Childhood BMI Statistics

BMI Statistics for Children in Primary 1 (updated annually)

Information is presented for children in Primary 1 (aged 4 to 6 years) from school year 2000/01 to 2008/09.  Data are available for 11 NHS Boards which participate in the CHSP-School system.  The BMI statistics cover approximately 70% of children in Primary 1 among the 11 participating NHS Boards, and approximately 62% of children in Primary 1 across Scotland.

Primary 1 Statistics for HIGH BMI (includes obesity statistics) ( 346KB)
    Updated 15 December 2009

Primary 1 Statistics for LOW BMI ( 271KB)
    Updated 15 December 2009

Estimated data completeness  ( 248KB)
    New at 15 December 2009

The Statistical Publication Notice   explains the key points from the 15 December 2009 release.

Key Points

Based on centile cut-offs on the 1990 UK growth reference charts used for population monitoring purposes:

1. In 2008/09, among the eleven participating NHS Boards, 19.8% of Primary 1 children were classified as overweight, including 8.0% obese and 3.9% severely obese.

2. In general: Levels of high BMI increased slightly, and very gradually, between 2000/01 and 2005/06. Over the last three years, levels of high BMI have decreased slightly and the percentages for 2008/09 are similar to those for 2000/01 (19.7% overweight, including 8.0% obese and 3.9% severely obese).  As the number of NHS Boards submitting data has increased since 2000/01 (from four to eleven Boards) the trend for "All participating NHS Boards" should be interpreted with a degree of caution.  However, a similar trend is observed among the Boards participating throughout the nine year period.

3. In Primary 1, levels of high BMI amongst boys tend to be slightly higher than those for girls.  In school year 2008/09, 20.2% of boys were classified as overweight (including 8.1% obese and 4.0% severely obese) compared to 19.4% of girls (including 8.0% obese and 3.8% severely obese).

4. Primary 1 figures for 2008/09 indicate that the most deprived areas have the highest percentage of children classified as overweight, obese and severely obese (21.2% overweight, including 9.2% obese and 4.7% severely obese) while the least deprived areas had the lowest percentage (16.7% overweight, including 6.2% obese and 2.6% severely obese), however this pattern is not clearly observed for all previous years.

5. Among participating Boards, the percentage of Primary 1 school children with low BMI (classified as underweight) was 3.4% in 2008/09.  Levels of low BMI have remained relatively stable over the period 2000/01 to 2008/09 at approximately 3% - 3.5%.

Historical Statistics

Height and weight for Pre-school children and those in Primary 7 and Secondary 3 are not routinely collected on the CHSP.  However, historical BMI statistics, for the limited number of NHS Boards who carried out reviews for children in these age groups, are available via the links below:

Pre-school (children under 5)   ( 110KB)            
Information is presented for children born between 1995 and 2001.
Last updated on 19 December 2006.

Primary 7 (children aged 10 - 12 years)  ( 82KB) 
Information is presented for school children in Primary 7, for school years 2000/01 to 2004/05.
Last updated on 19 December 2006.

Secondary 3 (children aged 13 - 15 years)  ( 83KB)
Information is presented for school children in Secondary 3, for school years 2000/01 to 2004/05.
Last updated on 19 December 2006.

Key points
Last updated on 19 December 2006.

BMI and statistics background notes

Calculating BMI statistics

Body mass index is calculated by dividing an individual's weight in kilograms by their height in metres squared.  For epidemiological purposes, an individual BMI is not meaningful in isolation, only in the context of the distribution of values for a population.  Individuals can be assigned to categories e.g. underweight (using their BMI or centiles derived from BMI) to gauge where they lie in relation to the rest of the population - in particular, whether they have an unusually high or low BMI.

In adults, BMI can be directly classified into different categories (for example, the World Health Organisation international classification of "underweight" is BMI < 18.5) since age in adulthood doesn't greatly affect weight in relation to height.  However, these adult BMI category cut-offs aren't appropriate for children since BMI changes markedly as a child ages.  A certain BMI at one age may be the norm but for another age the same BMI may be unusually high or low (indicating that the child is overweight or underweight).

Instead, for children, BMI can be converted into centiles, using UK 1990 growth reference data based on sex and age in months.  These centiles can then be used to categorise BMI as detailed in the table below:

Category	Definition	What this means
Very low BMI (very underweight)	Less than or equal to 2nd centile	Children whose BMI is within the bottom 2% of the 1990 UK reference range for their age and sex.
Low BMI (underweight)	Less than or equal to 5th centile	Children whose BMI is within the bottom 5% of the 1990 UK reference range for their age and sex.
Overweight	Greater than or equal to 85th centile	Children whose BMI is within the top 15% of the 1990 UK reference range for their age and sex.
Obese	Greater than or equal to 95th centile	Children whose BMI is within the top 5% of the 1990 UK reference range for their age and sex.
Severely obese	Greater than or equal to 98th centile	Children whose BMI is within the top 2% of the 1990 UK reference range for their age and sex.

The number of children within each of these categories can then be used to calculate the percentage of children reviewed who are: very underweight, underweight, overweight, obese and severely obese.  Children with a BMI within the 5th - 85th centile range are considered to be in the healthy weight range (although BMI may incorrectly categorise a small minority of children with heavy musculature as being overweight or obese).

