Total Body Potassium (TBK)

Total Body Potassium (TBK) is a method for measuring body composition. This procedure accurately determines the body's total cell mass (that is, the active growing tissues in the body), which in turn can be used to estimate fat-free or lean body mass. When this measurement is combined with measurements from the Total Body Protein, you can determine total organ and muscle mass.

purpose: to measure total cell mass

equipment required: extremely sensitive equipment which measures the gamma rays emitted from the naturally occurring radioactive isotope of potassium known as 40K.

procedure: The subject must first be shielded from naturally occurring radiation in the environment, using concrete, lead or steel. Once the external radiation is minimized, the subject’s natural radiation as 40K is measured through the use of scintillation counters. The radiated 40K is measured over a specified time period and from this TBK is estimated.

advantages: other than the initial outlay cost for equipment, this method is relatively simple to carry out, safe, and operational expenses are relatively small.

disadvantages: the required instrument is very expensive to install and has limited availability.

comments: The determination of TBK is based on the fact that the proportion of total potassium found in human tissues as 40K is constant at 0.0118% of total potassium. Therefore, by measuring 40K you can calculate total-body potassium. As potassium is distributed almost entirely within the intracellular compartment of fat-free mass, and using the ratio of total-body potassium to fat-free mass, once TBK is known you can calculate fat-free mass and total-body fat.

use: TBK is a classical method of quantifying total-body fat. It has mostly been replaced by newer more accurate techniques.

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