Disturbing new research has found BMI completely fails as an accurate measure of obesity in children, calling into question the foundation for all past and present government child obesity policies.

For nearly a generation, weight-to-height ratio charts and BMI for age and sex have been used to diagnose children with obesity, but they have now been proven to be an inaccurate measure since they don’t distinguish fat mass from muscle mass.

This is because muscle weighs more than fat, skewing the results.

Waist circumference-to-height ratio is a more reliable measure of obesity in children and adolescents than BMI according to a new study, published in the journal Pediatric Research and conducted by the University of Exeter in collaboration with the University of Bristol and University of Eastern Finland.

Dr Andrew Agbaje of the Children’s Health and Research Centre at the University of Exeter says: “Unlike BMI, the average waist circumference-to-height ratio in childhood, adolescence, and young adulthood does not vary with age and among individuals, which is why it might be preferable to BMI assessment in children and adolescent clinics as an inexpensive tool for detecting excess fat.

“It also means parents can easily and quickly confirm whether an increase in their child’s BMI or weight is due to excess fat, by examining their child’s waist circumference-to-height ratio.”

This study is the largest and the longest follow-up DEXA-measured fat mass and muscle mass study in the world, using the University of Bristol’s Children of the 90s data. The study included 7,237 children (51-percent female) aged nine years who were followed until the age of 24. Their BMI and waist circumference-to-height ratio were measured at ages nine, 11, 15, 17, and 24.

HCM editor, Liz Terry, said: "For decades we've been told childhood obesity is a growing epidemic, based on the measure of BMI and data gathered by family doctors.

"However, this new research clearly shows BMI is not an accurate measure and that the foundation on which childhood obesity policy is built is flawed – how do we know some children haven't – in fact – been becoming more muscular rather than obese?

"This situation is made far more worrying by the fact that parents of children with above-average muscle mass are being told by their family doctors that their children are obese, when in fact they are the opposite.

"If parents take this feedback at face value without reading the research – which will be the outcome in most cases – this is likely to lead to children being unnecessarily calorie-restricted. This in turn could lead to the development of eating disorders and distress around the whole subject of food. Real harm could be done.

"Children will fall into two groups, those who are genuinely obese and need support to reverse this in healthy and sustainable ways and those who are muscular and have a higher proportion of lean body mass. In these cases, their caloric needs are likely to be greater and calorie-restriction would be unhealthy and potentially even distressing for them.

"They would have less energy to be active and this in turn could lead to reductions in muscle mass and vitality and – ironically – in them becoming more obese.

"We need to be able to determine between these two groups so we can offer appropriate support to those that need it.

"We're calling on the government to carry out a formal review of the accuracy of BMI and the current system of reporting childhood obesity to ensure parents are not being given false readings about their children's body composition

"The review also needs to examine how current results have been fed through to policy, so this can be reviewed and revised based on new and accurate data.

"The research from the University of Exeter – and the accompanying method of measuring – gives us a better way forward when it comes to accurately assessing children's body composition and prescribing lifestyle interventions and must be adopted immediately.

"Urgent action is needed to ensure that we do not continue down a path where the fitter and more muscular children become, the higher their obesity readings, as is happening with the current system."