Physical Activity and Cardiovascular Risk Factors in Children: a Meta-Analysis Update

Background: Obesity and overweight in childhood can increase the risk of developing cardiovascular disease throughout live. Objectives: This study provides an update of a meta-analysis of randomized clinical trials (RCT) published in 2014, to assess the effects of physical activity interventions on preventing cardiovascular risk factors in childhood. Methods: This update combines data from the previous search with new data obtained from June 2013 to June 2020. Searches were performed on PubMed, EMBASE and Cochrane CENTRAL. The RCTs enrolled used interventions with physical activity longer than six months in school children aged 6-12 years, and evaluated body mass index (BMI), systolic blood pressure (SBP), diastolic blood pressure (DBP), total cholesterol (TC), triglyceride (TG), and low-density lipoprotein (LDL) and high-density lipoprotein (HDL) levels. Data analysis was performed using a random-effects model and a P value <0.05 was considered statistically significant. Results: A total of 28,603 articles were retrieved, and 17 RCTs (11,952 subjects) were included. Physical activity interventions were associated with reduction in SBP [−2.11mmHg (95%CI −3.67, −0.54), I²43%], DBP [−2.08mmHg (95%CI −3.68, −0,49), I²65%] and TG [-0.08mmol/L (95% CI -0.13, -0.03), I²0%], and increase in TC [0.17mmol/L (95%CI 0.04, 0.30), I²0%]. However, the interventions were not associated with reductions in BMI [−0.03 kg/m 2 (95%CI −0.17, 0.10), I²0%]. Conclusion: This update confirms and reinforces the beneficial effects of physical activity intervention in reducing systolic and diastolic blood pressure and TG levels.

To be included in the protocol, the RCT should meet the inclusion criteria and have at least one of the primary or secondary outcomes. The authors whose articles did not present the size of the intervention effect in the experimental and control group were contacted by e-mail in the attempt of obtaining missing information. In case of unsuccess, the studies were excluded. Studies with nutritional intervention were included only if the physical activity was the main intervention. For articles from multiple publications, the most recent publication was included, the previous ones were used for complementary information.

Information sources
The online databases searched were MEDLINE (accessed by PubMed), EMBASE and the Cochrane Central Register of Controlled Trials (CENTRAL). The following terms were used: "obesity", "overweight", "child nutrition disorders", "child", "school", "student", "exercise", "exercise therapy", "exercise movement techniques", "motor activity", "sports", "physical education and training", in addition to a highly sensitive strategy for the search for randomized clinical trials. 15 The search strategy for PUBMED is shown in Appendix I. The authors searched and verified reference lists of systematic reviews and previously published metaanalysis to identify primary studies. This review had no language restrictions.

Study selection and data extraction
For this update, titles and abstracts of all articles identified by the search strategy were evaluated by three independent researchers in duplicate (B.E, C.C.C. and S.M.B). Those abstracts that did not provide enough information about inclusion and exclusion criteria were selected for a full-text evaluation. Disagreements between reviewers were evaluated by a third reviewer and solved by consensus (L.C.P). The outcomes extracted were: BMI (Kg/m 2 − weight in kilograms divided by the square of the height in meters), SBP and DBP (mmHg), TC and triglycerides (TG) (mmol/L), HDL-c and LDL-c (mmol/L).

Assessment of risk of bias
To assess the internal quality of the studies, we evaluated each RCT according to the Cochrane Collaboration's tool for assessing risk of bias. 16 To be considered of good quality, the studies needed to present growing body of scientific evidence that recommends the practice of physical activity and points out the risks of sedentary behaviors. 9 In a previous meta-analysis published in 2014, 10 our group demonstrated that physical activity was associated with a reduction in triglycerides and blood pressure levels in school children and suggested that physical activity programs aiming at cardiovascular prevention should be stimulated in this population. Since the publication of this review, other clinical trials have been published. Therefore, this update aims to discuss recent evidence of mid-term effects of physical activity.

Methods
This systematic review is an update of the review published in 2014. All included studies in the 2014 review were considered in this update, in addition to studies published between 01 June 2013 and 15 June 2020. Details of our previous systematic review can be found in Cesa et al., 10

