The researchers tracked the body weight and height of nearly 5,000 children taking part in Children of the 90s, also known as the Avon Longitudinal Study of Parents and Children (ALSPAC), from birth until the age of 15.
They found that the number of children who changed from a healthy weight to being overweight and obese was much higher between the ages seven to 11 years (19 per cent), than between ages three to seven years (ten per cent) or 11 to 15 years (seven per cent).
Professor John Reilly of the University of Strathclyde said: 'This research gives us an important insight into the stages of childhood and adolescence when the environment is most obesity promoting.'
Dr Adrienne Hughes, University of Stirling, added: 'This research is consistent with our other recent study which showed that most excess weight gain occurs by mid-late childhood.'
Professor Debbie Lawlor of the University of Bristol concluded: 'These recent findings from ALSPAC suggest that mid-late childhood may be the best bet for childhood obesity prevention.'
Source: Bristol University