New research has found that 20 minutes of daily, vigorous physical activity over just three months can reduce a child's risk of diabetes as well as his total body fat - including dangerous, deep abdominal fat - but 40 minutes works even better.

Pediatric and adult studies have shown the metabolic benefits of aerobic activity, but had yet to dissect differences in the dose response, or the amount of activity needed to elicit a given benefit. 

The study published is in the Journal of the American Medical Association. 

Researchers observed 222 overweight, previously inactive 7- to 11-year olds in the Augusta, Ga., area and found that more exercise is better.

Research said their findings add to the body of evidence that physical activity improves children's health, that longer periods of exercise provide a greater benefit and that increased physical activity among overweight and obese children could stave off the onset of type 2 diabetes.

"Obesity is a growing public health crisis that is affecting youth throughout the United States, and we know that obesity can contribute to the development of type 2 diabetes," Michael Lauer, Director of the National Heart, Lung and Blood Institute Division of Cardiovascular Sciences of the National Institute of Health, said. 

A third of the study participants maintained their typically sedentary lifestyle; a third began a 20-minute heart-rate-raising, after-school exercise routine for three months; and a third exercised for 40 minutes after school. While their primary focus was insulin resistance, a risk factor for diabetes, the researchers also measured total body fat, visceral fat and aerobic fitness over the study's course.

Children who exercised for 40 minutes had a 22 percent reduction in insulin resistance versus the controls, while the 20-minute group experienced an 18 percent reduction.

The extra 20 minutes also helped the children lose more total body fat and visceral fat while fitness, which appeared driven by intensity rather than duration, gained a similar boost from both time periods. Benefits were gained without restrictive diets and worked equally well in black and white boys and girls.

"Regulation sports tend to have kids standing around a lot waiting for the ball. We had enough balls so everyone was moving all the time,"  said Catherine Davis, lead author. "They kept it fun, giving non-food rewards, such as small toys, for children who kept their heart rates high. It had to be fun or they would not keep coming."

She hopes the evidence of the solid health benefits of a fun, vigorous and relatively short exercise routine will be used to design public health interventions for a society in which one-third of elementary school children are overweight.

Childhood obesity rates in the United States have been climbing for more than a decade. While they seem to be plateauing, the unprecedented levels have serious consequences for children's health and longevity, Davis said. A primary example is the emergence of type 2 diabetes, previously considered an adult, lifestyle-related problem with serious implications for cardiovascular health and more. One of the first indications of trouble is increased insulin resistance, how much insulin the pancreas must produce to enable glucose circulating in the blood to become energy for the cells. In this study, researchers showed a benefit of 40 minutes of daily exercise on the disposition index - the ratio of insulin resistance to the body's ability to secrete insulin - a proven predictor of diabetes development in adults.

"When your body is no longer able to secrete enough insulin to overcome your body's resistance to it, that's when it becomes diabetes," Davis said. "Exercise basically gives the pancreas a break and could prevent or delay type 2 diabetes as long as people remain active." 

Davis noted that in insulin resistance, pancreases must work overtime producing extra insulin to convert excess blood glucose to usable energy. Without sufficient insulin, a vicious cycle results as energy-starved cells increase the appetite and people eat more creating even more glucose to convert. 

Longer-term and follow-up studies are needed to find out what happens with these children over time and how to help them sustain a healthy lifestyle.

In 2005, a federal panel, recommended 60 minutes or more of daily vigorous physical activity for school-age children.