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British Medical Journal
May 6, 2000
Establishing a standard definition for child overweight and obesity
worldwide: international survey.
Author/s: Tim J Cole
Abstract
Objective To develop an internationally acceptable definition of
child overweight and obesity, specifying the measurement, the reference
population, and the age and sex specific cut off points.
Design International survey of six large nationally representative
cross sectional growth studies.
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/[height.sup.2]).
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/[m.sup.2] for adult overweight and obesity. The resulting curves were
averaged to provide age and sex specific cut off points from 2-18 years.
Conclusions The proposed cut off points, which are less arbitrary and
more internationally based than current alternatives, should help to
provide internationally comparable prevalence rates of overweight and
obesity in children.
Introduction
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]
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. The ideal definition, based on percentage body fat, is
impracticable for epidemiological use. Although less sensitive than
skinfold thicknesses,[5] the body mass index (weight/[height.sup.2]) is
widely used in adult populations, and a cut off point of 30 kg/[m.sup.2]
is recognised internationally as a definition of adult obesity.[6]
Body mass index in childhood changes substantially with age.[7 8] At
birth the median is as low as 13 kg/[m.sup.2], increases to 17 kg/[m.sup.2]
at age 1, decreases to 15.5 kg/[m.sup.2] at age 6, then increases to 21
kg/[m.sup.2] at age 20. 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.[9] 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.[10] 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?
A reference population could be obtained by pooling data from several
sources, if sufficiently homogeneous. A centile cut off point could in
theory be identified as the point on the distribution of body mass index
where the health risk of obesity starts to rise steeply. Unfortunately
such a point cannot be identified with any precision: children have less
disease related to obesity than adults, and the association between
child obesity and adult health risk may be mediated through adult
obesity, which is associated both with child obesity and adult disease.
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The adult cut off points in widest use--a body mass index of 25
kg/[m.sup.2] for overweight and 30 kg/[m.sup.2] for obesity--are related
to health risk[1] but are also convenient round numbers. A workshop
organised by the International Obesity Task Force proposed that these
adult cut off points be linked to body mass index centiles for children
to provide child cut off points.[11 12] We describe the development of
age and sex specific cut off points for body mass index for overweight
and obesity in children, using dataset specific centiles linked to adult
cut off points.
Subjects and methods
Subjects
We obtained data on body mass index for children from six large
nationally representative cross sectional surveys on growth from Brazil,
Great Britain, Hong Kong, the Netherlands, Singapore, and the United
States. Each survey had over 10 000 subjects, with ages ranging from
6-18 years.
Centile curves
Centile curves for body mass index were constructed for each dataset
by sex Using the LMS method.[13] The fitted LMS curves allow an extra
centile curve to be drawn for each dataset, passing through the adult
cut off point for obesity of 30 kg/[m.sup.2] at age 18. Superimposing
the curves of the six datasets leads to a cluster of centile curves that
all pass through the adult cut off point yet represent a wide range of
obesity. 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.
The curve is representative of all the datasets involved but is
unrelated to their obesity--the cut off point is effectively independent
of the spectrum of obesity in the reference data. A curve for overweight
passing through 25 kg/[m.sup.2] at age 18 is obtained in the same way.
Results
Table 1 gives the centiles for overweight and obesity corresponding
to a body mass index of 25 and 30 kg/[m.sup.2] at age 18 for each
dataset by sex. The prevalence range at 18 years is 4.7-18.1% for
overweight and 0.1-4.0% for obesity.
