Methods used to select results to include in meta-analyses of nutrition research: a meta-research study

Objectives: To investigate the extent of multiplicity of results in study reports of nutrition research, and the methods specified in systematic reviews to select results for inclusion in meta-analyses. Methods: MEDLINE and Epistemonikos were searched (January 2018 - June 2019) to identify systematic reviews with meta-analysis of the association between food/diet and health-related outcomes. A random sample of these reviews was selected, and for the first presented ('index') meta-analysis, rules used to select effect estimates to include in this meta-analysis were extracted from the reviews and their protocols. All effect estimates from the primary studies that were eligible for inclusion in the index meta-analyses were extracted. Results: Forty-two systematic reviews were included, 14 of which had a protocol. In 29% of review protocols and 69% of reviews, at least one decision rule to select effect estimates when multiple were available was specified. In 69% (204/325) of studies included in the index meta-analyses, there was at least one type of multiplicity of results. Conclusions: Authors of systematic reviews of nutrition research should anticipate encountering multiplicity of results in the included primary studies. Specification of methods to handle multiplicity when designing reviews is therefore recommended.