The Prevalence of obesity in British children – is BMI telling us the whole story? Dr. David McCarthy RNutr Institute of Health Research & Policy London Metropolitan University 17 th February 2005
Body Mass Index (BMI) in adults - affected minimally by age popular, quick, use fixed cut-off points in children - greatly affected by age
Early childhood prevalence in the UK Obese 17% (BMI >95 th centile) Overweight 31% (BMI >85 th centile) Source: Reilly et al BMJ 319: year olds
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
Correlation of BMI with Fat Mass (kg) r 2 = 0.763, P< year olds
Correlation of BMI with Fat Free Mass (kg) r 2 = 0.514, P< year olds
Assessment requirements in children Simple measurement technique Strongly related to morbidity or risk (CHD) Biological/clinical definition Avoid misclassification
Central body fat accumulation in children Intra-abdominal adipose tissue Subcutaneous abdominal adipose tissue assessed by circumference and skinfold measurements
Is waist circumference in children linked to risk? WC related to an atherogenic lipid profile in yo (Flodmark et al. 1994) WC related to adverse insulin levels in 5-17 yo ( Bogalusa Heart Study, Freeman et al. 1999)
Waist circumference and blood pressure Jarrett, McCarthy et al. (unpublished observations in 4 and 5 year olds)
The Metabolic Syndrome
Waist circumference measurement Midway between the 10 th rib and the iliac crest WHO standard method Used by : McCarthy et al Freedman et al Moreno et al. 1999
Waist circumference measurement continued. At the level of the umbilicus Used for the waist circumference percentiles in Italian children, Zanolli et al. 1996
UK children's waist circumference centile study 8355 children aged 5-17 years mean and SD for waist circumference smoothed percentile curves constructed using the LMS method (Cole 1990)
n, 8355 McCarthy et al. 2001, EJCN Development of WC centile charts for the UK children
Boys 3-17 y © D.McCarthy2001
Girls 3-17 y © D.McCarthy2001
Boys aged y p<0.01 r 2 =0.689 n=349 Waist circumference vs BMI (cm)
Girls aged y p<0.01 r 2 =0.716 n=400 Waist circumference vs BMI (cm)
BMI-WC relationship SubjectAgeBMI WC BMI %ile WC %ile (y) (cm) A ~50th <9th B ~50th >98th C >91st ~50th D >98th>99.6th
Has upper body fatness increased in UK children? Comparison of data collected 10 and 20 years apart BSI and NDN surveys Children aged years
McCarthy et al BMJ 326: 624 NDNS boys NDNS girls BSI boys BSI girls
McCarthy et al BMJ 326: 624 NDNS girls NDNS boys BSI girls BSI boys
Changes over years in overweight and obesity based on BMI and waist circumference in British children aged years. Values are % exceeding 91 st centile (98 th centile) % prevalence of overweight % change (obesity)over time BSI 1977/87NDNS 1997 MaleFemaleMaleFemaleMaleFemale BMI (3.3)(1.6)(10.0)(8.3)(6.8)(6.6) WC (3.3)(3.1)(13.8)(17.1)(10.7)(14.5) McCarthy et al BMJ 326: 624
Is this also seen in younger children? Comparison of WC and BMI in children aged 2-5 years from the ALSPAC study with those in the BSI survey McCarthy et al Int J Obesity 29:
Key findings from ALSPAC study Mean WC greater in contemporary children Increases greater in girls Mean BMI slightly (but significantly) higher Proportional increase in WC exceeded that for BMI
Causative factors to consider – TV viewing displaces physical activity Increased energy consumption Role of advertising Reduced resting metabolism
Causative factors to consider – eating habits and energy intake
Energy dense drinks Cola – 11% sugar Orange juice – 10% sugar Juices perceived as being ‘healthier’
Conclusions and recommendations Several limitations of BMI use in children Excess upper body fatness more closely linked to morbidity than general fatness WC better than BMI in identifying obesity- related risk in children WC measurement should be taken routinely in obesity assessment in children More research needed into excess upper body fat accumulation in childhood