This randomized clinical trial with exceptional adherence and retention quantified the efficacy of monitored aerobic exercise training to reduce diabetes risk (i.e., insulin resistance) and other indices of cardiometabolic risk in sedentary, overweight and obese children, 28% of whom had prediabetes. A daily aerobic exercise intervention over 3 mo. showed clear dose-response benefits reducing diabetes risk, as assessed by insulin response to OGTT, fasting insulinsurrogate indices of diabetes risk integrating insulin resistance and β-cell function. The high-dose exercise intervention demonstrated significant benefit on the DIOGTT
, a surrogate index of diabetes risk integrating insulin resistance and β-cell function, which is an excellent predictor of diabetes incidence in adults.22,33
The reductions in fasting insulin moved most participants in the exercise groups from a high to a borderline high clinical category for insulin resistance.34
No intervention effects were detected on fasting glucose or the DIFI
. Dose-response improvements in detailed measures of fatness were observed, and the two exercise doses showed similar improvements on fitness. This extends the literature with quantified benefits, including weight loss, from closely monitored, selected doses of aerobic training with no dietary restrictions. No evidence for EE compensation was provided.
No difference in efficacy was noted between boys and girls, black and white children, or children with prediabetes vs normoglycemic children. These consistent effects of intervention do not conflict with cross-sectional race differences reported in the literature (lower visceral adiposity, greater insulin resistance, and higher disposition index in black children),35–37
but it contrasts with the prospective finding that black girls are less sensitive than white girls to the effects of physical activity on fat accretion.38
An effect modification for the effect of exercise on fasting insulin was detected for family history, but this appeared to be due to one extreme value, probably due to noncompliance with fasting. Therefore the cardiometabolic effects of exercise appear to be generalizable to overweight black and white boys and girls, regardless of prediabetes or family history of diabetes.
The increment of benefit between the control and low-dose conditions was larger than the additional benefit observed between low-and high-dose groups. The greatest benefit is obtained from a given amount of physical activity in the most sedentary people, with smaller benefits accruing to people who are already moderately active.39
The low- and high-dose groups showed nearly identical effects on fitness. A similar result on insulin resistance was obtained by the STRRIDE study, where the low-volume, moderate intensity group improved more than a similar volume, high intensity group, and had similar improvement to the high-volume, high-intensity group. Moderate as well as vigorous activity was linked with insulin sensitivity in a population study.40
Inflammation from a large volume or high intensity of exercise may impair insulin sensitivity.41
Fitness benefits may be gained based on intensity rather than volume of exercise.42
This study was powered to detect a dose-response gradient but was unable to distinguish between these daily volumes of aerobic activity, except for subcutaneous abdominal fat and BMI z-score, for which greater benefits were observed with 40 than 20 min daily vigorous activity.
Though several exercise studies have now utilized an 8–9 month training period, more than twice that of the current study, the 5 day/wk frequency in this study is rare.43
The Cochrane review of obesity treatment trials44
includes only 9 focused on physical activity in children under 12, and only 1 of comparable size (N = 218). In the larger studies, interventions consisted of clinical advice rather than monitored exercise. Most interventions were of similar or shorter duration. Physical activity interventions were of lower intensity and frequency (contacts with subjects from 1/mo to 3/wk) and few isolated exercise rather than combining it with dietary intervention. Nonetheless, the relatively short duration of intervention, and lack of follow-up assessment of possible lasting effects, are limitations of the current study. There were other limitations. Participants were not blinded to condition, because it was a behavioral intervention. Measurement staff were not blinded. The control group was not offered an attention-control intervention program; daily attention from adults and the minimal nutrition intervention may have affected outcomes in the exercise groups.
Large, well-conducted school-based studies have tested effects of physical activity on obesity in children, and have failed to reduce obesity, perhaps due to inadequate dose;45,46
one succeeded only in girls.47
The HEALTHY study was designed to reduce risk for type 2 diabetes using multiple school-wide strategies to improve nutrition and physical activity over 3 years; it improved adiposity measures and fasting insulin by a small amount. This efficacy study, with a more intensive, focused intervention, achieved 3X the effect on BMI z-score and 8X the effect on fasting insulin in overweight children in a short time. These results contrast with a similar exercise intervention in black girls that despite longer duration (10 mo.) and improved adiposity and fitness, did not reduce fasting insulin concentration.11
That study did not restrict enrollment to overweight or obese children, who are more insulin resistant and may be more sensitive to intervention than normal weight peers.
Twenty minutes of aerobic exercise per school day over just a few months showed benefits over the control condition on insulin resistance, fitness, and fatness. Thus, measurable health benefits could be achieved through a daily dose of safe, vigorous physical activity which could be achieved during the school day by providing daily fitness-focused physical education classes, recess, and other physical activity opportunities.48–52
However, to achieve the benefits of 40 min/d of vigorous physical activity (the basis for the 60 min/d recommendation for physical activity for free-living children),14
after-school physical activity programs may be necessary. Schools are the logical focus for such public health interventions.49
An ancillary study showed benefits of the exercise intervention on cognition and mathematics achievement, which may increase its appeal to educators.53
Inclusive, appealing interventions with fun, simple games that minimize barriers to participation will be most effective. Using heart rate as a physiological index of effort and providing contingent rewards for such exertion, rather than athletic performance, encourages even unfit children to exercise intensely enough to improve fitness and improve energy balance.