The value of adding 'grey literature' in evidence synthesis for equity-driven global health: a case study from Small Island Developing States.

Grey literature, defined as evidence 'not controlled by commercial publishers', can enhance the comprehensiveness, timeliness and balance of evidence reviews. It may be particularly important in global health research, where funding inequities and under-representation of local expertise in peer-reviewed research articles-often published by commercial publishers for substantial fees-can lead to evidence gaps in systematic reviews. However, searching, reporting and analysing grey literature poses conceptual and methodological challenges. We conducted a systematic scoping review of commercially published and grey literature examining food sources in Small Island Developing States (SIDS). The grey literature component aimed to expand understanding beyond built food environments and food retail and to promote epistemic justice by including traditionally excluded voices, knowledge and experiences. This might be especially important in SIDS, which, due to their small size, remote locations and limited global visibility, might not be well represented in commercially published literature. In this paper, we discuss how grey literature enriched our findings, offering detailed descriptions of diverse food sources, regional data and country comparisons less present in commercially published sources. These documents frequently reflected multisectoral initiatives and were developed through partnerships involving local stakeholders. Beyond highlighting the value of grey literature, we also provide practical guidance for its use in evidence synthesis. This includes key principles for effective searching and screening to support researchers aiming to incorporate grey literature in a systematic and transparent way. Our findings underline the importance of including grey literature to avoid excluding valuable perspectives and perpetuating structural imbalances in global health knowledge.