The prevalence of child obesity is increasing rapidly worldwide.1 It is associated with several risk factors for later heart disease and other chronic diseases including hyperlipidaemia, hyperinsulinaemia, hypertension, and early atherosclerosis.2–4 These risk factors may operate through the association between child and adult obesity, but they may also act independently.5 Because of their public health importance, the trends in child obesity should be closely monitored.Trends are, however, difficult to quantify or to compare internationally, as a wide variety of definitions of child obesity are in use, and no commonly accepted standard has yet emerged.Each z score substituted into equation 1 provides the formula for an extra centile curve passing through the specified point (dotted line in fig 1).
The hypothesis is that the relation between cut off point and prevalence at different ages gives the same curve shape irrespective of country or obesity.
If sufficiently similar the curves can be averaged to provide a single smooth curve passing through the adult cut off point.
Four of the datasets were based on single samples whereas the British and American data consisted of pooled samples collected over a period of time.
We omitted the most recent survey data from the United States (1988-94) because we preferred to use data predating the recent increase in prevalence of obesity.19 In practice this decision made virtually no difference to the final cut off points.
Regardless of centile or reference population, the cut off point can still be criticised as arbitrary.
A reference population could be obtained by pooling data from several sources, if sufficiently homogeneous.
Setting: Brazil, Great Britain, Hong Kong, the Netherlands, Singapore, and the United States Subjects: 97 876 males and 94 851 females from birth to 25 years of age Main outcome measure: Body mass index (weight/height2).
Results: For each of the surveys, centile curves were drawn that at age 18 years passed through the widely used cut off points of 25 and 30 kg/m2 for adult overweight and obesity.
Clearly a cut off point related to age is needed to define child obesity, based on the same principle at different ages, for example, using reference centiles.10 In the United States, the 85th and 95th centiles of body mass index for age and sex based on nationally representative survey data have been recommended as cut off points to identify overweight and obesity.11 For wider international use this definition raises two questions: why base it on data from the United States, and why use the 85th or 95th centile?
Other countries are unlikely to base a cut off point solely on American data, and the 85th or 95th centile is intrinsically no more valid than the 90th, 91st, 97th, or 98th centile.
which summarises the data in terms of three smooth age specific curves called L (lambda), M (mu), and S (sigma).