Background Grey literature, ”not controlled by commercial publishers”, is recognised to increase comprehensiveness, timeliness, and balance in evidence reviews. Grey literature may be particularly important in global health research, as global funding inequities and underrepresentation of local expertise in peer-reviewed literature may lead to important evidence gaps in systematic reviews. However, searching, reporting, and analysing grey literature comes with conceptual and methodological challenges. Using a worked example of a review of food sources in Small Island Developing States, our objectives were to (a) identify and propose solutions to these challenges; and (b) identify the unique value of grey literature in understanding this topic.

Methods We conducted a systematic scoping review of peer-reviewed and grey literature examining food sources in Small Island Developing States. The grey literature component aimed to ensure the inclusion of evidence on informal food sources and food sharing practices. A combination of MeSH terms and free-text terms representing the food source concept was applied when using publication repositories of 9 UN agencies and website search engines of 8 key international networks. We adapted existing standards of rigour; two researchers independently undertook double screening of all title, abstract and full texts, and 10 percent of documents were double charted.

Results We included 21 sources of grey literature, yielding 296 documents. These varied from blog posts and news stories to policy briefs and government reports. Their identification required expert input; for example, UN repositories were selected in consultation with a WHO librarian. We developed solutions to a range of methodological challenges. Broad and simple terms were purposively selected to allow for maximum sensitivity. Although comprehensive extraction and appraisal of type of research, study design, data collection tools and analysis were not possible as rarely fully reported, documents included detailed descriptions of informal food sources not covered in the peer reviewed literature. The grey literature documents reported data at a regional level and included country comparisons, largely reported on projects that used a multisectoral approach and developed by partnerships involving a wide range of processes and local stakeholders.

Conclusion The volume and nature of the grey literature identified added important new understanding to our review of food sources in Small Island Developing States. Research carried out in difficult-to-reach populations or settings and led by local experts is more likely to be represented in the grey literature and will be omitted from evidence synthesis unless a grey literature component is explicitly incorporated.