Measurement of excess body weight
Several anthropometric measures are commonly used to define excess body weight in human observational studies.1 Use of each of the parameters has advantages and disadvantages related to ease of use and confounding by other body habitus factors.
BMI, defined as weight (in kg) divided by the square of the height (in m), remains the commonest, internationally-recognised measure of the degree of body fatness, primarily due to its simplicity. The BMI classification of adult underweight, overweight and obesity statesrecognised by the World Health Organisation is as follows:2 underweight (<18.5 kg/m2); normal (18.5–24.9kg/m2); overweight (25.0–29.9kg/m2); obese class I (30.0–34.0 kg/m2); obese class II (35.0–39.9kg/m2); and obese class III (>40 kg/m2).
The term morbid obesity is still used and is generally defined as a BMI >40 Kg/m2. Super obese is often defined as a BMI >50 Kg/m2.
The use of BMI is not without limitation. As BMI is a function of the square of height, it underestimates the degree of adipose accumulation in tall subjects and leads to an overestimate in shorter subjects. Older people, with reduced lean body mass, will also have a BMI that underestimates the degree of adiposity.
BMI also fails to take into account differences in distribution between subcutaneous adipose tissue (SAT) and visceral (or abdominal) adipose tissue (VAT). Men have a greater propensity for central, abdominal fat distribution, whereas women characteristically have larger amounts ofSAT. VAT is increasingly understood to have a dominant biological role in mediating many of the complications of obesity.3
Other anthropometric measures, which better reflect differential fat distribution such as waist circumference (WC) and waist–hip ratio (WHR), are increasingly used in epidemiological studies of gastrointestinal cancer risk despite the increased complexity of obtaining these data and the inherent subjectivity in the measurement of these parameters by different operators.4 Radiological measurement of VAT is also possible but is less relevant to large-scale observational studies.1,5
References
1Ness-Abramof, R. & Apovian, C. M. Waist circumference measurement in clinical practice. Nutr. Clin. Pract.23, 397-404 (2008).
2WHO. World Health Organisation. Obesity and Overweight. Fact Sheet No 311, (2006).
3Despres, J.-P. et al.Abdominal Obesity and the Metabolic Syndrome: Contribution to Global Cardiometabolic Risk. Arterioscler. Thromb. Vasc. Biol.28, 1039-1049 (2008).
4Klipstein-Grobusch, K., Georg, T. & Boeing, H. Interviewer variability in anthropometric measurements and estimates of body composition. Int. J. Epidemiol.26 Suppl 1, S174-180 (1997).
5Kang, H. W. et al. Visceral Obesity and Insulin Resistance as Risk Factors for Colorectal Adenoma: A Cross-Sectional, Case-Control Study. Am. J. Gastroenterol.105, 178-187 (2009).
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