Body Composition Analysis
What is body composition?
Body composition is simply the amount of lean body mass and body fat that makes up total body weight. The lean body mass (LBM) includes the bones, muscles, water, connective and organ tissues. Body fat includes both essential.
DEXA (Dual Energy X-Ray Absorptiometry)
This method is generally considered the gold standard because of its reliability, precision and the fact that is based on three body components (fat,muscle and bone) as opposed to just two (fat and muscle) as in most other methods, including hydrostatic weighing. It allows fat distribution throughout the entire body to be read in a single scan. The equipment used is very expensive and requires the subject to lie perfectly still for 10 - 20 minutes while the scan is taken. DEXA is found mainly in research facilities.
Hydrostatic Weighing or Densiometry ("Dunk Tank")
Done correctly, this method is also accurate and often gives repeatable results. It too is considered a gold standard in body composition analysis. However, the test is somewhat subjective because it relies on the subject's ability to expel all oxygen from their lungs while submerged in a tank of water. Oxygen remaining in the lungs will skew the results. As well as being considerably inconvenient to the subject, the 'tank' is also expensive, depending on the type of equipment used and the underwater facility. In clinical settings, this procedure is repeated a number of times and an average is taken. Because of the expense, lengthy testing process and physical burden to the subject, this method is more suitable for research studies.
Conventional BIA
Bioelectrical Impedance Analysis uses a safe electrical signal to measure body impedance. The signal is conducted through the water contained in the body. Lean tissue has more water than fat tissue and allows the signal to pass easily. The fat tissue resists the signal, which is termed impedance.The single impedance measurement reflects the degree of resistance to the flow of the current in the body, water being a good conductor and fat a bad conductor.
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 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 measurments.
Skinfold Calliper
This is a highly subjective method of measuring body fat and relies on a trained and certified technician testing multiple sites on the body. Although the equipment is inexpensive and portable, this test depends on the skills of the attending technician and the quality of device used; therefore, the method may not be suitable for trending applications or obtaining repeatable results. The use of callipers is invasive to many people, leaving them uncomfortable with the necessary multiple-site skinfold pinches. In addition, the more obese the subject, the more difficult it becomes to "pinch" the skin correctly. Despite the contention that subcutaneous fat makes up about half of the total body fat there is no data to support this statement. Furthermore, because there is little information on the distribution of fat in the body of the population at large, the validity of using skinfold equations to predict body composition is restricted to populations from whom these equations were derived.
Body Mass Index
BMI is probably the most widely known method of determining whether a person is the correct weight for their height. BMI involves a simple calculation - a person's weight in kg is divided by their height in m². Although a simple and inexpensive method to use, it does not distinguish between lean body mass and fat mass and only has a modest correlation with actual body fat levels. The BMI calculation does not take different body types into account and can result in misclassifications (such as in the case of professional athletes and bodybuilders.)
NIR (Near Infra-Red)
A fibre optic probe measures tissue composition at various sites on the body. This method has become popular because it is simple, fast and non-invasive and the equipment is relatively inexpensive. However, studies have produced mixed results and a high degree of error has occurred with very lean and very obese people. Numerous sources report that more research is needed to substantiate this method.
