The epidemiology of obesity has for many years been difficult to study because many countries had their own specific criteria for the classification of different degrees of overweight. Gradually during the 1990s, however, the body mass index (BMI; weight/height2) became a universally accepted measure of the degree of overweight and now identical cut-points are recommended. This most recent classification of overweight in adults by the World Health Organization is shown in Table 2.1 (1).
In many community studies in affluent societies this scheme has been simplified and cut-off points of 25 and 30 kg/m2 are used for descriptive purposes. The prevalence of very low BMI (< 18.5 kg/m2) and very high BMI (40 kg/m2 or higher) is usually low, in the order of 1-2% or less. Already researchers in Asian countries have criticized these cut-points. The absolute health risks seem to be higher at any level of the BMI in Chinese and South Asian people, which is probably also true for Asians living elsewhere. There are some developments that indicate that the cut-points to designate obesity or overweight may be lowered by several units of BMI. This would of course greatly affect the estimates of the prevalence of obesity in these populations. For instance, the prevalence of overweight measured as BMI > 27kg/m2 in the 1989 China Health and Nutrition Survey (2) was 6% in the North, 3% in Central China and 1% in the South. If the cut-off point was lowered to 25 kg/m2 the prevalence would be increased to, respectively, 15%, 9% and 6%. In countries such as China and India, each with over a billion inhabitants, small changes in the criteria for overweight or obesity potentially increase the world estimate of obesity by several hundred million (currently estimates are about 250 million worldwide).
Much research over the last decade has suggested that for an accurate classification of overweight and obesity with respect to the health risks one needs to factor in abdominal fat distribution. Traditionally this has been indicated by a relatively high waist-to-hip circumference ratio. Recently it has been accepted that the waist circumference alone may be a better and simpler measure of abdominal fatness (3,4). Table 2.2 gives some tentative cut-points for the waist circumference. These are again based on data in white populations.
In June 1998 the National Institutes of Health (National Heart, Lung and Blood Institute) adopted the BMI classification and combined this with waist cut-off points (6). In this classification the combination of overweight (BMI between 25 and 30kg/m2) and moderate obesity (BMI between 30 and 35 kg/m2) with a large waist circumference (> 102 cm in men or 88 cm in women) is proposed to carry additional risk.
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