Meta-analyses of dietary exposures must consider energy adjustment: recommendations from a meta-scientific review

In observational studies of dietary exposures, the energy adjustment strategy directly alters the effect being estimated. Adjusting for total energy intake or expressing the exposure as a percentage of total energy changes the estimated effect to a substitution. This meta-scientific review aimed to investigate the extent to which meta-analyses of dietary exposures have considered this challenge and whether they might inadvertently pool incomparable effects. We identified all meta-analyses examining the relationship between 1) saturated fat and cardiovascular disease, 2) fish and cardiovascular disease. For each, we examined the two most recent and two most cited meta-analyses, and their constituent studies. We extracted information on research aims, estimated effects, and interpretations. The eight meta-analyses collectively included 82 studies and 144 unique models. 82% (n = 119/144) of models estimated substitution effects, but this was rarely recognised or discussed. Only one meta-analysis explicitly considered the energy adjustment strategy, only to determine eligibility for a sub-group analysis. No meta-analyses acknowledged that they may have pooled estimates for different effects. Such inappropriately pooled estimates result in quantities with obscure interpretations. Based on our findings, we offer recommendations designed to improve the conduct of future meta-analyses and, consequently, the quality of evidence that informs dietary recommendations.