2 B. Body Fluid Compartments and Investigational ... - Data-input.de

2
B. Body Fluid Compartments and Investigational Methods for Determining
Body Composition
The determination of bodily composition is possible
using different physical and chemical methods of
investigation. The principles of measurement and
1-Compartment Model 2-Compartment Model 3-Compartment Model 3-Compartment Model
with ECW and ICW
Weight
Measurement methods:
1. One-compartment measurement
The one-compartment model as the most basic
parameter corresponds to measurement of body
weight. An exactly determinable, very important
measurement is obtained, but if the weight changes
it is not possible to make a judgement about the
cause (change of water / fat?).
2. Two-compartment measurement
In this model the body is divided into fat and fat-free
mass. This division is historical and goes back to the
method of weighing under water (densitometry).
The density of pure fat (Total Body Fat, TBF) is 0.9
g/cm3, the density of the fat-free mass (Fat Free
Mass, FFM) is around 1.1 g/cm3. Densitometry is suitable
mainly for research purposes because of the
technical requirements. Anthropometry, infra-red
Fat Fat
Fat
Lean body mass
Weighing scales Anthropometry
Infra-red interactance
Non-phase sensitive BIA
DEXA
Fig. B.1 Schematic representation of the compartment models
the magnitude of results of the most important
methods are summarized in the following chapter.
ECM ECM
ECW
BCM BCM
Phase sensitive BIA
Total body potassium
Dilution methods
ECM = Extra-cellular Mass, BCM = Body Cell Mass, ECW = Extra-cellular Water, ICW = Intra-cellular Water
ICW
Phase sensitive
multi-frequency BIA
Total body potassium
Dilution methods
IVNAA
Fig. B.2 Densitometer of the University of Ulm
interactance and non-phase sensitive BIA measurement
are similar but not identical methods for twocompartment
measurement:

a. Anthropometry
The thickness of skin folds of
defined parts of the body is measured
with the help of a callipers
(mostly on the biceps and triceps, as
well as sub-scapular and supra-iliac). Conclusions
can be made about body fat from the thickness of
the skin folds.
b. Infra-red interactance
This method uses harmless infra-red light in order to
measure the percentage of total body fat on the
biceps of the dominant arm with the help of a near
infra-red probe.
Fig. B.3 Infra-red interactance measurement (Futrex)
c. Non-phase sensitive BIA measurement
This method measures the total resistance of the
body (impedance Z). By calculating body water, the
compartments for body fat and lean body mass can
be obtained. This does not give a determination of
the phase angle and, as such, a subdivision of impedance
into water and cellular resistance, so that no
judgement can be made about the body cell mass
BCM or the extra-cellular mass ECM with the nonphase
sensitive BIA.
The compartments measured using the last three
methods are known as body fat (BF) and as lean body
mass (LBM), in comparison to the fat-free mass (FFM)
which is obtained in the case of densitometry. The
difference is that densitometry measures pure
triglycerides, whereas body fat also contains 15-18%
water and approx. 2% protein.
3. Three-compartment measurement
The 3-compartment model arises by subdividing the
lean body mass into the body cell mass BCM and the
extra-cellular mass ECM.
The BCM is the sum of all aerobic oxidizing, metabolically
active cells, in other words the muscles,
internal organs and the CNS. The interstitial and
trans-cellular spaces are part of the extra-cellular
mass ECM, as well as the skeleton and connective tissue.
Large displacements between the BCM and ECM
can arise without changes to the bodyweight and
lean body mass. Measurements in the 3-compartment
model are possible with the following simple
methods:
a. Phase sensitive BIA measurement
The phase sensitive technique of measuring allows
the impedance Z to be differentiated into its two
components resistance R (water resistance) and
reactance Xc (cell resistance), so that a differentiation
can be made between body cell mass and extracellular
mass.
Fig. B.4 Phase sensitive mono-frequency device
COMPARTMENTS
3

4
b. Phase sensitive multi-frequency measurement
A further step towards making a more exact diagnosis
of body composition and nutritional status can be
achieved using phase sensitive multi-frequency
analysis. By using an additional determination of
resistance at low frequency (1 or 5 kHz), it is possible
to achieve a subdivision of the total body water TBW
into intra-cellular and extra-cellular water.
Deviations in mass of BCM and ECM can thus be estimated
differentially with regard to anabolic/catabolic
processes or shifts in water. Phase sensitive multifrequency
analysis is indicated in particular in
patients with disturbances in water balance, e.g. in
heart and kidney failure, oedema, patients undergoing
treatment with parenteral nutritional therapy
and intensive care patients.
Fig. B.5 Phase sensitive multi-frequency device
4. Other investigational methods for
determining body composition
a. Total body potassium
Fat is potassium-free and bones contain only traces of
potassium. For this reason, practically the entire content
of potassium in the body (98%) is to be found in
the cells of the BCM. The natural radioactive isotope
K40 is present wherever potassium is found at a level
of 0.012%. By measuring the isotope K40 it is possible
to draw conclusions about total body potassium and
thus the BCM. The measurement of K40 is done in a
whole body counting chamber.
b. DEXA (Dual Energy X-ray Absorptiometry)
DEXA is carried out using a special x-ray apparatus at
a very low radiation dosage. Two beams of photon
Fig. B.7 DEXA apparatus (Hologic Inc., USA)
radiation with different energy levels are used,
which gives the method its name. DEXA can be used
to differentiate between bone mass, fat mass and
fat-free mass.
c. Dilution methods
The dilution techniques are used mostly to determine
total body water TBW. Deuterated water is
used as a non-radioactive tracer; it can be given orally
or parenterally. About 2 hours after the dose, the
concentration of the tracer is determined in the
urine and blood using gas chromatography or mass
spectrometry, allowing a conclusion to be drawn
about the total body water. The extra-cellular fluid
can be determined selectively using bromide or sulphate.
d. IVNAA (In-vivo Neutron Activation Analysis) This is
probably the most expensive of all methods for
determining the body composition; but it also provides
the most differentiated assessment. A beam of
neutrons aimed at the body induces the emission of
gamma radiation with a characteristic spectrum. In
this way, single elements can be determined, such as
total body potassium or calcium. The corresponding
compartment can be calculated from the proportion
of these elements.
Further imaging processes for determining the composition
of the body are mentioned below:
Nuclear magnetic resonance technique (NMR)
Computer tomography (CT)
Magnet resonance tomography (MRT)
Photon absorption measurement
Fig. B.6 Potassium chamber of the University of Texas