Effects of Strength Training on Body Fat in Children and Adolescents with Overweight and Obesity: A Systematic Review with Meta-Analysis

Childhood overweight and obesity represent a growing public health problem worldwide. Since the 1980s, the global prevalence of overweight and obesity in childhood and adolescence has increased by 47%. The promotion of exercise is an important intervention to reduce the physical damage of obesity. The meta-analysis was conducted in accordance with the general guidelines for the reporting of systematic reviews and meta-analyses (PRISMA). The PubMed, SciELO, ScienceDirect and Google Scholar databases were searched from August to December 2021. The search yielded 722 titles published between 2000 and 2021. After screening the titles and abstracts, 64 duplicate articles were detected, and 27 articles were ultimately included in the systematic review, including 26 articles published in English and one published in Spanish. There was a statistically significant effect of the strength training interventions on the percentage of body fat, Test of 0 i = (p = 0.00, z = 6.92), Test of 0 = (p = 0.00, Q (9) = 42.63). The findings reveal that strength training has a positive impact on the treatment of body fat in children and adolescents with overweight and obesity.


Introduction
Childhood overweight and obesity is a global public health problem [1]. It is defined as a condition in which the body carries excessive and unhealthy amounts of body fat, which causes the individual to weigh more than their ideal weight [2]. This complex and multifactorial disease can begin in childhood and may be caused by a genetic-environmental interaction [3].
An imbalance between the energy expended (exercise) and the energy ingested (food) could lead to overweight and obesity. When food intake regularly exceeds calorie expenditure, unused energy is stored in adipose tissue or body fat [4]. Most people who eat and drink more than they use energy will produce adipose tissue to store excess energy [5]. There are many children who have a normal BMI but have a high percentage of body fat the effect of strength training on body fat in children and/or adolescents with overweight or obesity.

Study Search and Selection Strategies
The present meta-analysis was performed in accordance with the general guidelines for the reporting of systematic reviews and meta-analyses (PRISMA) See Figure 1. The PubMed, SciELO, ScienceDirect and Google Scholar databases were systematically searched from August to December 2021. The registration protocol was carried out in the Open Science Framework with the registration number 10.17605/OSF.IO/MNCNBI.
The following keywords were used in English and Spanish. Target population: Youth, youth, child, adolescent, puberty, boys, girls; Strength training: resistance training, muscle strength training, muscle strength program, muscle strength intervention, muscle strength exercise, weight training, strength and conditioning, concurrent training; Weight status: obese, overweight, body composition, waist circumference, fat, body mass, weight, skin fold. For studies that did not provide access to the complete document or studies that lacked the necessary descriptive statistics, the authors were contacted via email. The following keywords were used in English and Spanish. Target population: Youth, youth, child, adolescent, puberty, boys, girls; Strength training: resistance training, muscle strength training, muscle strength program, muscle strength intervention, muscle strength exercise, weight training, strength and conditioning, concurrent training; Weight status: obese, overweight, body composition, waist circumference, fat, body mass, weight, skin fold.
For studies that did not provide access to the complete document or studies that lacked the necessary descriptive statistics, the authors were contacted via email.

Inclusion and Exclusion Criteria
The inclusion criteria were as follows: (I) participants were aged between 5 and  19 years, (II) body fat was analyzed, (III) randomized controlled trials, (IV) a muscle  strength training was implemented and (V) original publications in English or Spanish  published from 2000 to 2021. The exclusion criteria were as follows: (I) the subjects had a pathological condition or disability that affected movement (e.g., cerebral palsy/dyspraxia), (II) the subjects were found to have a behavioral or neuropsychological condition and (III) plyometric, vibratory or neuromuscular training was used, or training specifically for rehabilitation purposes was implemented.

