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United States Department of Agriculture

Agricultural Research Service

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1 - Body Composition Lab
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Air Displacement Plethysmograph (BodPod)


The BodPod uses a technique called Air Displacement Plethysmography (ADP) to measure body volume , which in turn can be used to estimate percent body fat.

This technique relies on the physics of Boyle's Law, which states that pressure and volume vary inversely with one another. In other words, as pressure goes up, volume goes down, and vice versa . Monitoring pressure changes in a closed chamber allows one to calculate volume.


Bioelectrical impedance analysis (BIA)


The resistance to an applied electric current flowing through the body is related to the volumes of conductive tissues that the current passes through. This measurement can estimate water and fat ratios.

Bioelectrical impedance analysis is based on the conductive and non-conductive properties of various biological tissues. Most of the body's fat-free mass is composed of conductive tissues such as muscle, while fat is part of the non-conductive tissue mass. The volume of these tissues can be estimated from the measurement of the resistance to an applied electric current flowing through the body.

Water is a constant fraction of fat-free mass, usually about 73 percent. The water measurement can therefore be used to estimate levels of fat-free mass. Water ratios, however, can change with the onset of certain illnesses (mostly in the extra-cellular compartment). Accurate body water measurements using BIA can be important in studying disease.


 Computed Tomography (CT) Scan


Computed tomography produce cross-sectional scans of the body, which is particularly useful in determining ratios of visceral (intra-abdominal) fat versus subcutaneous fat. CT is also used to obtain true three-dimensional bone mineral density (BMD in g/cm 3 ), usually of the lumbar spine. The BMD measurement looks specifically at trabecular bone - the softer, spongy bone within a vertebra that is most closely associated with bone loss in osteoporosis and other bone diseases.



This small, portable, optical device enables researchers to measure the thickness of subcutaneous adipose tissue at any location in the body in a rapid, safe and non-invasive manner.



Ultrasound measurements operate by emitting high frequency sound waves that penetrate the skin surface and pass through the fat layer, bounce off the muscle or bone, and return to the ultrasound unit. The time for the transmission of the ultrasound wave is recorded and converted to a body composition measurement based on comparative data. This portable technology can be used to estimate the bone density of individuals using localized measurements, such as of the tibia and radius, as well as to estimate subcutaneous fat.

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Last Modified: 8/13/2016
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