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Total body potassium revisited

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There are a multitude of methods available today that can be used to assess body composition in humans, from pre-term infants to the oldest adults. A body composition method is usually focused at a specific component of body composition, such as fat mass, total body water (TBW) or body cell mass (BCM). Many methods have been developed over the past century, with one of the earliest measures of body composition being total body potassium counting (TBK), which was first recognized in 1950s. There were a large number of whole body counters (WBC) available in 1970s,1 but over time the use of TBK facilities has decreased and old counters have been decommissioned. With other more accessible body composition technologies becoming more commonplace, today TBK is a rarely utilized method. However, as no alternative technology provides a direct and precise measure of BCM,2 the need for accurate body composition techniques for clinical and public health demands that TBK counting as a method should be revisited.

BCM is the body’s actively metabolizing, oxygen consuming tissues3 and represents the body’s functional mass. The BCM contains more than 98% of the body’s potassium content. Natural 40K occurs in constant proportion to its stable isotopes. BCM measured using TBK counting is an ideal indicator of nutritional status in growth and disease because the potassium concentration of BCM is extremely constant and under careful control even in ‘non-steady’ states, and its measurement is independent of lean tissue hydration changes that occur with maturation and disease. Precise and accurate quantification of BCM allows the assessment of nutritional status at all stages of life, from loss of vital tissue with age or during disease,4 to growth studies in infants and children.5

Recent advances in WBC technology allow the technique to be more accessible and used in a wide range of settings without limit to the number and frequency of repeat measurements, making TBK counting an attractive technique to review. The WBC components have become simplified to the point that they are now ‘plug-and-play’, detectors have become more robust and of higher resolution, and importantly, homeland security applications have seen costs reduce to the point where WBC should be seriously reconsidered. The advances in technology provide the opportunity for new WBC to be customized and built to suit financial, clinical and population-specific needs, and allow wider access to BCM measurements. There is also at least one commercially available turnkey system that will allow TBK measurements throughout the entire life course.

The importance of body composition to health outcome is increasingly recognized in clinical practice, and the focus in body composition research is moving from methodology to clinical and public health applications. The interpretation of the clinical significance of BCM is most powerful when related to body size, with BCM adjusted for height (BCM index (BCMI)) permitting normalization of a subject’s TBK to a reference value, directly or as a Z-score.6 The BCMI Z-score can be used to monitor nutritional status longitudinally in patients through growth, nutrition intervention and disease progression, and be used to guide decisions regarding clinical care. Although BCM is an ideal interpretation of TBK, it is not the only body component that can be calculated from TBK.7 A ‘cellular’ four compartmental body composition model (extracellular fluid+BCM+extracellular solids) can provide unbiased body fat estimates using measures of weight, BCM and TBW.8 Body protein and skeletal muscle mass can also be predicted directly from TBK,9, 10 providing a non-invasive alternative method to determine these nutritionally important body components.

Measuring body protein accretion and BCM has become increasingly relevant to public health nutrition and its measurement by WBC has enormous potential. A persistent public health problem in many developing countries is low birth weight and healthy growth of infants. Low birth weight is associated with a higher infant mortality rate and a higher risk for chronic disease later in life. Although low birth weight is an easily measurable outcome, body composition, in particular BCM, is a measure of more functional significance. There is a need for TBK measurements during pregnancy and infancy in developing countries, where the problem may be most acute, to ensure that nutritional health issues during pregnancy and the first 1000 days are completely understood, allowing for correctly targeted nutrition practices and interventions. 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, and technological advances referred to above have made it feasible to be used in developing countries.

Another pressing area of public health interest is the evaluation of the nutritional treatment of severe acute malnutrition with ready to use therapeutic food (RUTF). RUTF has a high fat content, and can achieve a rapid catch-up growth rate in terms of body weight, but the composition of the catch-up growth remains unknown. If a high proportion of the catch-up tissue deposition was composed of fat, this could be an adverse outcome. Owing to difficulties with assumptions in conventional body composition methods aimed primarily at measuring fat-free mass or fat, TBK assessment may provide an alternative and accurate measure of protein deposition in these children.

The concept of BCM measurements from TBK counting is not new, having been initially reported more than 50 years ago. What is new is that today’s systems incorporate the latest advances in technology, therefore ensuring that TBK counting is a risk-free method, independent of measurement constraints and assumptions inherent in many other available methods. The benefits of TBK counting in assessing body composition and the potential of tailor-made WBC highlight that TBK should be revisited to meet the nutritional assessment needs of clinical and public health professionals throughout the entire life course.

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Correspondence to A J Murphy.

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This commentary is based on discussions during a Consultants' Meeting at the International Atomic Energy Agency, Vienna, 17–19 July 2013.

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Murphy, A., Ellis, K., Kurpad, A. et al. Total body potassium revisited. Eur J Clin Nutr 68, 153–154 (2014). https://doi.org/10.1038/ejcn.2013.262

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