Official child obesity figures include tens of thousands of children who aren’t clinically obese

New data obtained by Full Fact shows that official child obesity figures include tens of thousands of children who are not clinically obese, because they use a broader definition than doctors do.

This means the common claim that one in five children leave primary school with obesity—made by the health secretary Wes Streeting and NHS England, among many others—significantly overstates the proportion of children who are obese, if you define obesity clinically. The proportion of children leaving primary school who are clinically obese is more like one in seven.

Childhood obesity is a very important subject, with big implications for the health and wealth of the whole country—no matter which threshold is used to measure it. But it’s vital that the statistics on childhood obesity used by politicians, government and the health service are clear and accurate.

The new data, which we obtained earlier this year with a Freedom of Information request to NHS England, and which has since been published online, shows that in 2023/24 about 6% of 4-5-year-olds and 15.2% of 10-11-year-olds met the clinical thresholds for obesity. This compares to the 9.6% of 4-5-year-olds and 22.1% of 10-11-year-olds recorded as obese in the official NHS publication.

Full Fact’s analysis suggests this means 20,642 4-5-year-olds and 41,936 10-11-year-olds who are not clinically obese were counted as obese in the 2023/24 figures, which have since been used by ministers and the NHS.

The difference isn’t about how the children are measured. It’s the same height and weight data, gathered by the National Child Measurement Programme (NCMP). The distinction is which threshold is used to classify someone as obese.

The Department of Health and Social Care (DHSC) told us that it and NHS England are transparent on the use of population monitoring and clinical thresholds, and how they differ. It said that both are needed to assess children’s BMI, with population monitoring thresholds providing robust data on trends because they have been consistently used since the early 2000s.

The data for 2024/25 will be published next week. We do not yet know whether it will be reported differently.

What counts as obese?

Since 2006, the NCMP has measured the height and weight of every child in Reception (aged four or five) and Year 6 (aged 10 or 11) in English state schools. These measurements are then used to calculate each child’s body mass index (BMI)—essentially a number that captures how heavy someone is for their height.

In adults, a BMI above 25 is considered overweight, and anyone above 30 is considered obese. Children are more complicated however, as their bodies can vary widely between boys and girls at different ages.

To account for this, childhood categories are defined differently, by comparing a child’s BMI to the range of measurements for their age and sex that was recorded between 1978 and 1994 (known as the UK90 reference data). Children with a BMI that puts them high on the scale are classified as overweight or obese.

In practice, to the nearest whole number, this means that a child whose BMI puts them in the top 9% of the range for their age and sex in the UK90 data is considered clinically overweight by the NHS (also known as the top nine centiles), and a child whose BMI is in the top 2% is considered clinically obese.

These are the thresholds that doctors and nurses are advised to use by the National Institute for Health and Care Excellence (NICE), and which are applied by the NHS’s own childhood BMI calculator. Parents are sent letters about the weight status of their children based on these thresholds too.

But these are not the thresholds that the NCMP uses in the data it publishes on overweight and obese children every year. It categorises those in the top 15% of the range for their age and sex in the UK90 data as overweight, and those in the top 5% as obese instead—effectively including more children.

Why use different definitions?

We spoke to Professor Ken Ong, a specialist in childhood obesity at the University of Cambridge, who explained that for many years the 95th percentile was used both in the UK and internationally as the obesity threshold in children, largely as a statistical convention.

In some countries this remains the case. The US, for example, still uses the 95th centile, but this means something different because it uses a different reference population, based on data collected from American children between 1963 and 1980. So a 12-year-old boy who’s 150cm tall and weighs 54.5kg would be on the 98th centile in the UK and the 95th in the US, and be classified obese in both.

According to Professor Ong, the UK has now moved its clinical cut-off to the 98th percentile to reflect a more stringent definition, while the NCMP kept using the 95th.

He also told us there’s fairly good evidence about which BMI levels are harmful in adults, but an absence of good data when it comes to children. “So instead,” he said, “everyone around the world relies on an imperfect situation, of working out when a child is unusually large. That’s where 95th came from, because it’s one in 20. That’s our conventional statistical approach to distinguishing between what is normal and abnormal. So for decades we’ve used the 95th… Until for various reasons, the UK and some other countries moved to the 98th.”

