Table 2. Summary of the volunteers and the interventions.

Study	Volunteer Description	Training	Intervention
Chan 2014 [49] Yeung 2018 [56]	Peers: people with A1c < 8.0%.	Time commitment: 32 hours Content: Tutorials, case sharing, reflections, role playing, games, and activities. Focused on mindset, empathic listening, questioning skills, and counselling skills. Addressed factors that could influence blood glucose level (e.g. diet, exercise), self-monitoring of blood glucose, sick day management, foot care, emotional support, resources for information, and clinical care. Materials: Tutorial notes, reference materials Ongoing supports: NA Assessment: Pre- and post-training evaluation of diabetes knowledge and psychological-behavioural measures	Length of intervention: 12 months Time commitment: 2-hour face-to-face session, 15 minutes ≥ 12 phone calls Volunteer role: NA Patients per volunteer: 10 Volunteer-patient matching: NA Materials: Resource booklet, checklist for calls Mode: Telephone Stipend amount: US $500
Heisler 2019 [47]	Peers: people with a history of poor glycemic control (A1c ≥ 8.0%), but whose most recent A1c was < 8.0%	Time commitment: 2 hours (base training), 1 hour (iDecide training), 1.5 hour/month (follow-up meetings) Content: Motivational Interviewing-based communication skills (open-ended questions, rolling with resistance, eliciting ’change-talk’, and goal-setting and ’action planning’) Materials: iDecide (personally tailored diabetes medication decision aid) on iPads Ongoing supports: Monthly meetings with other peers to check-in and provide booster follow-up training Assessment: Self-assessment and random observation of phone calls by the study team	Length of intervention: 6 months Time commitment: 2-hour face-to-face session, ≥ 1 phone call/week Volunteer role: During the initial face-to-face session, peers and patients identified a behavioural goal and an action plan and generated a list of questions and concerns for health care providers. During follow-up phone calls, peers and patients brainstormed solutions to barriers and set new goals and action steps Patients per volunteer: 1–5 Volunteer-patient matching: NA Materials: Diabetes medication guides, iDecide Mode: Face-to-face, telephone Stipend amount: NA
Hsu 2021 [48]	Community health workers: four people selected from the community.	Time commitment: 4 hours Content: Periodontal disease and care, teaching and communication skills Materials: Training manual containing goals and contents of each lesson Ongoing supports: CHWs could contact research staff for support in the month prior to the intervention Assessment: Community health worker certification test	Length of intervention: 1 month Time commitment: 30 minutes x 4 face-to-face lessons Volunteer role: Peers taught patients about effective toothbrushing methods and tools Patients per volunteer: NA Volunteer-patient matching: NA Materials: Slide presentations, toothbrushing tools Mode: Face-to-face Stipend amount: US $70 per patient
Long 2020 [50]	Peers: patients with a history of poor glycemic control, but whose most recent A1c was ≤ 7.5% OR previous study mentees (with no A1c restriction).	Time commitment: 1 hour Content: Mentee’s story/motivations (to help set realistic goals and provide support), dealing with failure in an accepting manner, role-playing exercises, and sample questions Materials: NA Ongoing supports: Staff contacted peers 1 time/month to check in and provide training reinforcements Assessment: NA	Length of intervention: 6 months Time commitment: ≥ 1 phone call/week Volunteer role: NA Patients per volunteer: 1 Volunteer-patient matching: Based on age, race/ethnicity, sex, and insulin use. Materials: NA Mode: Telephone Stipend amount: US $20 per month for each month peers attempted to contact their assigned mentee
Sampson 2021 [51]	Peers: people with T2DM.	Time commitment: 14–17.5 hours Content: Impact of physical activity, diet, pre-diabetes, and lifestyle on T2DM. Also role-playing exercises Materials: NA Ongoing supports: NA Assessment: A mock call where the senior research associate assumed the role of trial participant to test peers in specific situation	Length of intervention: 46 months Time commitment: ≥ 18 phone calls. Volunteer role: Peers and patients discussed progress, goal achievement, action planning, and barriers to coping Patients per volunteer: ≤ 7 Volunteer-patient matching: NA Materials: NA Mode: Telephone Stipend amount: £350