Data Extraction
The data were extracted using an electronic form and included study characteristics (country, year of publication, sample), characteristics of the participants (age, nutritional status), components of the intervention (duration, type of training), outcome measures (body fat, BMI, waist circumference) and methodological quality. The study was carried out by two researchers, they evaluated the selection of titles and abstracts. Following that, full articles will use the inclusion and exclusion criteria. No automated tools were used in the process.

Evaluation of Methodological Quality and Risk of Bias
To evaluate the methodological quality and the risk of bias of the included studies, the "Quality assessment tool for quantitative studies" developed by the public health practice project was used [20]. The results of the evaluation led to a general methodological rating of strong, moderate or weak in eight sections: selection bias, study design, confounding factors, blinding, data, collection methods, elimination and dropouts, originality of the treatment and analysis. This evaluation tool has been proven to be valid and reliable. To verify the reliability, this evaluation was carried out in 100% of the included studies, and any disagreement was resolved by discussion between the two authors.

Data Analysis
The reference data were expressed as numbers, proportions, averages and standard deviations. The meta-analysis was performed with the statistical package Stata/IC14.2 (Texas, USA), and a random effects model was used to examine the decrease or increase in percentage of body fat with 95% confidence intervals (CI). The T 2, H 2 and I 2 statistics were used to evaluate the heterogeneity of the studies; heterogeneity was not determined between the studies if I 2 was less than 50%, and an I 2 value equal to or greater than 50% indicated heterogeneity. A value of H2 equal to 1 means that there is no heterogeneity, and its value increases when heterogeneity between studies increases. To identify publication bias, the effect sample sizes were plotted against standard errors.

Search Results
A total of 722 articles were identified from the PubMed, Science Direct, Scopus, SciELO and Google Scholar databases. In the screening phase, duplicates were eliminated, and the studies were filtered by screening the titles, abstracts and keywords, resulting in 361 references. A total of 169 studies were subjected to full-text analysis, with 55 studies being excluded because they were not focused on strength training, 49 being excluded because they did not specify the type of training applied, 36 being excluded because they did not evaluate variables related to body fat, and 2 being excluded because they did not specify the state anthropometric parameters of the children. Twenty-eight studies met the selection criteria .

Characteristics of the Studies
Across the 28 included articles, 1834 children and adolescents with overweight or obesity were analyzed, with an average age (years) of 12.5 ± 2.6 (min: 6 max: 19). The average duration (weeks) was 12 ±. 7.3, the average frequency (weekly) was of 3 ± 1.1 days, the average duration (minutes) of 60 ± 10.1 per session, and the average sets per exercise was 3 ± 1.3 with an average of repetitions per exercise of 12 ± 6.6. The studies were conducted in 12 countries: Brazil (10 studies), the United States (4 studies), Australia (3 studies), Canada (2 studies), France (2 studies), Chile (1 study), China (1 study), South Korea (1 study), Spain (1 study), Germany (1 study), Italy (1 study) and Austria (1 study). After the interventions in 92% of the studies, body fat was reduced, the remaining 8% did not present significant results, and the average attendance rate for the studies that measured this variable was 75%. For the studies that provided information, 15% presented a low intensity <50% maximum repetition (MR), 35% medium intensity 50-75% MR, and 50% a high intensity >75%.

Results of Methodological Quality
The Table 1 show the results of the methodological quality assessment, where the studies obtained a moderate score of 60.7%, a strong score of 35.7% and a weak score of 3.6%.

Results of the Meta-Analysis
Ten studies reported the necessary statistics to construct the forest plot and funnel plot [21,25,31,32,37,39,44,[46][47][48]. Using the random effects model, the results of the percentage of body fat were compared after the interventions with 95% CIs. The results of the meta-analysis revealed that ST is an effective intervention for the treatment of body fat (p = 0.00, z = 6.92), (p = 0.00, Q (9) = 42.63). The heterogeneity parameters of T 2 = 1.69, H 2 = 3.56 and I 2 = 71.94% indicated significant heterogeneity. See Figures 2 and 3.

