The Christchurch Obesity Prevention Project, called the "Ditch the Fizz" campaign, was run for a year in six junior schools in Southern England, where 644 children, aged between seven and eleven, participated in the study.
In the programme, kids were told to avoid drinking fizzy drinks and a healthy diet was promoted through four extra health education classes around the year.
At the end of the study, the authors found large differences in the proportion of overweight children in the control group and the intervention group.
However, two years after the end of the study, when the researchers re-measured the childrens' height, weight and Body Mass Index, they found the number of overweight children had increased in both groups, with the highest number still in the control group.
Three years later the differences between the two gap was no longer significant. The authors conclude that obesity is a very complex condition.
"It remains unclear whether specific interventions or those which focus on all aspects of the diet and physical activity are the most successful. Perhaps the true impact of any school based intervention can effectively only be evaluated if the interventions are continuous," British Medical Journal quoted the authors, as saying.
The study is published n the current online issue of British Medical Journal.