Children's Nutrition Research Center

Body Composition Techniques

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Total Body Potassium Counting

This extremely sensitive device measures the gamma rays emitted from an isotope of potassium known as 40K, which exists naturally in the human body at a known natural abundance (0.012%). This knowledge, plus the fact that potassium is only found inside body cells and is not present in stored triglycerides, makes 40K data an accurate index of the body's total cell mass (the active growing tissues in the body), which in turn can be used to estimate fat-free mass.

Although the terms lean body mass (LBM) and fat free mass (FFM) are often used interchangeably, it is more accurate to think of body weight (mass) as the sum of LBM (muscles, organs and other non-fat tissues such as bone) plus adipose (fat) tissue; or alternatively, as the sum of extractable fat (i.e., the pure fat component of adipose and other tissues) plus fat-free mass (FFM), with FFM including the water, protein and other non-fat components of adipose and other fat-containing tissues.

Dual Energy X-Ray Absorptiometry (DXA)

DXA was originally developed to determine bone mineral density and to aid in the diagnosis and treatment of osteoporosis. Later, the technique was expanded to include the analysis of fat mass and lean body mass in addition to bone mass. The basic principle of DXA data acquisition is based on the differences between bone and soft tissue attenuation at high and low x-ray levels. As an x-ray beam passes through the subject, detectors register the varying levels of x-rays that are absorbed by the anatomical structures of the subject. The raw scan data, which includes values of tissue and bone, are captured and sent to a computer. The computer generates an image of the body in pinpoint pixels, which can be 'counted' to assess bone status and fat distribution. The radiation exposure during DXA scanning is very low.

Air Displacement Plethysmograph (Bod Pod and Pea Pod)

Air-displacement plethysmography (ADP) measures human body composition by estimating body density (mass/volume). Body mass is combined with a measurement of body volume obtained using instruments called a BOD POD® and a PEA POD®. Estimates of fat and fat-free mass are generated. ADP offers several advantages over alternative reference methods. The technique is quick, non-invasive, safe and accommodates a wide range of subject types (e.g., children, obese, elderly, and disabled persons). The method does not use radiation (as does the DXA), is much faster than a whole-body potassium measurement (2 min vs. 20 min) and is more accurate than anthropometric assessments such as skinfolds. The PEA POD is designed to measure infants up to about 8 kg in weight. The BOD POD, with its pediatric option, can accurately assess body composition of children as small as 12 kg. The BOD POD will also accommodate adults weighing over 180 kg; thus, these instruments are well suited for longitudinal testing of a broad range of subjects.

Both instruments require a small change in pressure in the measurement chamber for a few seconds that is undetectable to the subject.  The magnitude of the pressure change is like that experienced when moving on an elevator, at normal speed, between the 3rd floor and ground floor of a building.

High-Resolution Peripheral Quantitative Computed Tomography (HR-pQCT)

HR-pQCT is a non-invasive technique that provides high resolution, quantifiable images of bone, focusing on the extremities. HR-pQCT can measure volumetric bone density and discriminate between cortical and trabecular bone. In addition, the microstructure of bone can be assessed (trabecular bone volume, number of trabeculae per millimeter, inhomogeneity of the network, trabecular thickness, cortical thickness and cortical porosity). Scan analyses involve segmentation of cortical and trabecular compartments. Analysis results provide bone density measurements (trabecular, cortical, and total volumetric), microarchitectural measurements (trabecular number, thickness, separation, and cortical thickness and porosity), and morphological outcomes (total bone area and cortical area).

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.