Subgroup Analysis
In an attempt to determine the causes of heterogeneity in our analysis, subgroup analyzes were performed according to the intensity of the studies (A: High and medium intensities >51% MR, B: Low intensities <50% MR). See Figure 4.

Discussion
According with the position statements of the American Academy of Pediatrics [10], UKSCA [17] and NSCA [18] on youth strength training suggest that it can have a positive impact on body fat, and the significant findings of this meta-analysis for body fat percentage are conclusive, supporting these claims. Regardless of dietary restrictions [50], being more effective at training in the gym [22,24,35,41,42] than home workouts [26,31]. Strength training showed a statistically significant effect on body fat percentage. Although this decrease in fat is expected, it is necessary to investigate in-depth studies considering the individual response to the intensity of strength training.
The training intensities that presented the best benefits for the reduction of body fat were high and medium [22,24,34,42,45], a possible cause may be that these intensities are close to muscle failure > 5 repetitions in reserve, this has been shown to be favorable [14], additionally if it is added to an adequate diet [22,37,51] these results are increased.
In the funnel plot (Figure 3), an asymmetry can be observed, which indicates publication bias. A possible cause is that due to the very specific research topic, there are a limited number of relevant studies; likewise, the existing studies have low sample sizes because the inclusion criteria are only children or adolescents with overweight or obesity.
It has been suggested that training periods of more than 14 weeks are required to observe effects on body fat and increases in lean muscle mass [43]. This suggests that the duration of the intervention for several of the studies may not have been long enough to invoke positive measurable changes.
Although more studies are required to provide a better understanding of the mechanism of a reduction in body fat due to a strength training intervention, it has been reported that a possible cause would be that the ST itself favors the loss of fat through the muscle. The muscle tissue releases Extracellular Vesicles (EVs) that present a potential mechanism through which the beneficial effects of exercise are transmitted to other tissues [52,53]. These EVs contain miR-1 absorbed by adipose tissue, promoting adrenergic signaling and lipolysis in adipose tissue, which favors fat oxidation [19]. Alternatively, it may simply be due to an increase in skeletal muscle mass and the resulting increase in basal metabolic rate [54]. In particular, this has been observed in adolescents [55].
On the other hand, it is also suggested that these changes could be due to increases in total energy expenditure that may have occurred simply when participating in an exercise intervention instead of an increase in metabolically active lean tissue. However, it is important to note that the data of the participants, children and adolescents included in the analysis may have had an impact on the results.
There are limitations in this study, that must be taken present into account when explain the results. First, the included studies used several different types of exercise intervention (exclusive strength training or strength training plus aerobic training) with great variability within the study interventions with respect to the number of participants (ranging from 18 to 304 participants). The duration varied from 6 to 52 weeks, the frequency varied from 2 to 5 times per week, and the programs also included a mixture of a series of 1 to 4 per exercise and repetitions with a range of 4 to 25 with low intensities, medium or high according to the 1MR, with different methodologies such as undulating periodization, training of a progressive nature with an increase in the number of sets, repetitions and resistance; in circuit, with linear periodization or with daily undulating periodization, circuit training; and with training at home or in the gym, added to the fact that the age of entry was not measured.
For the results, there were a variety of different measurement methods; for example, the percentage of body fat was measured by DEXA, BodPod, bioelectrical impedance, skinfolds and magnetic resonance. In the analysis, not all studies reported data to allow an exhaustive investigation, so limited conclusions can be drawn based on this level of additional analysis.

Conclusions
The results of this systematic review confirm that ST could be an effective intervention for the treatment of body fat percentage in the first 14 weeks of intervention with better longterm results (>36 weeks), in turn, high and medium intensities are beneficial for reducing body fat percentage. However, a more in-depth research is needed on ST intensities and their effect at the individual level in children and adolescents, these findings can be used to develop new methods for the treatment of childhood obesity.