Eligibility criteria
Eligible studies were randomized clinical trials (RCTs) with students aged from 6 to 12 years old, submitted to a single or main intervention of supervised physical activity. Population criteria were school children irrespective of body weight (normal-weight, overweight and obese). Interventions were exercises with a minimum of 150 minutes per week for at least 6 months. A control group with no intervention or undergoing lower-intensity exercise (e.g., ordinary physical education classes with a duration of less than 150 minutes per week) should be included. A minimum target of 150 minutes per week was chosen as it is the amount of physical activity a person should engage in to be considered physically active. 13,14 The variables included in the protocol were risk factors for the development of cardiovascular disease: systolic and diastolic blood pressure (SBP and DBP, respectively), body mass index (BMI), total cholesterol (TC), low-density lipoprotein cholesterol (LDL-c), high-density lipoprotein cholesterol (HDL-c), fasting glucose and C-reactive protein (CRP). a clear description of the generation of suitable sequences, including description of the allocation concealment, blinding of the researcher, participants, assessors and evaluators of results, analysis of intention-to-treat and description of losses and exclusions. Concealed allocation was considered if the terms were described as "central", "web-based" or "telephone randomization", or full explanation. For the intention-to-treat analysis, the number of randomized participants and the number of patients analyzed should be the same; exceptions were patients who were lost to follow-up or asked to leave the study (they withdrew their consent). Three reviewers independently performed a quality assessment and, for each criterion, the studies were classified as adequate, not adequate or unclear/unreported.

Data analysis
All analyses were conducted using Review Manager version 5.3 (Cochrane Collaboration). For continuous outcomes, if the unit of measurement was consistent across trials, results were presented as weighted mean difference, with 95% confidence intervals (CIs). Pooled-effect estimates were obtained using the final values. 17 Calculations were performed using a random-effect model and the statistical method used was inverse variance. Statistical heterogeneity of the treatment effects among studies was assessed using the Cochran's Q test and the inconsistency I² test, and values above 25% and 50% were considered indicative of moderate and high heterogeneity, respectively. 18 Sensitivity analysis was performed to evaluate differences in the intervention approach (intervention group: physical education classes + physical exercise program; control group: physical education classes or other types of intervention approaches). Heterogeneity between studies was also evaluated in terms of intensity and duration of the intervention and follow-up. A p-value < 0.05 was considered statistically significant. Funnel plots were constructed to assess the risk of publication bias (Appendix II).

Description of studies
In our previous systematic review, 10 23,091 potentially relevant citations were retrieved and from June 2014 to June 2020 another 5,512 articles were identified (total 28,603).
Six studies met the inclusion criteria, which were added to the previous 11 selected, 10 resulting in 17 studies included in this paper. Figure 1 shows the diagram flow of the studies in this review. Table 1 summarizes the characteristics of all studies (Appendix III).

Risk of bias
Of the six included studies, 47% had adequate sequence generation, 35% reported adequate allocation concealment, 29% reported adequate blinding of clinical assessors, 71% performed intention-to-treat analyses and 94% described losses to follow-up exclusions. Table 2 describes the risk of bias of the studies (Appendix IV).

Total cholesterol, cholesterol fractions and triglycerides
Three studies [Vandongen et al., 19 Walther et al., 32 Jones et al., 30 , n = 499, evaluated TC. Physical activity interventions were associated with TC increase when compared to less intensive or no intervention ( Figure 3), which is not different from previous findings. 10 Two studies , Jones et al., 30 Muller et al., 33 evaluated LDL-c (n = 287) and HDL-c (n = 328). Physical activity interventions were not associated with changes in LDL-c or in HDL-c when compared to less intensive or no intervention ( Figure 3). Triglycerides were evaluated in four [Kriemler et al., 22 Walther et al., 32 Jones et al., 2015; 30 Muller et al., 33 studies (n = 981). Physical activity interventions were associated with reductions in TG when compared to less intensive physical activity interventions or no intervention, similarly to what was previously shown. 10

Systolic and diastolic blood pressure
Six studies [Vandongen et al., 19 Kriemler et al., 22 Walther et al., 34 Jones et al., 20 Muller et al., 33 Aguilar-Cordero et al., 35 19 Kriemler et al., 22 Walther et al., 34 Jones et al., 30 and Aguilar-Cordero et al., 36 n = 1,170, evaluated DBP. Physical activity interventions were associated with reductions in SBP and DBP when compared to less intensive physical activity interventions or no intervention ( Figure 4). These results are not different from the previous data. 10

