The prevalence of obesity is increasing throughout the world at an unprecedented rate. To be a healthy BMI, as defined by the World Health Organization (WHO), is now to be in a minority in much of western Europe as well as the United States.  Indeed,  in many developing countries overweight and obesity are now so common that they are replacing more traditional problems such as undernutrition and infectious diseases as the most significant causes of ill-health (World Health Organization, 2000).

In 1995, there were an estimated 200 million obese adults worldwide and another 18 million under-5 children classified as overweight.  As of 2000,  the number of obese adults has increased to over 300 million. This obesity epidemic is not restricted to industrialized societies; in developing countries, it is estimated that over 115 million people suffer from obesity-related problems (World Health Organization, 2000).

As the proportion of the population with a low BMI decreases,  there is an almost symmetrical increase in the proportion with a BMI above 25. The WHO MONICA project compares obesity rates in 48 populations spread throughout the world (Berrios et al., 1997). In the period 1983 to 1986 these rates varied from less than 5 per cent in Beijing in China to about 20 per cent in Malta. Recent data suggests that the BMI distribution is moving upwards in China as in the rest of the world.
From 1989 to 1997 overweight (BMI 25–29.9 kg m^-2) doubled in females (from 10.4 to 20.8 per cent)  and almost tripled in males (from 5.0 to 14.1 per cent) (Bell et al., 2001). Some of the highest prevalence figures come from the Pacific region where in urban Samoans obesity has increased from 38.8 per cent in men in 1978 to 58.4 per cent in 1991 (World Health Organization, 2000).

Within the developed world the United States has led the obesity epidemic.

In the adult population in the United States the prevalence of obesity, as determined from the National Health and Nutrition Examination Survey (NHANES), has increased from 22.9 per cent in the period 1988–1994 to 30.5 per cent in 1999–2000 (Flegal et al., 2002). Corresponding increases have also occurred in overweight and in morbid obesity. Self-reported data (Behavioural Risk Factor Surveillance System) from much larger numbers of subjects confirm these worrying trends in the United States (Mokdad et al., 1999, 2003). Indeed, if weight gain continues at the current rate in the United States by 2008 39 per cent of the population will be obese (Hill et al., 2003).

The overall data on obesity prevalence masks other differences, including higher rates of overweight and obesity in non-Hispanic black women and in a number of minority ethnic groups. In the UK a number of surveys have documented the changes in obesity from 1980 to the current day (Prescott-Clarke and Primatesta, 1999; Figure 1.1). There has been a tripling in obesity prevalence even in this relatively short period of time, with the likelihood that the UK rates will continue to rise to attain those already existing in the United States.

  The age of onset of obesity is getting progressively younger (McTigue et al., 2002). This is reflected in the trends in overweight and obesity in children. In the US the prevalence of overweight (defined as at or above the 95th centile of BMI for age) increased from 10.5 to 15.5 per cent of 12–19-year olds between 1994 and 2000 and in the 2–5-year age group period the increase was from 7.2 to 10.4 per cent in this 6-year timespan (Ogden et al., 2002).

In England 9.0 per cent of boys aged between 4 and 11 years were overweight in 1994 compared with 5.4 per cent in 1984 (Chinn and Rona,  2001).  The corresponding figures for girls were 13.5 per cent (1994) and 9.3 per cent (1984). Though not all obese children become obese adults, a considerable proportion will do so (Kotani et al., 1997).

The continuing rise in childhood obesity is likely to lead to a massive increase in the prevalence of those co-morbidities linked to obesity.

Anthony H. Barnett
  Department of Medicine, University of Birmingham and Birmingham Heartlands and Solihull NHS Trust (Teaching), UK

Sudhesh Kumar
Professor of Medicine, Diabetes & Metabolism, Warwick Medical School, University of Warwick, UK