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The measurement of body composition

The knowledge of human body composition is involved with the technique of body measurement.

There are many ways to determine body composition, including:

-Hydrodensitometry(Underwater Weighing)
-Air Displacement Plethysmography
-Near-infrared interactance (NIR)
-Anthropometry (calipers)
-Dual-Energy X-Ray Absorptiometry (DXA)
-Bioelectrical Impedance Analysis (BIA)
-Computed Tomography (CT)
-Magnetic Resonance Imaging (MRI)

Hydrodensitometry (Underwater Weighing) is the traditional laboratory method for body composition measurement and regarded as classic "golden standard", or criterion-reference method for testing body composition. This technique is derived from Archimedes’s principle, which states that the density of the body is equal to the mass of the body divided by its volume. Simply put, muscle and bone are denser than water and there fore sink when immersed in water, whereas, fat tissue is less dense than water and floats. Therefore, a person with more LBM for the same total body mass weights more in water and has a higher body density and lower percentage body fat. Although hydrodensitometry is classic, it is time consuming, requires large equipment, amount of technical skill and substantial subject participation, including underwater submersion. So it’s not applicable in practical measurement.

The Air Displacement method (BOD POD) is similar to water displacement in that body volume may be measured by air displacement. One commercial system, Bod Pod, uses a dual-chamber plethysmograph that measures body volume by changes in air pressure within a closed two-compartment chamber. Once body density has been determined, percent body fat is calculated. This is often used as an alternative technique to hydrostatic weighing for individual that may experience difficulty with the under water weighing procedure. However, this equipment is expensive and is generally only accessible in research and athletic training facilities. In addition, there is a standard error of ±2.2 to 3.7% and more research data is needed.

Anthropometry (Skinfold calipers method) is a popular method of measuring body composition, measures subcutaneous fat, fat directly beneath the skin. This analysis is based on the principle that subcutaneous fat is directly related to one’s total fat in the body. It is important to remember that the amount of subcutaneous fat varies with gender, age , ethnicity, and other factors. General as well as population specific formulas are available, therefore it is important to make sure the proper formula is being used. There is ±3.5% standard error associated with this technique when using the appropriate formulas. Factors affecting skinfold measurement results include hydration, technician error, and recent exercise.

Near-infrared interactance (NIR) is based on the principles of light absorption and reflection. Using near-infrared spectroscopy information is collected about the chemical composition of the body. For body composition analysis purposes a light wand device is positioned on a body part, and the absorption of the infrared beam is measured. While the method is very portable and easy for both tester and subject much more research is need before NIR should be used as an acceptable means of body composition testing. Research shows an unacceptable standard error that varies more than ±5%.

Dual-Energy X-Ray Absorptiometry (DXA), typically used to measure bone density, is also used to measure body composition, separating bone nimeral content, fat free mass and fat. Unlike other methods, body fat  results from DXA scan can provide site-specific fat distribution. DXA use low level radiation, its machine is big and expensive and usually is accessible only in research and medical sttings. There is a standard error of ±1.8% and more research is needed.

Computed-Graphing methods including MRI,CT and ultrasound. These methods are popular and wide available in hospitals and clinics. But they are mostly used as other medical check utility. The MRI method may be the most precise one and give more information, it’s the modern “gold standard” in medical use. But scanning of a whole body consume a lot of time and money, it rarely used mainly for body composition measurement. To measure body compositions using these methods require special technical skills and computing software. So they are not popular in this field.

can calculate the volume of body precisely and need less subject participation. But it’s machine is big and expensive. The MRI method may be the most precise one and give more information, it’s the modern “gold standard” in medical use. But scanning of a whole body consume a lot of time and money, it rarely used mainly for body composition measurement. The newly developed DXA and CT, ultrasound

Bioelectric impedance analysis (BIA) is a popular method in healthcare and fitness industry. The popularity of BIA results from the ease of the method. The basic principle of BIA is LBM (FFM) is proportional to the electrical conductivity of the body. A small electrical current is passed through the body, then the impedance or resistance, to that current is measured. Fat is a poor electrical conductor containing little water, whereas, lean tissue contains mostly water and electrolytes and thus a good electrical conductor. Thus, BIA estimated total body water and uses assumptions about hydration levels and the exact water content of various tissues. The limitations to this method are technician skill, instrument accuracy, and most importantly subject factors such as hydration status, fluid distribution, and temperature.
BIA measurements are extremely easy, making it excellent for monitoring obesity level, pharmaceutical therapy, nutritional or exercise intervention, sports training or other body composition altering programs. They are also fast, reproducible, non-invasive and at the acceptable standard error (about ±2%).