The epidemic of obesity in UK children and adolescents began in the mid to late 1980s.¹ As in other parts of the world, overweight and obesity have become very common, and prevalence continues to increase. The most recent nationally available data from within the UK come from the Health Survey for England 2004:² in 2–10 year olds 16% of boys and 12% of girls were obese (BMI ⩾95th centile), and in 11–15 year olds a staggering 24% of English boys and 26% of English girls were obese in 2004.

SCALE AND IMPACT OF THE OBESITY EPIDEMIC

Though high, these estimates of obesity prevalence are probably conservative: a high BMI for age is a good simple means of diagnosing obesity in that it is characterised by high specificity (low false positive rate), but it is also characterised by modest sensitivity, a moderate false negative rate.^3,4 This means that there are many children with an excessively high body fat content but with BMI below any recommended BMI cut-point for defining obesity, such as BMI ⩾95th centile.^3,4 There is a large body of evidence, systematically reviewed and critically appraised,^5,6 showing that a high BMI for age is a better measure of obesity for clinical purposes, such as diagnosis of obesity in individual children, than for epidemiological applications such as surveillance of obesity. The other reason why current estimates of obesity prevalence using BMI are conservative, is the evidence on trends in increasing body fat content (as distinct from BMI) of British children,^7–9 as well as increasing central adiposity (e.g. rapidly increasing waistlines of British children and adolescents).^10,11 These increases in body fatness and in central fatness have probably affected much of the distribution, so that even …