The later years of primary school are the critical time for preventing obesity, researchers have found.
Action should be taken between the ages of seven and 11, they say; the idea that children naturally "grow out" of the overweight or obese stage as they become older is largely mistaken.
Researchers from Stirling, Strathclyde, Glasgow and Bristol universities tracked the weight and height of 4,283 children up to 15 years old.
There has been a large amount of research on the prevalence of childhood obesity, explained Stirling University's Adrienne Hughes, the lead researcher, but until now, none had examined when it was most likely to develop.
The results showed that the number of children who changed from a healthy weight to being overweight and obese was much higher between seven and 11 (19 per cent), than between three and seven (10 per cent) or 11 to 15 (7 per cent).
"Interventions to increase physical activity, reduce screen time and improve diet should take place at all ages, but our results indicate that mid-childhood should be a priority," Dr Hughes said. "It could be these behaviours are more of a problem in this age group."
The research group also found that the majority of overweight or obese children did not grow out of their condition by 15.
"We can speculate that if they don't grow out of it by 15, they won't grow out of it by adulthood," Dr Hughes said.
The researchers warned that prevention programmes - which were mainly school-based - "often had limited impact".
They frequently targeted too many types of behaviour and covered too short a time, said Dr Hughes, who is based in the university's school of sport. Prevention schemes should instead concentrate on the three main causes of obesity: lack of physical activity; excessive intake of sugary drinks; and high levels of time spent watching television and playing computer games.
School-based schemes would work far better if parents and community organisations were involved, Dr Hughes added.
The research group is now in the early stages of putting together an obesity programme aimed at primary school children.
The group's work was carried out with young people involved in the internationally-renowned Children of the 90s project. Based at Bristol University, it enrolled 14,541 pregnant women in 1991 and 1992.
PUT THROUGH THEIR PACES
Ways to improve the health and fitness of primary children should emerge from a new study, designed to tackle problems related to coronary heart disease in adulthood, such as obesity, high blood pressure and high cholesterol.
The University of the West of Scotland has been working with FAST Sports - an outdoor sport and fitness company - on a project involving 54 P6 and P7 pupils at Chatelherault Primary in Hamilton, South Lanarkshire.
Their fitness, body composition and blood biochemistry were measured before and after an eight-week exercise programme. Fitness levels improved markedly, but full results will only emerge in late autumn.
Julien Baker, head of the university's exercise and health sciences research unit, said he hoped that the research would lead to fitness strategies which could be implemented during school hours.