Discussion
The present study is the first update, since 2014, of the systematic review with meta-analysis including RCTs in children submitted to interventions involving physical exercises of at least six months' duration, differing from other systematic reviews 36       children may, for a period, be taller than those with adequate weight. Therefore, the relationship between weight and height does not change significantly until the child's growth rate decreases. In line with our original publication, it is necessary to reaffirm that BMI is not the best parameter to measure the effect of interventions performed in the clinical trials of this systematic review, precisely because an increase in the index is expected during the studied age range. In addition, only four new studies were added the original analysis of BMI, adding a total of 1,597 children to that of the original publication (n = 10,355). Thus, the observations made in the original review that there was little difference between the control and intervention groups are still valid, since all control groups received regular physical education classes whereas the intervention groups underwent additional exercise sessions. Moreover, to obtain more powerful results in reducing weight in overweight and obese children it is of utmost importance the combination of nutritional interventions (healthier eating habits and choices, lower caloric intake, etc). In this regard, Verjans-Janssen et al. conducted a systematic review of 18 studies where different approaches (physical activity, nutrition, diet, parental involvement and others) were used; 11 studies showed favorable results for BMI, six reported no change and one showed a negative outcome. 39 That review brought results of BMI's reduction, but, again, the review includes studies on combinations of multiple interventions, which differs from the current study where only studies on single physical activity interventions were included.
In addition, the present investigation corroborated the inverse relationship between blood pressure and physical activity in children and adolescents, as presented in our first review 11 and other studies. 40,41 Normal levels of blood pressure were expected as a result of improvement in cardiorespiratory fitness. 40 We may hypothesize that this positive effect may be carried into adulthood, and contribute to cardiovascular risk prevention, since we know that these risk factors (physical inactivity, increased blood pressure and excess weight) start in childhood and may persist throughout life. Furthermore, these positive findings may also indicate a protective role of these factors for hypertension in adults. 41 Liu et al.,42 in a longitudinal cohort study with adult participants in the Bogalusa Heart Study, who had been followed since childhood, demonstrated that the increase in BMI in childhood and adulthood and its overload throughout life are significantly associated with arterial stiffness in young adults, triggered by an increasing trend of hypertension. 42 In 2014, we reported that more physical activity in children was associated with a reduction in TG and an increase in TC. 11 For the new included outcomes LDL-c and HDL-c, the RCTs analyzed in this review did not show significant results, which can be justified by the small number of individuals evaluated. The only lipid component that showed positive repercussions after intervention with physical activity was TG. These results are in line with another meta-analysis that evaluated physical activity intervention in overweight children and adolescents. 43 High levels of TG are an independent risk factor for coronary heart disease, for their potential atherogenic effects. 44 Thus, the results demonstrated in the present study corroborate the importance of the practice of physical activity as a non-pharmacological measure to control elevations in TG levels. Unlike our findings, another systematic review with meta-analysis 45 involving children and adolescents between two and 17 years of age, demonstrated that exercise is associated with LDL-c reduction. However, the authors pointed out that greater reductions in LDL-c were associated with an older age of the adolescents, which could justify the difference in relation to our study, where children between 6 and 12 years were included.
The conflicting results between physical exercise and repercussions on HDL-c levels were also highlighted in another systematic review. 46 Stoner et al., 47 carried out a meta-analysis to assess the effects of physical exercise on cardiometabolic risk factors in adolescents with overweight or obesity and demonstrated no changes in HDL-c levels, in accordance with the present study. Longer periods of intervention with physical activity may be necessary, and aerobic exercise is more beneficial in comparison to regular strength training to improve blood lipids. 47 In disagreement with the present study, Costa et al., 45 demonstrated in a meta-analysis that supervised physical training was associated with a reduction in TC levels compared to no intervention. However, the authors showed that the longer the duration of the follow-up, the smaller the reductions, due to the adaptation of the TC levels. This inverse association could justify the absence of changes in TC levels in the present study, considering that only interventions of at least six months' duration were included. In addition, it is well-known that the peak of TC curves in healthy children aged between 9 and 10 years is approximately 15 mg/dL higher than in younger children and adolescents. 48 Considering that in the study by Costa et al. 45 the sample had a wider age range (between 2 and 17 years), it may not have been sensitive to possible associations between age groups and borderline cholesterol values .
This systematic review has limitations due to the poor methodological quality of the included studies. The absence of blinding can be considered one of the greatest limitations among these studies, although it is usual in non-pharmacological studies. Studies that involve physical activities and no pharmacological intervention have limitations regarding learning curves, introduction and stabilization of the intervention, and combination of other interventions that do not assess physical activity. However, the interventions used in the selected papers were similar, providing strength for the analysis of several components even in a small sample. In our first 10 and in the current systematic review, several studies have been excluded due to the lack of description of the interventions, resulting in a reduced number of RCTs included and of outcomes analyzed. However, considering that these physical activity interventions are of low cost and risk, the positive findings, in favor of the interventions, should be worthy of consideration even in a small sample.
Finally, this systematic review with meta-analysis has strengths that should be mentioned. The process of article selection was conducted with strict criteria, namely RCTs, studies with an intervention period longer than six months (27 weeks), and with outcomes already mentioned in our previous systematic review additionally to HDL-c and LDL-c. In addition, children and adolescents were included regardless of baseline BMI, and the original language of the studies was not considered as an exclusion criterion.

Conclusions
This update confirms the previous findings of the beneficial effects of physical activity interventions of at least six months' duration in reducing SBP, DBP, as well as TG levels. Although no changes in other cardiovascular risk factors (BMI, TC, HDL-c and LDL-c) were detected, we believe that regular physical activity classes in schools must still be encouraged as a way of preventing diseases and promoting health for children and adolescents, primarily during pubertal development and, later, for consolidation of healthy habits throughout life.