Child Health

Childhood Obesity

New information is presented for children in Primary 1 (aged 4 to 6 years) for school year 2006/07 (figures for school years 2000/01 to 2005/06 are also included)

Primary 1 statistics  (   606 KB)

Statistical Publication Notice

    Primary 1  - Key points

• Using the 1990 UK growth reference standards, in 2006/07, 21.0% of children in Primary 1 were overweight, including 8.5% who were obese and 4.3% who were severely obese. Although levels of high BMI decreased slightly from 2005/06 (22.0% overweight, including 9.1% obese and 4.4% severely obese) to 2006/07, overall, levels have gradually increased from those for 2000/01 (19.7% overweight, including 8.0% obese and 3.9% severely obese).
  
  
• In Primary 1, levels of BMI amongst boys tend to be slightly higher than those for girls. In school year 2006/07, 22.0% of boys were overweight (including 9.0% obese and 4.4% severely obese) compared to 20.1% of girls (including 8.0% obese and 4.1% severely obese).
  
  
• Primary 1 figures for 2006/07 indicate that the highest levels of overweight, obese and severely obese children are found in the most deprived areas (22.5% overweight, including 9.9% obese and 5.5% severely obese) while chidlren in the least deprived areas had the lowest levels (18.8% overweight, including 7.2% obese and 3.1% severely obese) however, this pattern is not clearly observed for previous years.
  
  
• The percentage of Primary 1 school children with low BMI was 3.0% in 2006/07, which is lower than the 1990 UK reference standard of 5%.

Height and weight for Preschool children and those in Primary 7 and Secondary 3 are no longer routinely collected on the CHSP. However, tables containing historical obesity statistics for children in these age ranges are available via the links below:

Preschool (children under 5)  ( 110KB)            
Information is presented for children born between 1995 and 2001.

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.

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.

Key points

BMI and statistics background notes

Calculating BMI obesity 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 reference data based on sex and age (in months). These centiles can then be used to categorise BMI as detailed in the table below:

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 normal weight range (althought BMI may incorrectly categorise a small minority of children with heavy musculature as being overweight or obese).

Reference standards

1990 UK 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 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).

Confidence intervals

The upper and lower limits for 95% confidence intervals have been included in our tables for all childhood obesity 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 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 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 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 Parternerships 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 obese Primary 1 school children for a particular NHS Board is statistically significantly different from the average percentage for all participating Boards. Consider the situation where the percentages of obese Primary 1 children 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 obese Primary 1 children 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 levels 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.

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.

Main contact: Child Health

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