A new report shows that encouraging children to avoid fizzy drinks has no long-term impact on their obesity.
The study, which is published online in the British Medical Journal (BMJ), shows that while teaching youngsters about healthy eating works in the short-term, the long-term effects can be minimal.
The report's authors suggest that healthy eating programmes need to be ongoing if their effectiveness is to be assessed.
The study is based on the Christchurch Obesity Prevention Project in Schools (CHOPPS) which ran for a year in six junior schools in southern England from August 2001.
A total of 644 children, aged between seven and 11, took part in the study which focussed on discouraging children from consuming fizzy drinks and promoting a healthy diet through extra health education classes.
At the end of the study, the authors found a "significant difference" between the number of overweight children in the group who followed the programme compared with a "control" group.
Those in the control group were more likely to be overweight, showing the benefits of the programme.
But three years after the start of the study, the gap between the two groups was not significant.
"It remains unclear whether specific interventions or those which focus on all aspects of the diet and physical activity are the most successful," the authors said.
"Perhaps the true impact of any school-based intervention can effectively only be evaluated if the interventions are continuous."