Author: Dr Rachael Wood, Consultant in Public Health Medicine, NHS National Services Scotland, Information Services Division

Being overweight or obese means having excess body fat to the extent that increases the risk of health problems.

There is no one ideal measure of body fat, however body mass index (BMI) is broadly accepted as a reasonable indicator. BMI measures a person’s weight in relation to their height (calculated as weight [in kilograms] divided by height squared [in meters2]). It can be calculated for children aged 2 years or over.

Defining overweight and obesity in adults aged 18 years or over using BMI is straightforward. A BMI of 25 to 30 places an individual in the overweight category. A BMI of 30 or over classifies an individual as obese. Having a BMI this high has been shown to be associated with a range of health problems.

The situation is more complicated for children. Children’s BMI naturally changes as they grow up, and it is different for boys and girls. Fixed BMI cut off values therefore cannot be used to define overweight and obesity in childhood.

A child’s BMI must first be compared to the range of BMIs seen for children of the same age and sex in a chosen reference population to determine the child’s BMI centile. The centile reflects the percentage of children in the reference population that had a BMI lower than that of the child being considered. Fixed BMI centile values are then used to define overweight and obesity.

In the UK, the ‘UK90’ reference is usually used as the reference population for children aged 4 years and above. This reference is based on various surveys carried out across the UK between 1978 and 1994 and can therefore roughly be thought of as representing British children in the 1980s.

There is less agreement about the BMI centile cut off values that should be used to indicate overweight or obesity in childhood. Two sets of thresholds have been proposed for use in the UK: the first for monitoring the health of the whole child population (‘epidemiological thresholds’) and the second for assessing the health of individual children (‘clinical thresholds’). The epidemiological thresholds classify children with BMI centile of 85 to less than 95 as 'at risk of overweight'  and of 95 or over as 'at risk of obesity'. The clinical thresholds are more stringent and classify children with BMI centile of 91 to less than 98 as 'overweight', and of 98 or over as 'obese'.

The lack of agreement about centile cut off values reflects some uncertainty about exactly how high BMI/BMI centile has to be in childhood before it leads to increased health risks. Despite some uncertainty about the exact threshold at which risks increase, there is no doubt that high BMI in childhood is associated with short and long term health risks such as high blood pressure, type 2 diabetes, asthma, and joint problems. Overweight or obese children are also at high risk of becoming overweight or obese adults, with the associated health risks that brings.

In summary, when looking at statistics on childhood overweight and obesity, it is important to be aware of the reference population that has been used for comparison, and the BMI centile cut off values that have been used to define the overweight and obesity categories. This allows you to interpret the information in a meaningful way and to compare like with like.

 Using BMI centile to define child overweight and obesity

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