Couch potato

Junk food and the couch potato lifestyle have been blamed, but they are associated with adolescents and teenagers who are difficult to reform. It would be useful to identify the children at risk when they are much younger - before they even get to school - when what they eat and what they do is largely decided by their family.

John Reilly, reader in paediatric energy metabolism at the University of Glasgow's division of developmental medicine, and colleagues have studied more than 8,000 children aged seven in order to identify what would have predicted the obesity of some and the more usual weight of others in their first years of life.

Their study, published online today by the British Medical Journal, looks at 25 possible risk factors, including season of birth and the amount of time children spend being ferried about in the car - neither of which had any impact - and identifies eight as associated with childhood obesity.

The strongest indicators of later obesity were above average birth weight, parental obesity, sleep duration and television viewing. "Parental obesity may increase the risk of obesity through genetic mechanisms or by shared familial characteristics in the environment such as food preferences," the Glasgow team write.

What to many may seem the curious finding about children who do not sleep all night could have a number of explanations, they say. It may be to do with growth hormones, which are secreted when the child is asleep and build up muscle and lean tissue.

But it could equally - or also - be connected with the fact that children who go to bed late tend to snack in the evening, well after their traditional meal time. And there's a third possibility - that the children who sleep well are those who are tired out from physical exercise.

Television viewing, they say, may increase obesity risk because children tend to eat in front of the TV set, or because the habit of watching the box after meals prevents them from chasing around and burning off the calories.

Dr Reilly says his team's work with pre-school children suggests that parents do not realise how inactive their children are. "They say, 'they are always on the go'," he said. "They have the impression their children are always rushing around. We do a lot of measures and it is quite obvious that they are always on the go in the mental sense, talking eight hours a day, but not physically on the go."

It was hard to say whether children 20 years ago really were always up trees instead of in front of the computer as people remember, he said, because until four of five years ago there were no proper measures of children's activity. "But I'm strongly of the belief that I didn't just imagine the halcyon days when the sun shone and we were all out playing."

Smoking

Junk food did not come up as a particular risk factor, but it is notoriously difficult for parents to remember what their children have been eating, he said. Some of the other factors the team looked at, such as the smoking habits of mothers, may play a part but the evidence was inconclusive, although the study suggests it is possible that women who smoke when pregnant could be unwittingly helping to programme their child to have an increased appetite. The evidence on breastfeeding was also not conclusive, although seven-year-olds whose mothers did not smoke and who did breastfeed had less of a chance of being obese.

The scientists conclude that their research provides evidence that the environment of the baby and growing child does play a role in the development of later obesity and that something can be done about some of the factors, such as television and sleeping patterns.

"Prevention strategies for childhood obesity to date have usually been unsuccessful and typically focus on change in lifestyle during childhood or adolescence," they say. "Future interventions might focus on environmental changes targeted at relatively short periods in early life, attempting to modify factors in early childhood which are independently related to later risk of obesity."

The warning signs