Obesity is of serious concern because of the high mortality caused by its associated diseases. In fact, obesity now kills more people than any other condition. Diagnosis of obesity is usually done by calculating the body mass index (BMI), a measurement based on the observation that in a normal individual the body weight is about proportional to the height squared. The resulting figure is given in units of kilograms per meter squared (kg/m2 or kg.m-2), with obesity defined as a body mass index of over 30 kg/m2. The main reason that this scale is favoured by medical professionals and is used extensively in research is because it is quick, easy, non-invasive, and can also be calculated without the need to even have contact with the subject. As long as the height and weight are known, the body mass index can be calculated, even retrospectively.
However, within its simplicity lies the main problem with the body mass index scale, namely that it does not take into account lean body mass. Athletes for example can show up as obese on the scale because of the increased lean mass they possess. In contrast, individuals with abdominal obesity but little subcutaneous fat can appear to show no signs of disease. These problems were highlighted in one study1 that investigated the usefulness of the body mass index scale by assessing it against the World Health Organisation’s obesity definition of a body fat percentage over 25 % in men and over 35 % in women. In the cross-sectional study of 13601 subjects, obesity was present in 19.1 % of men and 24.7 % of women when using body mass index as a measure. However, when the percentage body fat was assessed with bioelectrical impedance, obesity was present in 43.9 % of men and 52.3 % of women.
The authors noted that the ability of body mass index to detect obesity diminished with age and that in the 25 to 29.9 kg/m2 range, body mass index failed to discriminate between body fat and lean mass. Therefore, as expected, the usefulness of the body mass index was weakened by its inability to provide a representation of lean mass or abdominal obesity. This weakness can be overcome by use of the waist to hip ratio, which is a measure of the ratio between the waist circumference and the hip circumference of the individual. An increase in the waist to hip ratio signifies and increased risk of abdominal obesity. The waist to hip ratio is useful because it identifies abdominal obesity, which is the condition that is of real interest when considering obesity in general. It does not take into account peripheral subcutaneous fat which is not associated with disease. However, because it is time consuming it is not favoured by medical doctors.
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