, however, current is supplied from the tips of the toes of both feet and the fingertips of both hands, and the voltage is measured on the heel of both feet and the thenar side of both hands. This method allows five different impedance measurements to be made – whole body, right leg, left leg, right arm, and left arm – by switching the part of the body in which the current is flowing and the location where the voltage is measured. When measuring impedance in the right leg, for instance, the current flows between the right hand and foot, and the voltage between both feet is measured.

For reference, the impedance measurement methods for the respective body parts are shown in the diagram below.

Current

Voltage

Whole body

Right leg

Left leg

Right arm

Left arm

Impedance measurement methods for the respective body parts

Reliability of body composition analysis via 8-electrode BIA

In deriving the body fat percentage, fat mass, and fat free mass for the whole body, the Body Composition Analyzer uses data acquired by DXA* from both Japanese and Western subjects as well as a regression formula derived through repeated regression analysis using height, weight, age, and impedance between right hand and foot as variables. Measurements of body fat percentage, fat free mass, fat mass, and predicted muscle mass for specific body parts also use a regression formula for each body part derived from repeated regression analysis using height, weight, age, and impedance for individual body parts (right arm, left arm, right leg, left leg, trunk) as variables, based on data acquired through DXA.

A high degree of correlation has been found to exist between the figures for body fat percentage, fat mass, and fat free mass for individual parts and for the entire body calculated with this predictive formula and the figures obtained by DXA, and the results are thus highly reproducible.

Body composition measurements attained by the 8-electrode method are therefore extremely reliable.

Measurement of body composition using DXA

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.

Body composition measurement results obtained by DXA (Lunar Co., Ltd; DPX-L)