CHILDHOOD obesity is much more widespread than current estimates suggest according to researchers in Glasgow who said current measurements overlook body fat.

Experts at the University of Strathclyde warned there are "large numbers of children and adolescents" whose weight is "apparently healthy" when their Body Mass Index (BMI) is calculated, but actually have "an excessively high body fat content".

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Professor John Reilly said BMI - which is based on measuring height and weight only - could be underestimating the scale of the problem by as much as half, putting the true scale of childhood obesity at 100,000.

Switching to a body fat calculation known as the deuterium dilution measure would provide a "far more accurate picture of the scale of the problem", but it would also be "more costly", he said.

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Prof Reilly, of Strathclyde's School of Psychological Sciences and Health, recently led a study of obesity in Africa which involved 1,500 primary schoolchildren across eight separate countries.

He found that only 9% of children were obese according to their BMI status, but this escalated to 29% when body fat was measured using the .

Prof Reilly spoke out as the Active Healthy Kids Global Alliance (AHKGA) report, which assesses trends in childhood physical activity in 49 countries was published.

The Strathclyde University expert was the Scottish lead for AHKGA study, which gave Scotland a D+ rating - placing the country in the lower half of the rankings.

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Slovenia had the best ranking of any of the nations, with a B, while England was given a C overall.

The USA which was awarded a D, while China was given a D-.

While Scotland scored a B for organised sport and physical activity, it was given an F for high levels of sedentary behaviour amongst youngsters.

Professor 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.

"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."