Around three in ten children aged between two and 15 are thought to be overweight or obese, according to the latest official statistics.
However this is based on the schools measurement programme which uses Body Mass Index, a calculation of a person’s weight in kilograms divided by the square of their height in metres and plotted on a growth chart to take into account their age.
Now experts have said this is leading to an underestimate of the extent of the childhood obesity problem because it does not take into account where on the body the children are carrying their extra weight.
If waist circumference was used as well as BMI, then four out of ten children would be classed as overweight or obese, the researchers said.
Fat around the middle is most hazardous to health and is known to increase the risk of type 2 diabetes and this is missed by BMI, researchers from Leeds Metropolitan University said.
They measured more than 14,500 children over three years, using BMI, waist circumference and waist to hip ratio.
They found six per cent of boys and 15 per cent of girls, 429 boys and 992 girls, were found to be overweight and would not have been identified using BMI alone.
Most shockingly 2,000 11-year-old girls were found to be so big around the middle that they were as fat as a fat adult female, in that their waist circumference exceeded 80cm at which point the risk of type 2 diabetes increases.
Senior lecturer Claire Griffiths said: “Although the choice of BMI as a measure of obesity in children is well-established, and even recommended, widespread use of BMI to assess fatness in children may conceal differences in body composition and central adiposity which potentially pose a greater health risk.
“The data could have serious implications for public health, suggesting that there is a need to understand the relationship between BMI and waist circumference, with growth and health risk.”
BMI is a controversial measure as it was designed to indicate obesity on a population basis and can be inaccurate for individuals who are extremely fit and muscular such as rugby players and athletes.
The schools measurement programme has been criticised and some experts have called for it to be scrapped as children who look healthy have been labelled as overweight and become concerned about their looks.
A spokesman for the Department of Health said: “It is widely agreed that body mass index is the best available method for assessing a child’s weight.
“It is recommended by both NICE and the WHO and is used routinely by healthcare professionals around the world.
“We will continue to keep the scientific evidence under review and use this evidence to broaden our understanding of obesity.“