Experimental Methods

Bioelectrical impedance (BIA) is based on differences in conductivity between bodily tissues (A). Water-containing tissues have low impedance since they are highly conductive because of the presence of electrolytes. Fatty tissues have greater resistance, and cell membranes function as electrical condensers. Since electricity of different frequencies flows preferentially in different compartments, the measurement of impedance, combined with phase displacement, permits conclusions about the three compartments: fatty tissues, lean body mass, and water. BIA is considered to produce reliable and well-reproducible values for healthy people. The simplicity of the method's use is advantageous: the four stick-on electrodes don't bother the patient. However, changes in plasma electrolytes, use of diuretics, or dextrose infusions can greatly disturb the results.

Measurements of conductivity, like the BIA, are based on the different conductivities of different tissues. Since the person to be measured has to be placed inside a magnetic coil, the method is not practical as a routine. Body composition can also be determined through various isotope dilution methods (B). These are used to determine just one compartment—total body water. The method is based on the assumption that fatty tissue is waterfree and hence cannot take up any electrolytes. By additionally defining that lean body mass has a constant 73.2% degree of hydration, all three compartments can be determined through appropriate calculations. The most commonly used isotopes are deuterium oxide (2H2O), tritium-labeled water (3H2O), and the potassium isotopes 42K and 43K. The respective isotope is injected, losses are measured in urine, blood levels are measured after an equilibration phase, and the resulting dilution factor is used to determine total body water. Measurements of total body potassium using the 40K method differ somewhat. The isotope occurs naturally at a level of 0.0118% of body potassium and can be determined using a whole body counter. An assumption is made that potassium is found in lean body mass at a fixed concentration of 8 mmol/kg. Just as with the injected isotopes, this makes it possible to calculate all three compartments. Isotope measurements are also subject to errors. The assumptions mentioned do not apply to pathological conditions like sepsis, stress, malnutrition, or obesity.

Underwater weighing (C) is considered the standard for determining body fat. The subject has to be submerged under water. The displaced water in the vessel corresponds to the body volume. If the body weight is known, the density (D) (in g/cm3) can be calculated. Since body water has a constant density of 1.0 g/ cm3, and the density of lean body mass and fat are also near constant, D can be used to estimate the respective proportions of the three compartments. Any change in density is interpreted primarily as a change in body fat content.

- A. Impedance Measurement of a Body Section with Tetrapolar Electrodes

Current source electrodes 1 kHz - 100 kHz

Detection electrodes

- A. Impedance Measurement of a Body Section with Tetrapolar Electrodes

Current source electrodes 1 kHz - 100 kHz

Detection electrodes

Isotope Dilution Method

- B. Isotope Dilution Methods -

known, constant concentration and constant hydration status

Fatty tissue:

no exchange

1. Isotope with known concentration

Lean body mass

Lean body mass

Dilution factor Water content

2. Isotope concentration in blood plasma

- C. Underwater weighing

Displaced water volume = body volume Body density (D) =

Body mass Body volume

Dwater=1.0g/cm3

lean body

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