As an NHS document from 2011 says:

• “Population monitoring thresholds are used for most published obesity and overweight prevalence figures, e.g. those using Health Survey for England and National Child Measurement Programme (NCMP) data
• Clinical cut‐offs are recommended by NICE for use in clinical settings with individual children. Also used for NCMP parental feedback and the NHS choices BMI calculator.”

In its notes on the data, NHS England claimed until recently that it did things this way on the recommendation of the National Institute for Health and Care Excellence (NICE). The notes said: “These cut offs are based on the assumption that around 15% of the baseline population were overweight and 5% were living with obesity in 1990. This approach is recommended by the National Institute for Health and Care Excellence.”

Yet this seems not to be correct. NICE told Full Fact that it recommends using the 2% and 9% cutoffs in clinical practice, and has not recommended anything different for population monitoring.

NHS England told us in July that this was a matter for the DHSC, which oversees the NCMP, and the DHSC did not answer our question on this point. However, the notes have since been corrected, in a change made earlier this month. They now say: “The use of UK90 reference data is recommended by the National Institute for Health and Care Excellence.”

The Health Survey for England, a different programme, also uses the population monitoring thresholds for children, and the NHS talks about this data as a measure of obesity in the same way.

Obese, or at risk of obesity?

Strangely, even the NCMP itself seems to acknowledge once, in a section of its website explaining definitions, that what it calls obesity everywhere else isn’t really a measure of obesity. The note says it uses the wider boundaries “to capture children in the population in the clinical overweight or obesity BMI categories and those who are at high risk of moving into the clinical overweight or clinical obesity categories [our italics]. This helps ensure that adequate services are planned and delivered for the whole population.” A DHSC note on its obesity data says the same.

Yet the NHS publications that include the NCMP data do not say that it measures those ‘with or at risk of’ obesity or being overweight. Instead they say that the figures show the “prevalence” of those categories. This contrasts with Public Health Scotland, which refers to children “at risk of” obesity and being overweight throughout its publications, and uses the 85th and 95th centiles as well, calling them the “epidemiological thresholds”.

Nor, until we obtained it with a Freedom of Information request, does NHS England appear to have ever previously published the data using the clinical thresholds, meaning people were unable to see how different the clinical data was. Again, Public Health Scotland does publish this.

Are we overstating obesity in children?

There’s not necessarily anything wrong with using wider thresholds for some purposes. Larger numbers may produce more reliable rates at a local level that make it easier to plan services, for example.

But it’s unclear why ministers and the NHS are classifying tens of thousands of children above the 95th centile as obese when, clinically, many of them are at risk of it, but not actually obese. Clinicians don’t consider children on the 95th centile obese. The NCMP itself doesn’t tell parents of children on the 95th centile that they are obese.

It’s also worth noting that when you compare levels of adult and childhood obesity in England, as currently measured in the Health Survey for England, the rate seems to plunge inexplicably when children reach 16.

This is a widely recognised weakness in this system. As Dr Christopher Snowdon, head of lifestyle economics at the Institute for Economic Affairs, has pointed out, it isn’t plausible that English children suddenly get much slimmer at the end of their childhood, or that a level of body fat that is very unhealthy at 15 suddenly becomes healthy shortly afterwards. If the adult obesity threshold is based on good evidence, then the same evidence suggests the NCMP’s population monitoring threshold for children is too low.

Dr Snowdon told Full Fact he believed the measure of childhood obesity used in the national prevalence statistics is “horribly flawed”.

Although the figures obtained by Full Fact reveal for the first time how many children who are not clinically obese are being counted in the NHS’s headline statistic, debate over how childhood obesity should be defined is nothing new.

In 2002, a government report on the Health Survey did not use what it called the “arbitrary” 85th and 95th cutoffs as its primary measure, saying there seemed to be “no indication that these cut-off points relate directly or indirectly to any physiological outcomes or health or disease risks”.

In 2007, Professor Tim Cole, who devised the growth charts themselves, told BBC News that the cutoffs were not based on clear evidence of which children were healthy and which were not. He told the BBC: “The idea that these numbers are cast in stone is absolute nonsense. It is all built on sand."

The authors of the government-commissioned independent National Food Strategy report, published in 2021, said the fall in obesity rates at 16 was “a quirk of data definition”. They decided not to use the famous “one in five” figure, saying “we believe the way this statistic is measured is problematic and probably worth rethinking”.