An obesity-prevention program tested in several Dutch schools was able to cut down teenagers' consumption of sugary sodas and curb body-fat gain, according to a new study.

The program, dubbed Dutch Obesity Intervention in Teenagers (DOiT), aimed to boost students' exercise levels, steer them away from junk food and sugar-sweetened drinks, and lower their odds of excessive weight gain. Students had 11 lessons on the topics, and schools were encouraged to increase gym classes and make cafeteria changes.

The results were at least partially positive, researchers report in the Archives of Pediatrics & Adolescent Medicine.

Over 20 months, students at 10 schools that ran the program reduced their soda intake, compared with their peers at eight "control" schools. Girls at the intervention schools also showed a smaller increase in body fat.

However, those benefits tended to wane over time. And certain other positive effects seen at the eight-month mark -- like less weight gain around the waistline in boys -- had disappeared by the 20-month point.

The findings suggest that such school-based programs can be effective, but that they need to be kept up, lead researcher Dr. Amika S. Singh, of the VU University Medical Center in Amsterdam, told Reuters Health.

It's clear that obese adults have a difficult time losing weight and keeping it off, Singh pointed out, so preventing excessive weight gain in children and teenagers is critical.