Over-eating rather than more sedentary living is almost entirely to blame for the rise in obesity in the developed world, according to research.

A study of the US obesity “epidemic” — a precursor of world dietary trends — suggests that there has not been any significant reduction in levels of exercise in the past 30 years. It concludes that the surge in obesity is a result of excessive calories.

Researchers at the World Health Organisation Collaborating Centre for Obesity Prevention, at Deakin University in Victoria, Australia, said that the findings would be reflected in other industrialised countries such as Britain.

Last year the largest British study into obesity, backed by the Government and compiled by 250 experts, concluded that excess weight had become the norm. It predicted that by 2050, 90 per cent of today’s children will be overweight or obese — costing taxpayers an estimated £50 billion.

The WHO estimates that in 2005 about 1.6 billion adults worldwide were overweight, of whom at least 400 million were obese.

Health professionals and campaigners welcomed the latest findings, presented at the European Congress on Obesity in Amsterdam, as evidence of the need to focus on diet and the availability and overpromotion of highly calorific foods.

The Government is currently waiting for advice from the Food Standards Agency on how to use labels to alert shoppers to the unhealthy content of food.

For the WHO-Deakin study, researchers calculated what Americans should weigh today based on their current, higher food intake, and compared it with their actual weight. If they weighed more than projected it would suggest reduced physical activ-

ity.Researchers found that adults weighed less than could be expected from their diet, suggesting that, if anything, they had increased their physical activity over time.

Among children the tests showed a 100 per cent match, leading researchers to conclude that changes in physical activity had had no impact on children in the US growing fatter.

Professor Boyd Swinburn, chairman of population health at Deakin University, said that the findings would be similar for other developed countries in showing that changes in exercise played a minimal role. “This is a call to focus public health attention more on the energy intake side,” he said. “There is no evidence that a marked reduction in physical activity has been a contributor to this epidemic.”

He said that US children had grown on average 9lb heavier and adults were 17lb more. For the US population to return to leaner 1970s levels children would have to cut their intake by about 350 calories a day — equal to a can of fizzy drink and a small portion of fries, and adults by about 500 calories — about the same as a Big Mac burger. Alternatively children would have to walk an extra 2½ hours a day, and adults nearly two hours.

“Getting everybody to walk an extra two hours a day is not really a feasible option for countering the epidemic,” Professor Swinburn said. “We need to limit our expectations of what an increase in physical activity can achieve.” He said that the findings did not seek to negate the value of exercise for weight control.

A spokesman for Sustain, which campaigns for better food, said that the research supported the drive to reduce availability of energy-dense products. The study follows research presented to the obesity conference on Thursday from Peninsula College of Medicine and Dentistry in Plymouth, which suggested that encouraging children to do more exercise at school did not increase their overall activity levels.

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