The risk of children becoming obese could be halved with 15 extra minutes of moderately vigorous exercise each day, study results have suggested.

All that is needed is a short game of football or a walk to school brisk enough to get slightly out of breath. The effects are greater in boys than in girls, but both sexes benefit.

The findings point to a lack of exercise, rather than gluttony, as the key to obesity in young people.

Researchers were surprised to find that boys have just 25 minutes of activity each day on average, and girls only 16 minutes.

The data comes from the Children of the 90s project, which has followed a group of children born in Avon in the 1990s. The Avon Longitudinal Study of Parents and Children is one of the biggest and most ambitious cohort studies ever attempted and is producing some of the best evidence on the effects of diet and lifestyle on disease.

Researchers fitted 5,500 children aged 12 with activity meters to measure how much exercise they took. The children wore the meters around their waists, taking them off only to sleep, bath or swim. Their body fat was measured using an X-ray emission scanner, which can distinguish between fat and muscle. The results are published in PLoS Medicine.

Professor Chris Riddoch, of the London Sport Institute at Middlesex University, one of the project leaders, said: “We know that diet is important, but what this research tells us is that we mustn’t forget about activity. It’s been really surprising to us how even small amounts of exercise appear to have dramatic results.”

The boys who took the most vigorous activity were more than 30 times less likely to be obese than those who took the least. An extra 15 minutes a day of moderate and vigorous physical activity halved the risk of obesity.

Among girls the effects were less dramatic, but still significant. The most active fifth of girls reduced their risk of obesity by two thirds compared with the least active fifth.

Professor Andy Ness, of the University of Bristol, said that the most important activity was the kind that got the children slightly out of breath, or in a sweat. “Recommending an extra 15 minutes of vigorous activity a day may not sound very much, but it is actually double what the average 12-year-old girl does,” he said. “In the context of what they are doing, it is quite a lot.”

Why the effects should be so much greater in boys remains puzzling. “It could be physiological differences but I think that’s unlikely,” Professor Ness said. “The other possibility is that boys and girls use activity differently. Boys tend to use activity as the main weight control mechanism, while girls tend to control their weight by eating less.”

He said that surveys and food production statistics suggested that total calorie intakes had not increased. Yet obesity was rising, so it was reasonable to suggest that this was the result of burning less energy.

“Lots of opportunities for activity are factored out of children’s lives these days,” he said. “There are more sedentary opportunities — sitting in the car, watching television, playing computer games. There’s less walking to school, and when they get home Mum and Dad don’t want them wandering off into the woods or playing in the streets.”

– Inactive lifestyles are costing the NHS more than £1 billion a year, according to research published in the Journal of Epidemiology and Community Health. They were responsible for 3 per cent of all deaths and illnesses in 2002, researchers found.

The cost includes hospital stays, outpatient appointments, drugs, community care and visits to GPs. The study concludes: “There is an economic case for developing policies and interventions that promote physical activity.”

Under the microscope

– The Children of the 90s Study is one of the most ambitious attempts to work out the links between diet, lifestyle and child health

– In 1991 more than 14,500 pregnant women were enrolled. They, their partners and subsequent children are being studied

– The project started with 20 staff and core funding of £6 million from the Wellcome Trust and the Medical Research Council

– Today it still has almost 14,000 children enrolled. The number of staff has grown to 150

– Because children have been followed from the beginning, data is far more secure than in most cohort studies, and can be linked to DNA sample data to test interactions between genes and environmental influences

– Since the age of 7, all children have been invited to attend a clinic regularly to be weighed and assessed

– Outside researchers can use the data, but must seek independent funding. So far 160 scientific papers have been published

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