Sazlina 2015 [52]	Peers: older adults (≥ 60 years) with successfully managed T2DM who lived in the same community as participants.	Time commitment: 2 days Content: Interactive discussions, simulations, and role-plays to improve peers’ ability to provide support through telephone and face-to-face contacts Materials: NA Ongoing supports: Two fortnightly and two monthly debriefing meetings over the 12-week intervention Assessment: Assessments by the research team at monthly clinic visits with their peers	Length of intervention: 3 months Time commitment: 3 face-to-face meetings, 3 phone calls Volunteer role: Peers and patients discussed barriers and motivations, and peers encouraged patients to become empowered to increase their physical activity to self-manage their diabetes Patients per volunteer: 3–5 Volunteer-patient matching: NA Materials: NA Mode: Face-to-face, telephone Stipend amount: NA
Siminerio 2013 [53]	Peers: patients who had previously attended diabetes self-management education selected based on their communication skills and willingness to participate (no A1c restriction).	Time commitment: 2–3 hours Content: Information about active listening, empowerment, and behavioural approaches. Also role-playing exercises to practice skills Materials: NA Ongoing supports: Peers could contact a Certified Diabetes Educator or their Primary Care Provider Assessment: Human subject modules of the associated universities	Length of intervention: 6 months Time commitment: ≥ 5 phone calls Volunteer role: Peers and patients engaged in a patient-centred discussion regarding the patient’s behavioural goal and barriers to achieving their goal Patients per volunteer: NA Volunteer-patient matching: NA Materials: Telephone scripts, behavioural goal forms Mode: Telephone Stipend amount: NA
Sreedevi 2017 [54]	Peers: three people with T2DM (RPG < 250 mg/dL) selected from the community based on their adherence to treatment, and capacity to be a successful mentor.	Time commitment: 2 days Content: A physician explained diabetes, glycaemic control, and medications and their synergies with physical activity. A nutritionist explained the nutritional aspects of diabetes. A psychologist provided training in communication skills, empathy, and confidentiality Materials: Training manual (based on peers for progress handbook) Ongoing supports: NA Assessment: NA	Length of intervention: 3 months Time commitment: 45–60 minutes x 1 face-to-face meeting/week, 1 phone call/week Volunteer role: During the initial session, peers collected treatment details and went over the functions of peer support. During follow-up sessions, peers and patients discussed diet, exercise, medication, emotional stress, diabetes symptoms, foot care, and more. During the final sessions, the peer conducted a final process assessment Patients per volunteer: 13–14 Volunteer-patient matching: NA Materials: Diary (for patients) Mode: Face-to-face, telephone Stipend amount: NA
Tang 2022 [55]	Peers: patients with A1c < 8.0%.	Time commitment: 30 hours Content: Knowledge, skills, and strategies to address (1) assistance in daily self-management, (2) social and emotional support, and (3) linkage to clinical care. In particular, skills in motivating and empowering patients, active listening, goal-setting and action planning, and problem solving Materials: NA Ongoing supports: NA Assessments: Formative and summative assessments of five domains: diabetes-related knowledge, empowerment-based facilitation, active listening, goal-setting, and perceived self-efficacy. Also ’spot check’ phone calls made by the research team to peers	Length of intervention: 12 months Time commitment: 1 face-to-face meeting, 29 phone calls Volunteer role: Peers and patients discussed challenges, feelings and questions about self-management, solved problems, and set goals and develop action plans Patients per volunteer: NA Volunteer-patient matching: Based on gender and geographical proximity. Materials: NA Mode: Face-to-face, telephone Stipend amount: CAD $400 following training, then $20 per participant per month [86]

Abbreviations: CHW = Community Health Worker; NA = Not Available; T2DM = Type 2 Diabetes Mellitus