Evaluating the social return on investment of nature-based social prescribing to alleviate loneliness: a randomized controlled trial in Barcelona.

Introduction: Loneliness is associated with adverse health outcomes, including cardiovascular diseases, depression, and premature mortality, and is unequally distributed according to social determinants. Prescribing individuals to community-based interventions in nature, often described as nature-based social prescribing (NBSP), has emerged as a promising intervention to address loneliness. This study aims to quantify the social return of investment (SROI) of the "Friends in Nature" (FiN) intervention designed to alleviate loneliness. Methods: Following key guidelines, we performed a SROI study for a NBSP intervention compared to usual care alongside the FiN trial conducted in Barcelona within the EU-H2020 RECETAS project. The SROI ratio, which measures the social value generated per monetary unit invested, and the net present value, which reflects the difference between social value and investment costs, were estimated. Results: A social return of EUR 1.48 per EUR 1 invested in the NBSP intervention was generated. The net present value was EUR 8,585 per FiN and EUR 8,496 per UC participant. The SROI ratios remained positive and above one in most sensitivity and scenario analyses, ranging from EUR 0.83 to 13.18 per 1 EUR invested. Discussion: We conducted a SROI analysis to quantify the broader benefits of investing in a NBSP intervention to reduce loneliness. This SROI ratio enables decision-makers to consider social value alongside financial value, thereby facilitating more comprehensive comparisons of public health investments. Clinical trial registration: Trial Registration: ClinicalTrials.gov, ID: NCT05488496. Registered 29 July 2022.