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Highlights • We compared the effects of volunteer- and expert-led weight-loss intervention. • Participants were instructed to maintain a well-balanced, low-energy diet. • The completion proportions was significantly higher in the expert-led group. • The degree of body weight change was similar for both groups. • Such programs could be an alternative strategy for low-cost obesity management., This study compared the effect of volunteer- and expert-led versions of a community-based weight-loss intervention in a non-randomized comparative trial conducted in Ibaraki, Japan from 2016 to 2017. Participants were 145 Japanese adults with overweightness or obesity, aged 20–69 years, with 77 in a volunteer-led group and 68 in an expert-led group. Both groups received the same program content and intervention period. Community volunteers were trained in four or five 3-hour training sessions while experts were highly trained and experienced professionals in the fields of exercise and nutrition prescription. Participants were also instructed to maintain a well-balanced, low-energy diet. The primary outcome measure was body weight change. In the volunteer- and expert-led groups, 58 of 77 (75%) and 61 of 68 (95%) participants completed the 12-week intervention, respectively. The mean (95% confidence interval, CI) weight loss of the volunteer-led group was 6.4 (95% CI: 5.6–7.2) kg, corresponding to 8.9% of initial body weight, while that of the expert-led group was 6.3 (95% CI: 5.5–7.1) kg, corresponding to 8.2% of the initial body weight. The proportion of participants who completed the course was significantly higher in the expert-led group (P