Changes in BMI category of children between the first and final years of primary school, 2023 to 2024

1. Main messages

This report presents analysis examining how primary school children have moved between body mass index (BMI) categories from reception (age 4 to 5 years) to year 6 (age 10 to 11 years). This report links records for children who were in reception in 2017 to 2018 and in year 6 in 2023 to 2024. Clinical thresholds are used to categorise child BMI¹.

In summary, more children moved from a healthy weight in reception to a higher BMI category (overweight, living with obesity or severe obesity) in year 6 (91,803 which is 17.6% of all children) than the number who moved the other way from a higher BMI category in reception to a healthy weight in year 6 (17,144, 3.3% of all children). This is reflected in an increase in the proportion of children living with obesity between reception and year 6.

The other findings in this summary concentrate on the proportion of children who were a healthy weight in reception and whether they remained a healthy weight in year 6 or moved to a higher BMI category.

The majority of children in reception were a healthy weight (84.8%) in 2017 to 2018, of these:

the majority (77.6%) remained a healthy weight in year 6, when they were measured again in 2023 to 2024; this proportion was higher among girls (79.7%) than boys (75.5%) and among children living in the least deprived areas (84.7%) than the most deprived (71.8%)
the proportion remaining a healthy weight in year 6 was also higher among White British children (79.1%), White and Asian children (79.8%) and Chinese children (80.1%), and among children from the East of England (79.3%), South West (79.8%) and South East (79.9%)
20.8% moved from a healthy weight to a higher BMI category; this proportion was higher among boys (23.1%) than girls (18.4%), higher among children living in the most deprived areas (26.4%) than the least deprived (13.4%), and higher among children from Asian and Black ethnic groups

This report updates the previous analysis which was based on the pre-pandemic cohort who were in reception in 2013 to 2014 and year 6 in 2019 to 2020. Compared with this earlier cohort:

a lower proportion of children remained a healthy weight between reception and year 6 (77.6% compared with 78.8%)
a higher proportion of children moved to a higher BMI category between reception and year 6 (20.8% compared with 19.7%)

2. Introduction

This report is the second national analysis of the National Child Measurement Programme (NCMP) data which assesses how BMI category has changed in individual children during primary school from reception year (aged 4 to 5 years) to year 6 (aged 10 to 11 years). The first report was published in 2022.

National, regional and local authority data is analysed in this report and presented in the obesity profile. Data presented in the obesity profile combines 3 annual cohorts of NCMP data to deliver robust estimates at local authority level, and therefore, England prevalence and inequalities analysis in the obesity profile will have different values to those presented in this report which are based on the latest cohort.

Since the 2013 to 2014 data collection year, the NCMP IT system has collected data items (name, NHS number, date of birth and postcode) to enable the linking of a child’s measurements in reception and their subsequent measurement in year 6.

Clinical BMI thresholds, recommended for use when assessing individual children, were used to assign children to a BMI category as the analysis tracks individual children. Therefore, proportions of children within each BMI category will be different from published population level prevalence estimates that use the population monitoring BMI thresholds. Further information about the clinical BMI thresholds is in the methods section of this report (chapter 7). For this analysis, children living with obesity are presented as 2 groups:

children living with obesity excluding severe obesity and
children living with severe obesity

Children living with severe obesity are at greater risk than their peers of immediate and long-term serious health conditions.

Throughout the report, statistically significant differences are determined by comparing to the average for all children in England. Where a movement between BMI categories for a subgroup of children is described as higher, lower, bigger, or smaller it is significantly different from the same movement for all reception children. However, note that not all significant differences are commented on but are included in the spreadsheet published with this report.

The charts in this report are interactive. Users can hover over the charts to see more details. Data throughout the report is presented for children in reception who have moved to year 6. All the data used in this report is available to download from the gov.uk website.

3. Changes in BMI category

Figure 1a and 1b show the change in child BMI category from reception to year 6. Figure 1a shows the data as a bar chart, and figure 1b shows the flow of movement between BMI categories between reception and year 6. The thickness of the line linking 1 BMI category to another represents the number of children moving between the BMI categories. Only lines where 200 or more children moved out of a BMI category in reception are shown on figure 1b. Two hundred measurements equates to around 0.05% of all measurements. This simplifies the chart and makes it easier to see movements which are experienced by larger groups of children.

