Whole body counting (also called total body counting) measures the amount of naturally radioactive potassium 40 (40K) in the body (total body potassium or TBK). Potassium 40 is a naturally occurring isotope found in a known amount (0.0118%) in intracellular water and is not present in stored triglycerides.
The determination of TBK uses the principle that the proportion of total potassium found in human tissues as 40K is constant at 0.0118% of total potassium. Therefore, by measuring 40K, it is possible to calculate total-body potassium.
As potassium is distributed almost entirely within the intracellular compartment of fat-free mass, it is possible to calculate fat-free mass and fat mass using:
TBK is a classical method of quantifying total-body fat. It has mostly been replaced by newer more accurate techniques. However, as part of a multi-component body composition model, TBK provides a precise and accurate assessment of nutritional status at all stages of life, from loss of vital tissue with age or during disease to growth studies in infants and children.
The participant is in a supine position on a bed between two arrays of sodium iodide detectors that are inside a room with shielding using concrete, lead or steel, such as 20 cm thick steel walls surrounded by one -metre-thick concrete. This shielding is necessary in order to filter naturally occurring radiation in the environment.
Once the external radiation is minimized, the participant’s natural radiation as 40K is measured through the use of scintillation counters (an instrument for detecting and measuring ionizing radiation). As the participant is measured by the counter, the gamma rays emitted by the potassium at 1.46 MeV are detected by sodium iodide crystals in either single or multi-detector configurations (Figure 1). Depending on the scanning system, measurements are taken over a few minutes to an hour.
Figure 1 Example of a multi-detector whole body counter designed to monitor body potassium in infants, children, and adults. Distance between detector arrays can be varied to maximize counting efficiency and precision.
Source: Image and information from the Body Composition Laboratory at Baylor College of Medicine, Houston, TX.
After measurement of 40K radiation from the body (as described above), calculation of total body potassium (K) for the data is required. Fat free mass can be estimated from the total body K based on any one of several conversion factors which may vary in men from 2.46 to 3.41g K per kg of fat free mass and in women from 2.28 to 3.16 grams per kilogram. Total body fat can be calculated by subtracting FFM from body weight. Prediction models can be used to derive BCM, total body protein and total body skeletal muscle mass.
An overview of the characteristics of TBK is outlined in Table 1.
The main strength relates to its accuracy. However, it is costly and requires a specific environment in which it can be used. It is limited too in terms of its availability.
It may provide additional precision over anthropometrics and bioelectrical impedance methods (BIA) as the SEE of skeletal muscle in adult studies (1.5 kg) has been shown to be smaller than for simple anthropometric measures (2.2 kg) and bioelectrical methods (2.7 kg). The new technology with TBK counting, unlike many body composition methods, is safe, accurate and feasible in the first 1000 days of life and in pregnancy.
Table 1 Characteristics of whole body counting of TBK.
|Number of participants||Small|
|Researcher burden of data collection||Medium|
|Researcher burden of coding and data analysis||Medium|
|Risk of reactivity bias||No|
|Risk of recall bias||No|
|Risk of social desirability bias||No|
|Risk of observer bias||Yes|
|Suitability for field use||Not suitable|
|Participant literacy required||No|
Considerations relating to the use of whole body counting of TBK for anthropometry in specific populations are described in Table 2.
Table 2 Anthropometry by whole body counting of TBK in different populations.
|Infancy and lactation||Suitable|
|Toddlers and young children||May not be suitable in children between 18 months to 4 years due to lack of cooperation as it might be difficult to persuade them to stay still during the counting times (5-20 minutes). Children at these ages are often measured while asleep and for clinical applications sedation might be required.|
|Other (obesity)||Suitable for most clinical populations but not for those that have a condition that affects levels of potassium|
Refer to section: practical considerations for objective anthropometry
Extremely sensitive equipment, which measures the gamma rays emitted from the naturally occurring radioactive isotope of potassium known as 40K, is required.
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