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United States Department of Agriculture

Agricultural Research Service

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1 - Body Composition Lab
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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 treatment of osteoporosis. More recently, the technique has been 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.



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Last Modified: 8/13/2016
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