of 191 persons for the successful implementation of the program, or the ones who delivered the main part of

the program (not experts, not teachers, not parents/adult carers). Peers could be trained professionally by specialists or teachers but training had to occur before the intervention or separately and independently from the peer-led intervention.

Intervention contents

We included all studies with peer-led programs that involved diet and nutrition, physical activities (sport, exercises…), lifestyle and social support. We included interventions from all types of settings including (but not limited to) schools, out of school-hour care, home, community, childcare or preschool.

Types of comparison

We accepted studies which compared peer-led intervention with a non-peer-led (teachers, experts, parents) active control group or with a non-active control group who received usual care or regular school curriculum.

Types of outcome measures

We only included the studies with one or more of the following obesity outcomes, presenting a baseline and a post-intervention measurement, or a change from baseline. Obesity outcomes in our systematic review are

 Body Mass Index (BMI, kg/m2): A measure of weight adjusted for height, calculated as the child’s weight (in kilograms) divided by the square of their height (in metres).39 _{A BMI value is}

a relative number, especially for children with younger age, because the cut points that define overweight and obesity in growing children varying with age and sex, and require BMI-for-age reference standards39 _{(national and international reference standards are available, such as}

those from World Health Organization (WHO)40 _{and International Obesity Taskforce (IOTF)}41_).  BMI z-score (or BMI standard deviation scores): Also a measure of weight adjusted for height (BMI) but adjusted for child age and sex with an appropriate reference standard, and expressed as the number of standard deviation scores from the reference mean for the children age and sex.

 BMI percentile (or BMI-for-age percentile): A BMI percentile indicates the percentage of children in the BMI reference who have a lower BMI for the child’s age and sex. It is similar to BMI z-score in that it requires a reference standard to determine the corresponding percentile of a BMI value. It is similar to BMI z-score but easier to understand and interpret in clinical settings.

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 Waist circumference (cm)42 43_{: Is a measurement of the abdomen taken at the level of the} umbilicus using stand methods recommended by WHO, CDC or NHS. It is a good indicator of

body fat, especially internal fat deposits, and an indicator of the likelihood of developing weight-related metabolic disease.

 Body weight: Is measured by a scale in either kg or pound, with precision of at least 0.2 units.

Search methods for identification of studies Electronic searches

We performed searches in January 2018 for relevant studies in 8 databases MEDLINE, EMBASE, CINAHL, AMED, Cochrane Central register for controlled trials (CENTRAL), Web of Science, PsycINFO, and WHO Global Health Library. We searched the databases since their inception date until January 19th_{, 2018. Details of the search strategy are included in the appendices.}

Other resources

In March 2018 we also performed searches on following websites to identify other potential studies that may have been missed in the database searches.

 Registry of clinical trials of the United States National Library of Medicine at

https://clinicaltrials.gov/

 European Clinical Trial Registry at https://www.clinicaltrialsregister.eu/ctr-search/search

 WHO International Clinical Trials Registry Platform (ICTRP) at

http://www.who.int/ictrp/search/en/

 UK Clinical Trials Gateway at https://www.ukctg.nihr.ac.uk/#popoverSearchDivId

 Canadian Health Evidence at https://www.healthevidence.org/

 Google Scholar at https://scholar.google.com

We also checked the reference lists of systematic reviews relevant to our study (identified from the searches above), and the reference lists of the final studies included (once we finished screening the full-text) for information on peer-led interventions to identify potential additional studies for consideration.

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Selection of studies

We included studies that matched all the above criteria (randomized controlled design, children aged less than 18 years, peer-led intervention duration of four weeks or more, with a non-peer-led comparison, and obesity outcome measures. We did not limit the time of publication of the studies but did limit the language to English.

We rejected studies that violated any of the five main criteria such as those articles which did not have a controlled design (i.e. observational or cohort design, or reviews of RCT). Papers were also rejected if they did not aim to reduce or prevent child obesity or had no obesity outcomes in the objectives; or if there was no involvement of peers in any activity of the intervention; or if the participants were exclusively above 18 years, pregnant teenagers/young adults, or children less than three years; or if the duration of the trial was less than four weeks.

When a title or abstract could not be classified as “included” or “excluded”, we retrieved the full text of the article for further investigation. Two review authors independently assessed the studies for inclusion and resolved any disagreement by discussion, and when necessary in consultation with a third review author.

Data extraction and management

We developed the data extraction form based on our criteria of inclusion (design, participants, interventions, comparison and outcome measures) with subgroup characteristics in the design (as cluster RCT or regular RCT), participants (overweight/obesity only, mixed weight status), interventions (totally or partially peer-led), comparison group (non-peer led active control, non-active control) and outcome measures (BMI, BMI z-score, BMI percentile, waist circumference, body weight). We also extracted the peer leaders’ information which covered the number peer leaders, their gender, age, and the ratio of peer leaders to participants.

We first checked all included articles for the list of subgroup headings and then extracted the content in each article according to the extraction form including the quantitative outcomes (mean, SD, or 95%CI) for analysis in an Excel spreadsheet.

Assessment of risk of bias in included studies

We evaluated the risk of bias of included studies based on the tool provided by The Cochrane Collaboration44_{. The tool covered seven categories of bias: selection (random sequence generation}

and allocation concealment), performance, attrition, detection, reporting, and other bias (the last one to capture other potential threats to validity). Each category was judged as being at high, low, or

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