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What is the problem?
Recent surveys have found almost a quarter of children at school entry to be overweight or obese, leading to suggestions that this is where preventive interventions should be focused. However, others dispute this, as tracking of body mass index (BMI) from preschool age is weak, with 63% of obese toddlers reverting to overweight or healthy weight at later ages,1 while BMI z scores relate weakly to adiposity in younger compared with older overweight children.2
This paper explores the possibility that this lack of persistence of early obesity may be the result of a classification artefact. This issue arose when an obese toddler, seen by a specialist, was reported to have improved, despite no change in BMI. The explanation for this was evident on the BMI chart. This is illustrated with a fictitious example in figure 1, where a child’s centile normalises over time, despite no reduction in BMI. We are used to the idea of children catching down towards normality, as other children grow taller or heavier around them, as the whole growth curve rises over time. However, here the median BMI is the same at both ages. Can it be right that extreme BMI centiles are crossed at so much lower levels in younger children than in mid-childhood, when the average BMI is not increasing?
A boy aged 3 with a BMI of 20 is close to the 99.6th centile (severe obesity). If he remains with the same BMI until the age of 9, by then he is within the normal range. Over the same period, the median BMI has not risen and BMI is 25% above the median at both ages. BMI, body mass index.
How is obesity in childhood currently defined?
We still have no properly validated upper threshold for unhealthy BMI in childhood. The upper thresholds for healthy adult BMI …
Footnotes
Contributors CMW set up the GMS cohort, conceived, designed and undertook the analysis, and drafted the manuscript. She is its guarantor.
Funding There was no specific funding for this analysis. The GMS cohort has been supported by a wide range of grants over time, notably the Henry Smith Charity and Sports People Aiding Research for Kids (SPARKS), the Gateshead NHS Trust R&D, Northern and Yorkshire NHS R&D, Northumberland, Tyne and Wear NHS Trust, the National Prevention Research Initiative (incorporating funding from British Heart Foundation; Cancer Research UK; Department of Health; Diabetes UK; Economic and Social Research Council; Food Standards Agency; Medical Research Council; Research and Development Office for the Northern Ireland Health and Social Services; Chief Scientist Office, Scottish Government Health Directorates; Welsh Assembly Government and World Cancer Research Fund), the Children’s Foundation, and the Scottish Government Health Directorates Chief Scientist Office.
Competing interests None declared.
Patient consent for publication Not required.
Provenance and peer review Not commissioned; externally peer reviewed.