Defining Childhood Obesity - Body Mass
Index (BMI)
What is Obesity?
Obesity is a condition of excess adiposity, so that in principle childhood
obesity ought to be simple to define. It just needs a scale to measure
adiposity and some age-specific cut-off points to specify how much adiposity
is "excessive". Yet in practice this is not simple - adiposity
is difficult to measure with any accuracy, and there is no objective basis
for setting the cut-offs. Instead body mass index (BMI) is often used
as an imperfect proxy for adiposity, while the cut-offs are based on an
arbitrarily selected centile from an age-specific BMI distribution. For
example the US recommended cut-off for obesity is the 95th centile of
the US BMI reference.
Defining Childhood Obesity
This approach has led to a profusion of definitions for childhood obesity
that are mutually incompatible, and which make it impossible to compare
prevalence rates for obesity from one country to another. The International
Obesity TaskForce (IOTF) cut-offs (Cole et al, BMJ 2000) were introduced
to get round this, by providing a definition that was sufficiently politically
acceptable to be used internationally. It is also based on BMI centiles,
but uses data pooled from six large nationally representative surveys,
and its cut-offs are linked to the universally accepted adult overweight
and obesity cut-offs of BMI 25 and 30 kg/m2. Each survey contributes a
centile curve that passes through the adult cut-off at age 18, and these
centiles are averaged across surveys. The resulting cut-offs are broadly
independent of the obesity levels of the contributing surveys, and are
not linked to any one country.
Childhood Obesity Worldwide
The paper has been widely cited, and several studies have now reported
prevalence rates based on the IOTF cut-offs. Results from the UK, Australia,
China, Russia, USA and Brazil make clear that a) childhood obesity is
rising throughout the world; b) there are substantial differences in prevalence
between countries, being particularly high in urban China, and c) extrapolating
to the future suggests a major public health problem in the making.
BMI & Obesity
But the BMI trends may actually have underestimated the scale of the problem.
BMI fell slightly during the 1980's before rising steeply in the 1990's,
which probably reflected an initial shift of body mass from muscle to
fat, with the loss of muscle masking the increase in fat. Waist circumference,
which measures abdominal fat, has increased much more rapidly than BMI
over the same period, implying an even steeper obesity trend.
Source: Professor Tim Cole, Institute of Child Health, London, UK (2002)
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