The
Science
Body Composition Testing
Having tools to accurately access body composition,
and body fat in particular, has taken on greater importance
with the increased prevalence of obesity, development
of guidelines that recommend increasingly more aggressive
therapies for severe obesity, and to assess lipodystrophy
in AIDS and with AIDS therapies.
At the same time there is a trend to greater awareness
of the benefits of body fitness with many new centres
opening.
An ideal diagnostic test of body fat would precisely
and accurately quantify and localise adipose tissue
stores. For now, we cannot determine where in the body
the fat is – only the total average body fat.
The test would be:
- Easy and quick to perform
- Have low risk
- Low cost
- High predictive value
The measurement would be sensitive enough to detect
clinically important changes in body fat that occur
with weight loss and weight gain. The traditional measures
of body fat are based on the premise that total body
weight is the sum of two compartments; fat mass and
fat-free mass (FFM). Direct measures of FFM, total body
water, or body density are used along with total body
weight to estimate the absolute and relative amount
of body fat. In the last decade, more sophisticated
methods have been developed to separate total body weight
into four compartments: fat mass, body cell mass, extracellular
water, and skeletal mass
 Bio-Electrical
Impedance Analysis
Bio-electrical impedance analysis (BIA) measures the impedance
or opposition to the flow of an electric current through
the body fluids contained mainly in the lean and fat tissue.
Impedance is low in lean tissue, where intracellular and
extracelllular fluid and electrolytes are primarily contained,
but high in fat tissue. Impedance is thus proportional
to body water volume (TBW). In practice, a small constant
current, typically 400 uA at a fixed frequency, usually
50 kHz, is passed between electrodes spanning the body
and the voltage drop between electrodes provides a measure
of impedance. Prediction equations, previously generated
by correlating impedance measures against an independent
estimate of TBW, may be used subsequently to convert a
measured impedance to a corresponding estimate of TBW.
Lean body mass is then calculated from this estimate using
an assumed hydration fraction for lean tissue (NOTE: Bodystat
is unique in using their own regression equation for this
calculation and not the assumed 73.2% used by other manufacturers).
Fat mass is calculated as the difference between body
weight and lean body mass.
The impedance of a biological tissue comprises two
components, the resistance and the reactance. The conductive
characteristics of body fluids provide the resistive
component, whereas the cell membranes, acting as imperfect
capacitors, contribute a frequency-dependent reactive
component. By measuring the impedance at 50 kHz and
200 kHz and by applying predictive equations, it is
possible to estimate both extra-cellular water (ECW)
and TBW respectively and by deduction, intra-cellular
water (ICW). ECW can be related to extra-cellular mass
(ECM) and ICW to body cell mass (BCM).
Validation
of the Technology
Bio-impedance analysis has been correlated most frequently
against either hydrostatic weighing or isotope dilution
as the "gold standard". Most commercial machines
are supplied with proprietary prediction equations the
details of which are hidden from the user within the software
of the machine. In addition, many prediction algorithms
have been published in the bio-medical press. All prediction
equations include height, as a surrogate measure for the
inter-electrode distance, but may also include, weight,
sex or age as variables. Many studies have been undertaken
not only to validate particular prediction equations but
also to identify other variables which may improve the
quality of the predictor. The overall precision of any
given prediction equation is the sum of the precisions
associated with each independent variable.
Advantages of BIA
- Measures fat-free mass and calculates fat mass
- Some models include measurement or calculation of
body cell mass, total body water, intracellular water
extracellular water and 3rd Space Water
- Safe, non-invasive, fast, and inexpensive
- Lightweight, portable devices which can be used
at the bedside
- May be useful to assess total body water in individuals
with altered metabolic function
- Excellent consistency for repeated measurements.
TOP
|
