Alison Morton. Nov, 2002. Body Mass Index [online]. Seacroft and St James's University Hospitals, Leeds, UK. Available from http://www.cysticfibrosismedicine.com.

In 1869 Quetelet a Belgian astronomer observed that in adults of normal build weight was proportional to the square of the height. Some hundred years later Keys et al (1972) made a similar observation and named the relationship Body Mass Index or BMI.

BMI or Quetelet index is defined as: BMI = Weight (kg)/Height2 (m)

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BMI is widely used to define obesity (WHO, 1998). BMI can be used as part of a global clinical and nutritional assessment in adults. However, it is important to remember that BMI does not distinguish between lean body mass and body fat. Indiscriminate use of BMI can lead to misinterpretation and misdiagnosis of obesity ie individuals with well developed musculature such as body builders, heavily muscled boxers may have a BMI above 30 but they are not obese. In addition, in certain clinical conditions such as liver disease, BMI may be normal or high but this may reflect ascites or oedema and indeed the patient may actually be clinically malnourished despite a normal BMI.

BMI is most widely used and accepted in assessing adults. However an Expert Consensus Group of the Royal College of Paediatrics and Child Health recently advised the use of BMI in preference to % weight for height in children (www.rcph.ac.uk). There do however remain concerns about the use of BMI and its interpretation in a paediatric population. In addition to the problems seen in adults, if a child is stunted due to malnutrition or delayed puberty their BMI may be normal but this does not reflect his/her nutritional status.

There is no world wide consensus about the use of BMI in the assessment of paediatric patients with CF: The Cystic Fibrosis Trust 'Nutritional Management of Cystic Fibrosis, 2002' recommends a combination of BMI and percentage weight for height and percentage weight and height for age. The North American consensus report on nutrition (Borowitz et al, 2002) recommends a combination of % ideal body weight and the use of BMI centile charts but not BMI in isolation and the European Consensus Report (Sinaasappel et al, 2002) states BMI values must be interpreted on the basis of comparison with age and gender specific centiles but that no advantage has been shown of using BMI rather than weight for height to document malnutrition in children and adolescents with CF.

BMI is widely used as part of routine assessment of nutritional status in adult patients with cystic fibrosis but it forms only part of the 'ABC… ' of nutritional assessment: anthropometry, biochemistry, clinical and dietary assessments.

BMI has been used in the definition of nutritional failure in patients with CF but again there is variability in the three reports stated above on the criteria and cut off points used to define nutritional failure or the need for nutritional interventions.

References

Borowitz D, Baker RD, Stallings V. Consensus Report on Nutrition for Pediatric Patients with Cystic Fibrosis. J Pediatr Gastoenterol 2002; 35; 246-259.

Keys A, Fidanza F, Karvonen MJ, Kimura N, Taylor HL. Indices of relative weight and obesity. Journal of Chronic Diseases 1972, 25, 329 - 43.

Nutritional Management of Cystic Fibrosis. Report of the UK CF Trust Nutrition Working Group. London, CF Trust, 2002.

Sinaasappel M, Stern M, Littlewood J, Wolfe S, Steinkamp G, Heijerman H, Robberecht E, Doring G. Nutrition in Patients with Cystic Fibrosis: a European Consensus. Journal of Cystic Fibrosis 2002; 1 (2); 51-75.

WHO. Obesity. Preventing and Managing the Global Epidemic. Geneva: WHO, 1998