Lack of exercise is not to blame for the childhood obesity crisis, researchers have found, as overweight children become less physically active, not the other way around.
Debates have been raging in the scientific communities about whether lack physical activity or poor diet is the greater cause of obesity.
Now a controversial study has found for the first time that children who are obese do less exercise because they are already overweight. The findings suggest that junk food and overfeeding from an early age is responsible for childhood obesity rather than children increasingly staying inside to watch TV and play video games.
The study could have important implications for government policy which is based on increasing childhood levels of exercise and education about healthy eating.
One of the authors, Prof Terence Wilkin, professor of Endocrinology and Metabolism at Peninsula, said: "The implications for strategies designed to prevent obesity are profound, as the data very clearly point to overnutrition.
"Children may tolerate more activity if they shed weight, but more activity does not lead to weight loss.
"I hope Andrew Lansley (Health Secretary) will take notice of this. I think it has profound implications for where we target the limited resources that are left for childhood obesity. Where it has been targeted for the last ten to 15 years has not worked."
He said there are very close links between mothers and daughters and fathers and sons where if the parents is obese the child is ten times more likely to be obese by the age of five, clearly showing that early eating patterns are very important in the development of obesity.
The EarlyBird study has been running at Peninsula Medical School in Plymouth and has already published findings that increasing a child's activity in school means they do less outside of school hours meaning children have a set level of activity they will do.
Other studies have shown that increasing physical activity in children to reduce obesity led to weight loss of on average just 90g or 3oz over three years.
In the study 200 schoolchildren were tracked for an average of three years between the ages of seven and ten.
They wore electronic devices that measured their physical activity levels for seven consecutive days at the same point in each year and their body fat percentage was calculated.
It was found that changes in their body fat preceded changes in their physical activity level but not the other way around.
If the children had a body fat percentage ten per cent higher than average at age seven they went on to reduce the amount of time they spent doing moderate or vigorous activity by four minutes per day by the time they reached age ten.
However more physical activity at age seven did not correspond to a reduction in body fat percentage in the following years.
The research paper, published in the Archives of Disease in Childhood, concluded: "The image of the “couch-potato” child who is obese because he is sedentary runs deep in Western consciousness. However, the possibility that the reverse obtains, that his fatness is the cause rather than the result of his inactivity, has far-reaching implications.
"Although there may be many benefits to physical activity the findings of this study, coupled with the limited success of physical activity interventions aimed at improving body mass index, imply that public health strategies may need to target energy intake to curb the year-on-year rise in childhood obesity."
The paper said: "Our findings suggest that rather than giving children ever-increasing doses of physical activity, we should first question the basic paradigm that more physical activity leads to less fat. If childhood fatness is not the result of physical inactivity, the implication may be that excess energy intake underlies fatness and inactivity.
"It is still not clear what type of intervention is the most likely to succeed in preventing childhood obesity.
"Future intervention studies should, perhaps, focus more on reducing energy intake than on increasing energy expenditure."
The authors suggested that overweight children avoid sport and exercise because they are embarrassed about their weight.
But it was also suggested that activity in these children can cause joint and muscle pains and that they become breathless and tired earlier than normal weight children.
Tam Fry, member of the Child Growth Foundation, which supports Prof Wilkin's EarlyBird study, said the results were 'fascinating'.
"I don't think the findings are suggesting that exercise is a waste of time rather that pouring massive amounts of money into school sport may not have the effect everyone is hoping for. It means we would be better off spending money on healthy eating from an early age."