Scientists have questioned the assumption that a lack of exercise causes fatness in children.
The study suggests that physical inactivity appears to be the result of fatness, instead of its cause.
Researchers said the findings indicate that nutrition, rather than exercise, is the best way of tackling childhood obesity.
The EarlyBird team followed more than 200 children in Plymouth over three years, monitoring their fat and exercise levels at regular intervals.
They found that body fat levels had an effect on physical activity, but that varying activity did not lead to any changes in fatness.
The paper, published in the Archives of Disease in Childhood, suggests that overweight children may think about their body negatively, shying away from sports and exercise as a result.
It concluded: "Physical inactivity appears to be the result of fatness rather than its cause.
"This reverse causality may explain why attempts to tackle childhood obesity by promoting physical activity have been largely unsuccessful."
Dr David Haslam, from the National Obesity Forum, cautioned that the wider health benefits of exercise for children must not be overlooked.
He told the BBC: "The EarlyBird team really force us to question our comfortable assumptions regarding childhood obesity.
"What we, as clinicians must do, is nod reverently at their work, learn lessons from it, and reappraise our own practices accordingly.
"What we shouldn't do is take the paper at face value and allow lean children to be as lazy as they please, as that would be a catastrophic mistake."
A Department of Health spokesman added: "We are committed to tackling childhood obesity and this study provides some useful messages on the importance of a child's early years and the impact this can have on their future health and behaviour.
"We will consider this evidence alongside other research which has different findings on the link between physical activity and weight when we are developing our policy to produce better public health outcomes."