A high intake of sweetened carbonated drinks probably contributes to childhood obesity, and there is a growing movement against soft drinks in schools. But until now there have been no studies showing that efforts to cut children's soft drink consumption would do any good.
The study, outlined this week on the Web site of the British Medical Journal, found that a one-year "ditch the fizz" campaign discouraging both sweetened and diet soft drinks led to a decrease in the percentage of elementary school children who were overweight or obese.
The improvement occurred after a modest reduction in consumption -- less than a can a day.
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.
The group said carbonated drinks provide only a fraction of children's daily calories and therefore should not be blamed for the childhood obesity epidemic.
However, other experts 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 anti-obesity school programs — some costing millions of dollars — have been disappointing. Such programs, which included reducing dietary fat or trying to get kids 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 both regular and diet sodas and stressing the benefits of a healthy diet, while 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 program involved a one-hour session given to each participating class four times during the school year.
The first session focused on good health and the importance of drinking water. The children ate fruit to emphasize the sweetness of natural products and each class received a tooth immersed in cola to show its effects on teeth.
The second and third session involved a music competition in which classes were challenged to produce a song with a healthy message.
The final session involved art presentations and a classroom quiz based on a TV game show.
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 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 diet.
But experts said the important point was that the program reduced obesity rates through nutrition education.
Soft drink consumption has increased enormously in the United States and in Europe over the last three decades, and children are becoming increasingly overweight around the world.
The World Health Organization said that although the change in obesity in the study was small, the intervention was also modest.
"This is a promising finding," said Derrek Yach, who spearheads the agency's anti-obesity effort. "We would hope to see larger studies with more intensive interventions ... What happens when you combine this with the removal of vending machines? I'm sure you'd see even bigger beneficial effects."