An education programme which successfully cut the level of obesity in children by teaching them about healthy eating and discouraging fizzy drinks was no longer effective three years after the intervention came to an end, according to a study published on the British Medical Journal website.

The authors say the original study provided hope that simple interventions could have an impact on obesity levels but they now believe these sorts of programmes need to be continuous if their long term effect is to be properly assessed.

Ten percent of the world's school age population is estimated to be overweight. In the UK it has been forecast that by 2010 nearly one in five boys and one in four girls will be obese.

Janet James and colleagues say a number of studies have shown the relationship between drinking soda and obesity, a link which has also been acknowledged by the World Health Organisation.

The Christchurch Obesity Prevention Project, also known as the "Ditch the Fizz" campaign, was run for a year in six junior schools in Southern England from August 2001. 644 children, aged between seven and eleven, took part. The project focussed on discouraging children from drinking fizzy drinks and promoting a healthy diet through four extra health education classes spread over the year.

At the end of the study the authors found significant differences in the proportion of overweight children in the control group and the intervention group.

Two years after the end of the study the researchers re-measured the childrens' height, weight and Body Mass Index, converting them into standard scores (Z scores). The Z score takes into account the child's age and gender and represents the deviation compared to an average child of the same gender and age.

They found the number of overweight children had increased in both groups although the prevalence was still higher in the control group. Three years later this gap was no longer significant.

Obesity is a very complex condition, say the authors, but they conclude: "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."