• What is assessed?
• How is the assessment conducted?
• When is this method used?
• How are estimates of body composition derived?
• Strength and limitations
• Populations
• Further considerations
• Resources required
• Instrument library
• References

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:

1. TBK via measurement of ⁴⁰K
2. The known ratio of TBK to fat-free mass

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 It has mostly been replaced by newer more accurate techniques.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.

TBK has been used primarily as a marker for body cell mass (BCM), total body protein and can also be used to estimate FFM. In general, the accuracy of the whole body measurement of TBK is 1–5% for adults, decreasing slightly for the preterm infant. This method was originally applied for predicting total body fat in the 1960s. Although the TBK-fat method is no longer in use, studies support the continued use of TBK for total body protein, body cell mass and skeletal muscle in both adults and children.

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.

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.

1. Any jewellery should be removed before counting, as possible presence of trace radioactivity in the metal.
2. No need for fasting before measurement as most food contains about the same amount of potassium concentration as the whole body.
3. As the presence of radioactive contamination on the participant’s body and clothing from atmospheric radon gas can affect the performance of the counters, it is recommended that participants wear clean clothes before undergoing the measurement.

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.

A method specific instrument library is being developed for this section. In the meantime, please refer to the overall instrument library page by clicking here to open in a new page.

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