Reference standards

1990 UK growth reference standards

In 1995, new reference growth curves for the weight and height of UK children were published, replacing the Tanner-Whitehouse reference curves used since the 1960s.  The new curves represent UK children in 1990 and are widely accepted as the reference for growth screening for the UK.  The reference data used were collected between 1978 and 1990 (and therefore represent weight relative to height before the recent rise in levels of obesity in children) and were obtained by combining data from 11 distinct surveys that were representative of children in England, Scotland and Wales.  From this national dataset, BMI reference curves for children and young people were established providing BMI centiles covering birth to 23 years of age.

We have used 1990 UK Reference data and Cole's LMS method to produce BMI centiles for Scottish children (Cole TJ, Freeman JV and Preece MA.   Body mass index reference curves for the UK, 1990. Arch Dis Child 1995; 73 : 25-9) as recommended in SIGN guidance.

International growth reference standards

The World Health Organisation has published international reference standards for infants and children.  These reference standards are derived from growth data from the Multicentre Growth Study relating to approximately 8500 children from six different countries around the world (Brazil, Ghana, India, Norway, Oman and USA).

Use of international reference standards allows international comparisons to be made (Scottish figures derived using these standards aren't available here).

Thresholds for defining very low BMI, low BMI, overweight, obese and severely obese

These pages look at trends in the percentages of children classified as very low BMI, low BMI, overweight, obese and severely obese, as defined according to centile cut-offs from the UK 1990 growth reference standards charts (very low BMI < =2^nd centile, low BMI < =5^th centile,  overweight > =85^th centile, obese > =95^th centile, severely obese > =98^th centile).

The BMI centile cut-offs used to derive the percentages overweight, obese and severely obese in these pages, are those recommended in SIGN guidance for the purposes of population monitoring and epidemiological research.  Use of the corresponding thresholds recommended by SIGN for clinical practice (overweight > =91^st centile, obese > =98^th centile, severely obese > = 99.6^th centile) would result in lower percentages for overweight, obese and severely obese and BMI centile would be only one of a variety of factors taken into consideration before a clinical diagnosis is made.

In the UK, it is generally agreed that the most appropriate clinical cut-off for classifying individual children as being underweight is < =2^nd centile.  There is no agreed definition of underweight for population monitoring purposes but a reasonable threshold would be < =5^th centile (see Cole TJ, Flegal KM, Nicholls D, Jackson AA. Body mass index cut offs to define thinness in children and adolescents: international survey. BMJ 2007; 335: 194-7 and Dinsdale H, Rutter H et al. National Child Measurement Programme: Detailed Analysis of the 2006/07 National Dataset. National Obesity Observatory publication, June 2008).

Confidence intervals

The upper and lower limits for 95% confidence intervals have been included in our tables for all childhood BMI distribution percentages.  These have been produced using the Wilson method for proportions (Wilson EB. Probable inference, the law of succession, and statistical inference. J Am Stat Assoc 1927, 22, 209-12).

A confidence interval gives some indication of the precision of an estimate by providing an "error term", which when added or subtracted from the estimate gives a range of values within which the estimate lies.  For example, if we think of the estimate as being the percentage of Primary 1 children who are classified as obese, say 20%, with an error term of 0.5%, the confidence interval would be (19.5%, 20.5%).  We can be sure that the percentage of Primary 1 children who are classified as obese is between 19.5% and 20.5%.

The size of the "error" term, and so the width of the confidence interval, depends on the variability of the percentage of Primary 1 school children who are classified as obese (the larger the variability, the larger the error term and the poorer the precision) and also the sample size (in this case, the number of reviews).  The larger the number of reviews, the better the estimate is and the greater the precision.  This should be borne in mind when examining confidence intervals for areas with a relatively small number of reviews (figures for some Community Health Partnerships and smaller NHS Boards).  The confidence intervals for e.g. Western Isles NHS Board are very wide because the estimates for this NHS Board are based on small numbers, so they should be interpreted with caution.

It is also possible to use confidence intervals to gain some indication of whether e.g. the percentage of Primary 1 school children classified as obese for a particular NHS Board is statistically significantly different from the average percentage for all participating Boards.  Consider the situation where the percentages of Primary 1 children classified as obese in NHS Boards 'X' and 'Y' are below the average percentage for all participating Boards.  The confidence interval for NHS Board 'X' includes the average percentage but the confidence interval for NHS Board 'Y' does not (the upper bound of the NHS Board 'Y' confidence interval is lower than the average percentage).  We can say that we are 95% confident that the percentage of Primary 1 children classified as obese in NHS Board 'Y' is statistically significantly lower than the average percentage for all participating Boards. However, the percentage for NHS Board 'X' is not significantly lower.

Other reports on BMI distribution in children:

Obesity in Scotland - An epidemiology briefing (2007)
This report, produced by the Scottish Public Health Observatory, includes a chapter on obesity in children.

National Child Measurement Programme: results from the school year 2008/09
Established in 2005, the National Child Measurement Programme (NCMP) in England weighs and measures children in Reception (4-5 years) and Year 6 (aged 10-11 years) to assess overweight and obese levels.

Health Indicators Report - December 2004 - A Focus on Children
This report, produced by NHS Quality Improvement Scotland, explores trends in the percentage of under and over weight children between the late 1960s and early 1990s for both primary school (aged 4-6 years) and secondary school (aged 13-15 years).

Clinical Outcome Indicators Report - November 2003 
This report, produced by NHS Quality Improvement Scotland, highlights that in recent years the percentage of Scottish children estimated to be obese was higher than expected.