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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
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.
BMI is a measure to check if your weight is healthy for your height. For most adults, an ideal BMI ranges from 18.5 to 24.9.