Representatives of the soft drink industry contested the implications of the results.
The study ''reduced the average daily consumption of carbonated soft drinks by about 150 milliliters, or 35 calories -- half the reduction was in diet carbonated soft drinks. This represents about 2 percent of a child's calorie intake, not a significant amount," the British Soft Drink Association said in a statement.
However, other specialists were impressed.
''If a simple targeted message aimed at kids can decrease development of obesity, by whatever means, that's groundbreaking," said Dr. David Ludwig, who runs a pediatric obesity clinic at Children's Hospital in Boston but was not connected with the study.
Previous studies of antiobesity school programs -- some costing millions of dollars -- have been disappointing. Such programs, which included reducing dietary fat or trying to get children to exercise more, largely failed to show any meaningful impact.
The investigators studied 644 children, aged 7 to 11, in six primary schools in Christchurch, England, during the 2001-2002 school year. Half the classes participated in a program discouraging soda drinking and stressing a healthy diet; the other half did not. All students kept a diary of their soft drink consumption over one Thursday, Friday, and Saturday at the beginning of the experiment and again for another three days at the end.
''They were told that by decreasing sugar consumption they would improve overall well-being and that by reducing the consumption of diet carbonated drinks they would benefit dental health," said the scientists, diabetes doctors and nurses at the Royal Bournemouth Hospital in southern England.
The percentage of overweight and obese children increased by 7.5 percent in the group that did not participate and dipped by 0.2 percent among those who did.
Consumption of soft drinks dropped by an average of 0.6 glasses a day among the targeted children, but increased by 0.2 glasses a day among the children outside the program.
All the children drank more water than before. They had been told it improves concentration.
It was not possible to prove the weight improvements were linked to the decline in soda consumption because the children may have changed other aspects of their diets. But specialists said the important point was that the program reduced obesity rates through nutrition education.