Table 1 Centiles and z scores for overweight and obesity
corresponding to body mass index of 25 kg/[m.sup.2] and 30 kg/[m.sup.2]
at age 18 years in six datasets, derived from fitted LMS curves
Males
% above cut
Country Centile z score off point
Body mass index 25 kg/[m.sup.2]
Brazil 95.3 1.68 4.7
Great Britain 90.4 1.30 9.6
Hong Kong 88.3 1.19 11.7
Netherlands 94.5 1.60 5.5
Singapore 89.5 1.25 10.5
United States 81.9 0.91 18.1
Body mass index 30 kg/[m.sup.2]
Brazil 99.9 3.05 0.1
Great Britain 99.1 2.37 0.9
Hong Kong 96.9 1.86 3.1
Netherlands 99.7 2.71 0.3
Singapore 98.3 2.12 1.7
United States 96.7 1.84 3.3
Females
% above cut
Country Centile z score off point
Body mass index 25 kg/[m.sup.2]
Brazil 84.8 1.03 15.2
Great Britain 88.3 1.19 11.7
Hong Kong 90.2 1.29 9.8
Netherlands 93.5 1.52 6.5
Singapore 93.0 1.48 7.0
United States 83.5 0.97 16.5
Body mass index 30 kg/[m.sup.2]
Brazil 98.0 2.06 2.0
Great Britain 98.8 2.25 1.2
Hong Kong 98.2 2.10 1.8
Netherlands 99.7 2.73 0.3
Singapore 99.0 2.33 1.0
United States 96.0 1.76 4.0
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Figure 1 presents the centile curves for overweight for the six
datasets by sex, passing through the adult cut off point of 25 kg/[m.sup.2]
at age 18. Figure 2 gives the corresponding centile curves for obesity
in each dataset, passing through a body mass index of 30 kg/[m.sup.2] at
age 18. The curves are reasonably consistent across countries between
ages 8 and 18, although those for Singapore are higher between ages 10
and 15. This is due partly to an increased median and partly to greater
variability.
[Figures 1-2 ILLUSTRATION OMITTED]
Table 2 and figure 3 show international cut off points for body mass
index for overweight and obesity from 2-18 years, obtained by averaging
the centile curves in figures 1 and 2. From 2-6 years the cut off points
do not include Singapore because its data start at age 6 years.
[Figure 3 ILLUSTRATION OMITTED]
Table 2 International cut off points for body mass index for
overweight and obesity by sex between 2 and 18 years, defined to pass
through body mass index of 25 and 30 kg/[m.sup.2] at age 18, obtained by
averaging data from Brazil, Great Britain, Hong Kong, Netherlands,
Singapore, and United States
Body mass index 25 Body mass index 30
kg/[m.sup.2] kg/[m.sup.2]
Age (years) Males Females Males Females
2 18.4 18.0 20.1 20.1
2.5 18.1 17.8 19.8 19.5
3 17.9 17.6 19.6 19.4
3.5 17.7 17.4 19.4 19.2
4 17.6 17.3 19.3 19.1
4.5 17.5 17.2 19.3 19.1
5 17.4 17.1 19.3 19.2
5.5 17.5 17.2 19.5 19.3
6 17.6 17.3 19.8 19.7
6.5 17.7 17.5 20.2 20.1
7 17.9 17.8 20.6 20.5
7.5 18.2 18.0 21.1 21.0
8 18.4 18.3 21.6 21.6
8.5 18.8 18.7 22.2 22.2
9 19.1 19.1 22.8 22.8
9.5 19.5 19.5 23.4 23.5
10 19.8 19.9 24.0 24.1
10.5 20.2 20.3 24.6 24.8
11 20.6 20.7 25.1 25.4
11.5 20.9 21.2 25.6 26.1
12 21.2 21.7 26.0 26.7
12.5 21.6 22.1 26.4 27.2
13 21.9 22.6 26.8 27.8
13.5 22.3 23.0 27.2 28.2
14 22.6 23.3 27.6 28.6
14.5 23.0 23.7 28.0 28.9
15 23.3 23.9 28.3 29.1
15.5 23.6 24.2 28.6 29.3
16 23.9 24.4 28.9 29.4
16.5 24.2 24.5 29.1 29.6
17 24.5 24.7 29.4 29.7
17.5 24.7 24.8 29.7 29.8
18 25 25 30 30
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Discussion
Our method addresses the two main problems of defining
internationally acceptable cut off points for body mass index for
overweight and obesity in children.[11 12] The reference population was
obtained by averaging across a heterogeneous mix of surveys from
different countries, with widely differing prevalence rates for obesity,
whereas the appropriate cut off point was defined in body mass index
units in young adulthood and extrapolated to childhood, conserving the
corresponding centile in each dataset.
Although less arbitrary and potentially more internationally
acceptable than other cut off points, this approach still provides a
statistical definition, with all the implied advantages and
disadvantages.[14] Our terminology corresponds to adult cut off points,
but the health consequences for children above the cut off points may
differ from those for adults. Nonetheless, the cut off points based on a
heterogeneous worldwide population can be applied widely to determine
whether the children and adolescents they identify are at increased risk
of morbidity related to obesity.
