‘CRUDE” measurements have failed to reveal the scale of childhood obesity in Scotland as it grows to affect more than 100,000 youngsters, researchers claim.

Strathclyde University experts say reliance on body mass index (BMI) is masking the true scale of the public health problem affecting young people.

They now warn “large numbers” of “apparently healthy” children and teenagers have “excessively high” levels of body fat when assessed through other measures.

The BMI ratio is based on height and weight, but the team says alternative calculations are “far more accurate” in assessing the nation’s waistline.

Leading a study of obesity in Africa, which involved 1500 primary schoolchildren across eight separate countries, Professor John Reilly of Strathclyde’s School of Psychological Sciences and Health found 9% could be defined as obese by BMI (9%), but this increased to 29% using the deuterium dilution method, which measures excessive weight by total body water.

He is now calling for that measure to bump BMI from health charts.

Reilly said: “BMI is a straightforward and cost-effective way of measuring obesity in children. It has become widely-used in national surveys and in public health information but it is a very crude proxy measure.

“Large numbers of children and adolescents with an apparently healthy BMI for their age have an excessively high body fat content. Childhood obesity is at least twice as prevalent as reported in national surveys and official publications.

“In fact, more than 100,000 Scottish children and young people will have obesity at present.”

He went on: “The deuterium dilution measure would be more costly and would take longer than BMI – three to four hours compared with 15 to 20 minutes for BMI – but it would present us with a far more accurate picture of the scale of the problem.

“It needs to be properly studied and could be worth the consideration and investment.”

The Strathclyde team was involved in the biennial Active Healthy Kids Global Alliance (AHKGA) report, which assesses global trends in childhood physical activity in almost 50 countries and provides “report cards” on the physical activity and health of children.

Exercise levels here were far below the recommended rates, while screen time was amongst the worst in the world.

Scotland’s D+ rating placed it in the lower half of the rankings, compared to England’s C.

Reilly said: “There is no longer any room for complacency about childhood obesity anywhere in the world; urgent measures will be required to prevent and control the problem.”

Dr Mark Tremblay, AHKGA president, said: “We all have a collective responsibility to address these cultural and social norms – particularly screen time – because inactive children are at risk for adverse physical, mental, social and cognitive health problems.”