Definition, classification of obesity, Body Mass Index (BMI) as a measure for assessing obesity

Obesity is characterized by excess adipose tissue. Quantification of adipose tissue mass can be achieved by a number of laboratory methods including underwater body measurement and body fat content estimated by the dual energy of X – ray absorption (DEXA). In addition, the development of new technique, such as magnetic resonance imaging (MRI) and computed tomography (CT), has provided researchers with opportunities to describe human adiposity in more detail. However, these methods require costly equipment and are difficult to implement in epidemiological studies, although some exceptions exist, such as bioelectrical impedance.

In large-scale population surveys, body weight adjusted for stature (body mass index) is commonly used as a surrogate for body fat content. These indices are defined as different combinations of weight and height, such as weight divided by height and weight expressed as a percentage of mean weight for a given height and sex. The most widely used is Quetelet’s index, better known as body mass index (BMI), which is body weight (kg) divided by height squares (m²). This index has been shown to correlate weakly with height and strongly with body fatness.

Although the correlation between BMI and body fat adjusted for height is high (γ=0,82-0,91), BMI fails to distinguish between lean body mass and fat. Thus, the relationship between BMI and body fatness varies according to body composition proportions. For instance, the percentage of body fat mass is higher in women than in men with a similar BMI. In addition, body fatness has been shown to increase with ageing, meaning that a given BMI may correspond to a greater body fat content in older subjects compared with younger ones.

Any age-related change in height has an influence on BMI as well. In adults, height is lost with normal ageing in a recent study, an average height loss of 3 cm from age 30 to 70 years was estimated to account for an artificial increase in BMI of 0.7 kg/m² for men.

In women, the height loss averaged about 5 cm over the same 40 – year period, accounting for an increase of 1.6 units in BMI. During growth in childhood and adolescence, not only does height increase but body composition changes as well, thus classification of obesity according to BMI is complicated. Because the age of onset of puberty varies, international BMI-based estimates of overweight in children and adolescents are rendered even more difficult to determine (World Health Organization, 2000). The need for these estimates has, however, been emphasized. Consequently, internationally based cut-off points for children have recently been published.

Despite its limitations, BMI provides a simple and the most useful population-level measure of obesity in adults. A BMI of 30 kg/m² is widely recognized as a cut-off point for obesity. The latest classification of overweight according to BMI (table) was introduced in a WHO report published in 2000 (World Health Organization, 2000). BMI-based classification of overweight and obesity has been well received by the research community, making comparisons for obesity prevalence between or within populations feasible. However, in some studies, alternative cut-off points have been used.

For example, obesity has been classified on the basis of the BMI distribution in the reference population, the 85% being the cut-off point for overweight and the 95% for obesity, or subjects with a relative weight index (100 × weight divided by ideal weight) larger than 130% have been considered to be obese.

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