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The role of body composition analysers in general practice

Jeremy Krebs
MB ChB FRACP
Medical Research Council Clinical Scientist
Endocrinologist

Susan Jebb
SRD PhD
Medical Research Council Scientist
Head of Nutrition and Health Research

The traditional method of measuring body fat is using the body mass index (BMI). However, this method assumes that any excess weight is mainly fat, and can give a false estimate of body fatness, especially in those with a well-developed musculature such as bodybuilders or athletes. Errors also occur in certain ethnic groups and older adults. More recently, waist circumference has been used as a proxy measure of abdominal fatness.
Since the risks of obesity relate to the presence of excess body fat, a direct measure of body fat mass would be a useful screening tool. Reference methods such as dual-energy X-ray absorptiometry (DXA), underwater weighing or isotope dilution techniques are the gold standards, but are available only in the research setting.(1) Until recently the only "bedside" method was the measurement of skinfold thickness, but this is limited by the practical difficulty of accessing the appropriate sites and the measurement differences incurred by different practitioners.
Bioelectrical impedance analysis (BIA) measures the voltage drop across the body when a small current is passed between two electrodes. Lean tissue acts as an efficient conductor, whereas fat acts as an insulator - hence the measured impedance depends on the proportion of fat and lean tissue. Most commercial systems also incorporate weight, height, age and gender to improve the prediction of body fat. Several BIA methods are now available, such as the tetrapolar system, where electrodes are placed on the skin of the forearm and lower leg, with the subject in the supine position, the hand-to-hand system, where the subject grips the electrodes, one in each hand, and the bipedal system where the subject stands barefoot on two electrode plates. The latter has the advantage of simultaneously measuring body weight and can be most easily incorporated into routine clinical visits.
These various methods of BIA have been validated against reference methods and provide reasonably accurate estimates of fat mass in groups of subjects across a range of body fat mass.(2) However, the absolute accuracy of BIA is affected by a number of factors, especially hydration status, and will produce unreliable results in pathological conditions such as oedema or ascites.
In general practice, BIA can be used to track changes in fat mass: for example, to monitor the impact of a weight loss programme. Such measurements avoid the short-term fluctuations from changes in water balance. For patients who embark on an exercise programme, BIA can be used to assess the impact on body composition and will detect the loss of fat and increase in lean tissue at times when weight may remain stable.
Currently there are no data from prospective longitudinal studies to provide clear information on the healthy range of body fat. However, Table 1 shows typical ranges for body fat that correspond to the traditional BMI ­cut-offs for "overweight" and "obese".(3)

[[NIP02_table1_113]]

References

  1. Jebb SA. Measurement of body composition from the laboratory to the clinic. In: Kopelman P, Stock M, editors. Clinical Obesity. Oxford: Blackwell Science; 1998. p. 18-49.
  2. Jebb SA, Cole TJ, Doman D, Murgatroyd PR, Prentice AM. Evaluation of the novel Tanita body-fat analyser to measure body ­composition by comparison with a four-compartment model. Br J Nutr 2000;83:115-22.
  3. Gallagher D, Heymsfield SB, Moonseong H, Jebb SA, Murgatroyd PR, Sakamoto Y. Healthy percentage body fat ranges: an approach for developing guidelines based on body mass index. Am J Clin Nutr 2000;72:694-701.

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