The more aerobic exercise that overweight or obese children got in a randomized trial, the less likely they were to develop prediabetic insulin resistance, researchers said.
Among 209 children, age 7 to 11, who were assigned to undertake 20 or 40 minutes per day of aerobic exercise, or to no intervention, a dose-response relationship was apparent between the amount of exercise and reductions in time-averaged insulin levels relative to a usual-care control group, reported Catherine Davis, PhD, of the Medical College of Georgia in Augusta, Ga., and colleagues.
More exercise was also associated with larger reductions in total body fat and in visceral fat, the researchers reported in the Sept. 19 obesity-themed issue of the Journal of the American Medical Association.
Although exercise generally is known to be metabolically beneficial to overweight and obese children, this is the first pediatric study to demonstrate a dose-response relationship between exercise and avoidance of adverse metabolic outcomes, the researchers indicated.
Davis and colleagues recruited overweight or obese school children from the Augusta area from 2003 to 2006 to participate in the study for up to 15 weeks. A total of 222 entered the study, with 209 completing one semester.
Children included in the trial had a baseline body mass index of at least the 85th percentile for their age and were considered sedentary. The latter was defined as no participation in a regular physical activity program providing more than 1 hour of exercise weekly.
They were randomized 1:1:1 to their usual level of physical activity or to participate in after-school exercise programs that provided 20 minutes (low dose) or 40 minutes (high dose) of aerobic exercise every school day for a semester. The activities varied, with several running and jumping games employed, such as modified forms of basketball and soccer.
Children in these programs were rewarded for intensity of effort, rather than for conventional athletic performance measures such as points scored or team wins. Davis and colleagues noted that such a reward system would "encourage even unfit children to exercise intensely."
Additionally, the families of the children enrolled in the study were offered monthly lifestyle education classes on healthy diet, physical activity, and stress management. However, there were no dietary restrictions in placed on the participants.
Primary outcome measures included the area under the curve for serum insulin during an oral glucose tolerance test (insulin AUC); aerobic fitness as measured by peak oxygen consumption; percent body fat as measured by dual energy x-ray absorptiometry; and visceral fat measured with MRI scans. Outcomes were calculated on an intention-to-treat basis.
At baseline, the mean BMI was 25.9 kg/m2, which translated to the 97th percentile for the overall sample. Some 85% were considered obese. Mean body fat percentage was 40.5% and participants carried an average of 33.4 cm3 of visceral fat. The authors stated said 28% of the children had prediabetes.
Mean insulin AUC at baseline was 16.3 mU/mL.
After adjusting for age, sex, race, and Tanner growth stage, insulin AUCs declined by a mean of 2.96 mU/mL relative to control in the low-dose exercise group and by 3.56 mU/mL versus control in the high-dose group (both P<0.05 versus control, P=0.64 for high versus low dose).
Body fat percentage dropped by 0.84 points in the low-dose group relative to control and by 1.42 points relative to control in the high-dose group (both P<0.05 versus control, P=0.13 for high versus low dose).
Visceral fat volume was reduced by 2.77 cm3 relative to control with low-dose exercise and by 3.86 cm3 with high-dose exercise (both P≤0.01 versus control, P=0.32 for high versus low dose).
For the remaining primary outcome, aerobic fitness, both exercise groups showed improvement of 2.4 mL/kg/min relative to control (P<0.05), with no meaningful difference between them.
These improvements with exercise in all four primary outcomes were apparent relative to baseline levels within the study arms as well as relative to the control group.
Although significant differences in outcomes between the low- and high-dose groups were generally not found, Davis and colleagues interpreted the general pattern as showing "a dose-response gradient."
Limitations to the study included its short duration, lack of follow-up after the interventions ended, and its open-label design, the researchers indicated. Also, the daily snack, which was offered only to the exercise groups, may have affected outcomes.
Primary source: Journal of the American Medical Association
Davis C, et al "Exercise dose and diabetes risk in overweight and obese children: a randomized controlled trial" JAMA 2012; DOI: 10.1001/2012.jama.10762.