Agreement of the centile curves
The major uncertainty with our approach, and the test of its validity,
is the extent to which the centile curves for the datasets are of the
same shape. Figures 1 and 2 show that although the agreement is
reasonable it is not perfect.
Nothing obvious explains Singapore's unusual pattern of overweight in
puberty. Omitting it from the averaged country curves would lower the
cut off points for both sexes by less than 0.4 body mass index units at
age 11-12. Therefore, even though Singapore looks different from the
other countries, its impact on the cut off points is only modest.
Because there is no a priori reason to exclude Singapore, and because so
little is known about growth patterns across countries, we have chosen
to retain it in the reference population.
Extending the dataset
We recognise that the reference population made up of these countries
is less than ideal. It probably reflects Western populations adequately
but lacks representation from other parts of the world. The Hong Kong
sample may, however, be fairly representative of the Chinese, and the
Brazilian and US datasets include many subjects of African descent.
Although additional datasets from Africa and Asia would be helpful, our
stringent inclusion criteria of a large sample, national
representativeness, minimum age range 6-18 years, and data quality
control, mean that further datasets are unlikely to emerge from these
continents in the foreseeable future. To our knowledge no other
available surveys satisfy the criteria. It is not realistic to wait for
them because there is an urgent need for international cut off points
now. Also, our methodology aims to adjust for differences in overweight
between countries, so it could be argued that adding other countries to
the reference set would make little difference to the cut off points.
None the less, further research is needed to explore patterns of body
mass index in children in Africa and Asia.
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Puberty
The body mass index curves in figure 3 show a fairly linear pattern
for males but a higher and more concave shape for females. This sex
difference can also be seen in the individual curves of figures 1 and 2
reflecting earlier puberty in females. The sensitivity of the curve's
shape to the timing of puberty may affect the performance of the cut off
points in countries where puberty is appreciably delayed,[15] although
delays of less than two years are unlikely to make much difference.
Conclusions
Our analysis provides cut off points for body mass index in childhood
that are based on international data and linked to the widely accepted
adult cut off points of a body mass index of 25 and 30 kg/[m.sup.2]. Our
approach avoids some of the usual arbitrariness of choosing the
reference data and cut off point. Applying the cut off points to the
national datasets on which they are based gives a wide range of
prevalence estimates at age 18 of 5-18% for overweight and 0.1-4% for
obesity. A similar range of estimates is likely to be seen from age
2-18. The cut off points are recommended for use in international
comparisons of prevalence of overweight and obesity.
What is already known on this topic
Child obesity is a serious public health problem that is surprisingly
difficult to define
The 95th centile of the US body mass index reference has recently
been proposed as a cut off point for child obesity, but like previous
definitions it is far from universally accepted
What this study adds
A new definition of overweight and obesity in childhood, based on
pooled international data for body mass index and linked to the widely
used adult obesity cut off point of 30 kg/[m.sup.2], has been proposed
The definition is less arbitrary and more international than others,
and should encourage direct comparison of trends in child obesity
worldwide
We thank Carlos Monteiro (Brazil), Sophie Leung (Hong Kong), Machteld
Roede (the Netherlands), and Urea Rajan (Singapore), for allowing us
access to their data.
Contributors: TJC had the original idea, did most of the statistical
analyses, and wrote the first draft of the paper. TJC, MCB, KMF, and WHD
provided the data. KMF did further analyses of the US data. All authors
attended the original childhood obesity workshop, participated in the
design and planning of the study, discussed the interpretation of the
results, and contributed to the final paper. TJC will act as guarantor
for the paper.
Funding: This work was supported by the Childhood Obesity Working
Group of the International Obesity Task Force. TJC is supported by a
Medical Research Council programme grant.
Competing interests: None declared.
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(Accepted 21 January 2000)
Department of Epidemiology and Public Health, Institute of Child
Health, London WC1N 1EH
Tim J Cole professor of medical statistics
International Obesity Task Force Secretariat, London NW1 2NS
Mary C Bellizzi health policy officer
National Center for Health Statistics, Centers for Disease Control
and Prevention, Hyattsville MD 20782, USA
Katherine M Flegal senior research scientist
Division of Nutrition and Physical Activity, Centers for Disease
Control and Prevention, Atlanta GA 30341-3724, USA
William H Dietz director |
  
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