In England:

the majority of children were a healthy weight in reception (84.8%); 77.6% of these children remained a healthy weight in year 6, and 20.8% moved to a higher BMI category
of the 14.1% of children who were in a higher BMI category the majority (76.6%) remained in a higher BMI category and 23.4% moved to a healthy weight

Figure 1a and 1b: Changes in child BMI category between reception aged 4 to 5 years and year 6 aged 10 to 11 years

Movements with less than 200 measurements are not shown on this chart. 200 measurements equates to less than 0.05% of all measurements.

Bar charts are used in the remainder of the report as they make it easier to compare movements between groups, for example between boys and girls, than comparing 2 Sankey diagrams (flow diagram as shown in figure 1b). However, they do not show the number of children in different BMI categories so comparisons back to figure 1b are useful as a reminder that the majority of children are in the healthy weight category followed by the overweight and obesity (excluding severe obesity) categories. The severe obesity and underweight categories contain the smallest number of children.

4. Changes in BMI category by demographics

This section of the report looks at how patterns of change in BMI category varied between different demographic groups for children who were in the latest cohort of children and were in reception (aged 4 to 5 years old) in 2017 to 2018 and in year 6 (aged 10 to 11 years old) in 2023 to 2024.

4.1 Sex

The patterns of change in BMI categories between reception and year 6 were different between boys and girls. Figure 2 shows the changes in child BMI category by sex compared with all children in this analysis:

a larger proportion of girls (79.7%) than boys (75.5%) who were a healthy weight in reception remained a healthy weight in year 6, and a lower proportion of girls (18.4%) than boys (23.1%) moved to a higher BMI category

Figure 2: Changes in child BMI category between reception aged 4 to 5 years and year 6 aged 10 to 11 years, by sex

4.2 Ethnic group

There are known inequalities in BMI category by ethnicity, with children from Black and some Asian ethnic groups having higher prevalence of obesity than children of White British ethnicity. Data is presented here for the 17 ethnic groups recorded in the NCMP data. Figure 3a to figure 3e shows how patterns of change between BMI categories differ by ethnic group. Users can click on the tabs above the chart to view the data for each BMI category.

Compared with all children who were a healthy weight in reception, higher proportions of children remained a healthy weight for the following ethnic groups:

Chinese (80.1%)
White and Asian (79.8%)
White British (79.1%)

Compared with all children who were a healthy weight in reception, higher proportions of children moved from healthy weight to a higher BMI category for the following ethnic groups:

Bangladeshi (27.8%)
Indian (22.4%)
Pakistani (26%)
Any other Asian background (23.9%)
Black African (26.5%)
Black Caribbean (27.2%)
Any other Black background (26.7%)
White and Black African (24.9%)
White and Black Caribbean (26.2%)
Any other ethnic group (24%)

Figure 3a to 3e: Changes in child BMI category between reception aged 4 to 5 years and year 6 aged 10 to 11 years, by ethnic group

Figure 3a: Changes in weight status for healthy weight children in reception by ethnic group

Figure 3b: Changes in weight status for children living with severe obesity in reception by ethnic group

Figure 3c: Changes in weight status for children living with obesity (excluding severe obesity) in reception by ethnic group

Figure 3d: Changes in weight status for overweight children in reception by ethnic group

Figure 3e: Changes in weight status for underweight children in reception by ethnic group

4.3 Deprivation

There is a strong correlation between BMI category and deprivation, with children living in the more deprived areas experiencing a higher prevalence of obesity. Data is presented using the Index of Multiple Deprivation (IMD), a measure of relative deprivation for small areas. The small areas are divided according to their deprivation rank into 10 equal groups (deciles), ranging from the most deprived areas (decile 1) to the least deprived areas (decile 10). Only the most and least deprived areas have been commented on in this report, but data for all deciles is available in figure 4a to 4e and the spreadsheet published with this report.

Compared with all children who were a healthy weight in reception:

Higher proportions of children living in the least deprived areas (84.7%) and lower proportions of children living in the most deprived areas (71.8%) remained healthy weight in year 6
higher proportions of children living in the most deprived areas (26.4%) and lower proportions of children living in the least deprived areas (13.4%) moved to a higher BMI category

Figure 4a to 4e: Changes in child BMI category between reception aged 4 to 5 years and year 6 aged 10 to 11 years, by deprivation decile

Figure 4a: Changes in weight status for healthy weight children in reception by deprivation

Figure 4b: Changes in weight status for children living with severe obesity in reception by deprivation