Discussion
The impact of school-based PA interventions on obesity is inconsistent across studies.[20,53–55] In the present meta-analysis of 18 studies, we found an overall significant impact of long-term PA intervention on reducing BMI gain among primary school children (2.23 kg/m2 greater in the intervention group than in the control group). Therefore, long-term PA intervention should continue to be stressed as one of the core strategies in battling childhood obesity.
In our study, the PA&N intervention durations that lasted for at least one year had a positive impact on children's ΔBMI. Several reviews shared the same conclusion as our analysis.[56–59] Lavelle et al concluded in a recent review, when the intervention duration of the studies involved ranged from 1 to 72 months, that school-based PA interventions are effective in promoting healthier BMI among children under 18 years old.[60] A synthesis analysis of existing systematic reviews and meta-analysis from Khambalia et al also stated that PA intervention lasting for over one year, especially combined with diet intervention, significantly reduced body weight in children.[59] However, the similar trends were not always found in some other reviews.[30,61–64] In a meta-analysis with a total of 18,141 children included and the interventions lasting for a minimum of six months, Harris et al. declaimed that school-based PA interventions do not improve body composition,[64] although PA is one of the key components of a healthy lifestyle and contributes to many aspects of health.[65] Despite the positive effect of PA interventions on physical activity behaviors, i.e., increased MVPA and prolonged time students spent on PA, Lonsdale et al declared in his review paper that PA interventions had little effect on BMI.[63]
In our results, weekly PA intervention for both ≤100 min and >100 min reduced children's BMI increment significantly, while the reduction of BMI increment was higher in the group with intervention ≤100 min. A common sense would be an intervention introducing longer time PA should be more effective on reducing BMI increment. However this was not supported by our results. Our results, in addition to indicate the shorter PA time the better, tended to indicate that a well-designed intervention program should be implementable as well as scientifically sounds. An intervention simply using long PA time as the intervention strategy might result in a poor implementation, and then lead to poor outcome. To improve the interest and spirit of children's attitude to get a higher intensity and longer duration of MVPA in primary school children, more appropriate and enjoyable PA interventions involving a variety of activities are needed.[66,67]
The large variation in characteristics of participants, such as a wide age span (6 to 18 years), large range of intervention duration (from 1 month to 6 years), and PA intervention intensity (from additional 4-min walking/running per day to 3-h MVPA) not only led to the inconsistent conclusions regarding the impact of PA intervention across the studies, but could also hinder the discovery of the real impact. For example, the impact of the long-term intervention on BMI, if any, might be comprised by those studies with short intervention duration, as the significant reduction of BMI is unlikely to happen in a short-term. This reinforces the importance of investigating the effect of long-term PA intervention on BMI among children with a narrower age span, i.e., elementary students, in order to yield a more promising conclusion.
Along with the strict inclusion criteria in our analysis, a high overall heterogeneity in the recruited studies was observed and is considered as one of the limitations of the study. Seven studies identified by sensitivity analysis have the greatest impact on the overall BMI change,[33,38,41,42,46,49,52] with the BMI SMD as -2.23 kg/m2 (90 % CI: -2.92, -1.56) for the 18 studies and -2.0 kg/m2 (90 % CI from -0.30, -0.09) for the remaining 11 studies. Some characteristics of the seven studies, including study duration, intervention type, PA duration, age range, sample size, year conducted, and geographic area accounted for the high heterogeneity. For instance, Angeloupoulos et al had the shortest intervention duration (12 months), a relatively short PA duration (60 min/week) but the widest age range (from grade one to six);[41] studies from Manios et al were conducted in early 90's which is about one decade earlier than the rest of the studies,[33] when the obesity problem had not yet become as serious as it is now.[68] The sample size of Magnusson et al.[42] is only 166 and Llargues et al.[38] is the only one that was conducted in an African country. These variations could also lead to the high heterogeneity. In the study of Lohman et al.,[49] the details regarding the age range of included children and the measurement of PA level were not available, which may also contribute to the heterogeneity among studies. As a result, multiple factors, such as the age of participants, the type of intervention and the duration, sample size, socio-economic variables, etc., collectively contributed to the high heterogeneity. After the exclusion of the seven studies, the heterogeneity was reduced but still remained relatively high, which can also be found in some former synthesis of systematic reviews and meta-analysis.[59] Analysis from Shijun Li suggested that I2 was only suitable for testing heterogeneity amongst small sample size trials.[69] As the current meta-analysis involved more than 19,700 children, I 2 might not be an appropriate variable to assess heterogeneity in this study.
Another limitation of the analysis was about PA intensity. It would be more straightforward if a recommendation regarding the best practice of PA intensity could be provided. Unfortunately, as the included studies used inconsistent measurements or definition of PA intensity (e.g., different questionnaires or different electronic equipment were used to collect the data about PA intensity), we were unable to specify the PA intensity in our analyses. This limitation also existed in previous studies, too.[11] Further work is needed to establish more comparable PA measurement standards that can be used in different studies.
The asymmetric funnel plot indicated that studies included in our analysis may have publication bias. This may be due to the publication preference that reports with positive or significant outcomes are more likely to be published or reported.[70] The low quality of included studies (low JSS) was also of concern in the current analysis.
Reduced level of PA and increased sedentary lifestyles have greatly contributed to the rapid increase of childhood obesity prevalence.[71] The competitive society and score-oriented education strategy and long school hours have further edged out the exercise time of school-age children.[72] For example, although the Chinese government has initiated a recommendation that students should achieve at least 60 min of after-class PA per day,[73] only 14.5 % of the students reached this goal in 2011.[74] In the United States and Canada, children and adolescents are also falling short of benchmarks for PA and fitness.[75,76] Our results reinforce the importance of PA interventions in the school settings to battle the prevalence of childhood obesity. More venues for PA exercises and more feasible and attractive curricular and extracurricular physical exercises should be made available to ensure the quality and quantity of PA among school children. How to effectively implement the school PA, policy should be carefully determined.
BMC Public Health. 2016;16(205) © 2016 BioMed Central, Ltd.
