Validating weight optimization’s Impact on participants via Social Impact Bonds

Abstract Background To promote health for everyone, even the health-unconcerned, Okayama City has established the Social Impact Bond (SIB), which funds and executes social programs for defined outcomes, with third-party assessment. This study examined the normalization of participants’ physiques three years later to verify the effectiveness of SIB. Methods We conducted a study of a quasi-experimental design, non-randomized controlled trial. A total of 8,251 eligible people applied for the program. Of these, 1,599 were enrolled in the National Health Insurance System (NIH), and 731 of them were included in the intervention group in 2018. Among 705 non-participating matched members and randomly selected, 170 were used as the control group. Whether or not the standard physique in the endpoint (EP) FY2021, was used as the dependent variable, and the participation as the independent variable. Age, gender, and physique at baseline (BL) were used as covariates for logistic regression analysis. Results A total of 532 patients underwent health checkups for both BL and EP, and the follow-up rate was 60.5% in the intervention group and 52.9% in the control group. Examination of between-group differences in BL revealed that the mean age was 66.7 years significantly higher in the males’ intervention group and 60.6 years in the control group (p < 0.01). The crude odds ratio (OR) of maintaining or achieving standard BMI in the participating group was 0.90, and the adjusted OR was 1.23 and 1.31 after propensity score matching, both were not significant. On the other hand, when restricted to people with thinness, the pre-adjusted OR was 5.00 and the adjusted OR was 12.43. Conclusions Participation in the project contributed about 20% to 30% to weight optimization, and it could not be denied that it was brought about by chance. However, when limited to thin physiques, project participation was about twelve times higher, and the probability of having an appropriate physique was increased. Key messages • SIB might affect the BMI optimization for older people in improving thinness. • Efforts will be needed to encourage participation among younger age groups.