Schools have been trying for better nutrition, more exercise and improved education about healthy living to help kids who are at risk for obesity keep weight off, compared to kids in schools without similar programs.

The program, however, failed to reduce the overall number of obese and overweight school kids. The numbers fell by four percent over three years whether the 42 middles chools in the study had the health programs or not, researchers report.

"The intervention, surprisingly, did not result in a [population-wide] reduction in overweight or obesity," said lead researcher Gary D. Foster, a professor of medicine and public health and director of the Center for Obesity Research and Education at Temple University, Philadelphia. "What's surprising is that the control group improved as well," he said.

While the programs did not seem to have much influence on kids who were not already overweight or obese by sixth grade, it did impact children who were already having weight troubles.

For those kids, "there were small [weight-loss] effects of about 3 percent. Although that's small, it's enormous given the rates of obesity in this country," Foster said. "This was an unexpected but fortunate finding -- that we are actually impacting the kids at the greatest risk."

The discoveries are to be presented Sunday at the annual meeting of the American Diabetes Association in Orlando, and they are being published online in the New England Journal of Medicine.

Foster's team randomly selected about 4'600 students from 42 schools to go on a diet, exercise and information program, or a program where they have their height and weight assessed. The study targets schools with high levels of minority children, because studies have shown them to be at a high risk for obesity.

The selected children were offered healthier food choices throughout the school in the cafeteria, at snack bars and vending machines, and during class events. These included lower-fat, higher-fiber foods; more fruits and vegetables; and an emphasis on water, low-fat milk and drinks without added sugar.

Additionally, there were longer and more intense periods of physical exercise for the school kids, and activities and awareness campains to promote healthier living.

The research found that kids who were already obese at the programs outset lost a significant amount of weight, as shown by a reduction in waist size, compared to other children.

Actually, kids in program schools who were overweight or obese in sixth grade had 21 percent fewer chance of being obese by the end of eighth grade, compared with students in schools without initiatives. The program also lowered fasting insulin levels, a key indicator for diabetes risk, the researchers found.

The children from both types of schools had the same average blood sugar levels and the same amount of students with elevated blood sugar, the study noted.

Foster said that it is intriguing that the rate of overweight and obesity dropped by 4 percent regardless of the presence of an anti-obesity program or not. "This is potentially good news -- that the rates of childhood obesity appear to be declining [naturally]," he said.

Other factors may be involved in the weight decrease even among those kids putting more attention on their weight. The fact that researchers were measuring the kids' height and weight may have made the youngsters more self conscious, he said.

"Something has changed and we've got to figure out what that something is," Foster said.

Director of the Prevention Research Center at Yale University School of Medicine, Dr. David L. Katz, said that "the results of this study are open to interpretation, and thus conclusions are apt to reside in the eyes of the beholder."

People who do not put much stock in school-based intervention will point to a lack of effort overall. "However, those who perceive value in school-based interventions -- and I am among these -- will focus on the obvious pattern in an array of secondary outcomes, including reductions in body fat and insulin levels, which in turn would be expected to reduce diabetes risk," Katz said.

The program's effect on obese children is important, according tp Foster.

"There is a strong signal here that this [program] can make a difference," Foster said. "If we can reduce the risk of type 2 diabetes in children by reducing the rates of obesity and reducing waist circumference, that's an important outcome," he said.