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The Use of the Body Mass
Index in Under 18 year oldsDr. David McCarthy RNutr
Institute ofHealth Research & Policy
London Metropolitan University28th March 2004
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Body Mass Index (BMI)
in adults - affected minimally by age
popular, quick, use fixed cut-off points
in children - greatly affected by age
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Drawbacks of BMI in children
Age-dependent
Correlates with both fat
mass and fat-free mass Low sensitivity
no indication of body
fat distribution Cannot identify secular
trends
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BMI
40302010
FatMas
s(kg)
40
30
20
10
0
Correlation of BMI with Fat Mass (kg)
r2= 0.763, P
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Cut-off points
US 85th
and 95th
centiles UK 91st and 98th centiles
IOTF centiles relating to BMI of 25and 30 at age 18 years
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Childhood prevalence
in the UK
Obese 17% (BMI >95th centile)
Overweight 31% (BMI >85th centile)
Source: Reilly et al. 1999
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Waist circumference
in adults
Indicator of intraabdominal fat
related to risk for NIDDM,hypertension and CVD
waist:hip and waist:height ratios
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Is waist circumference in
children linked to risk?
WC related to an adverse atherogenic
lipoprotein profile in 12-14 year olds(Flodmark et al. 1994)
WC related to adverse insulin
concentration in 5-17 year olds(Bogalusa Heart Study, Freeman et al. 1999)
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Waist circumference
and blood pressure
Jarrett, McCarthy et al. (unpublished
observations, 2002), 4 and 5 year olds)
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The Metabolic Syndrome
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WC percentile charts
in children
Italian children (Zannolli & Morgese
1996)
Spanish children (Moreno et al. 1999)
Cuban children (Martinez et al. 1994)
British children (McCarthy et al. 2000)
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Waist circumference measurement
Midway between the 10th rib
and the iliac crest
WHO standard method
Used by :
McCarthy et al. 2000
Freedman et al. 1999
Moreno et al. 1999
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Waist circumference measurementcontinued.
At the level of the umbilicus
Used for the waist circumference
percentiles in Italian children,
Zanolli et al. 1996
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UK children's study
8355 children aged 5-17 years
mean and SD for waistcircumference
smoothed percentile curvesconstructed using the LMS method(Cole 1990)
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4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Age (years)
40
50
60
70
80
Waistcircumference
(cm)
Boys
Girls
Waist in children
n, 8355 McCarthy et al. 2001, EJCN
Development of WC centile charts for the UK children
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What about clothing effects? Preferable to measure
over skin
need to take account of
vest, T-shirt or school
shirt
subtract 0.5 cm from
reading
problem in measuringover tunics
22Waist circumference vs BMI
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Waist Girth WsGr
70605040
BMI
22
20
18
16
14
12
Boys aged 6.0-6.99 y
p
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Waist Girth WsGr
8070605040
BMI
24
22
20
18
16
14
12
10
Girls aged 6.0-6.99 y
p
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BMI-WC relationship
Subject Age BMI WC BMI %ile WC %ile
(y) (cm)
A 7.7 15.2 48 ~50th 98th
C 7.1 19.1 53 >91st ~50th
D 7.1 20.0 67 >98th >99.6th
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Has upper body fatness
increased in British children?
Comparison of data collected10 and 20 years apart
BSI and NDN surveys
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Ch 10 20 i BMI d i t
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Changes over 10-20 years in mean BMI and waist
circumference in British children aged 11-16 years.
Mean SD Score (SD) Mean increase
over time (SE)
BSI 1977/87 NDNS 1997
Male Female Male Female Male Female
BMI -0.05 -0.15 0.42 0.38 0.47 0.53
(1.02) (0.99) (1.13) (1.09) (0.06) (0.06)
WC 0.00 0.00 0.84 1.02 0.84 1.02
(0.99) (1.00) (1.02) (1.33) (0.06) (0.06)
McCarthy et al. 2003. BMJ326: 624
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How should I use the WC
percentile charts?
In conjunction with BMI centile charts
as a second line point of reference for
referral
routinely
epidemiology
research
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Further issues
Cut-off points yet to be linked to risk or
Fat Mass
What effect does measurement error
have upon validity of WC percentiles?
Relationship between BMI and WC
centile
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Conclusions
Waist circumference important in children
Simple technique
WC correlates with risk factors for CHD
further work required on validation of
technique
other countries could begin to develop WC
percentiles in their childhood population
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Genetics Prenatal diet
Leg length?
Infant feeding
Current diet
Physical
activity
central obesity
Catch-up
growth
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Acknowledgements
Karen Jarrett
Adam Collins
Prof. Tim ColeSandra Ellis
Pauline Emmett
Jean Golding andthe ALSPAC
team