Restricting Meta-Analyses’ Source Studies to Non-consumer Reference Groups Has Significant Effects on the Link between Processed Meat and Type II Diabetes

Abstract

ObjectivesTo compare estimates of the relative risk (RR) of type-2 diabetes mellitus (T2DM) from processed meat consumption between observational studies including only non-consumers in the reference exposure groups (ZB) vs studies including both no- and low-amount processed meat consumers (mixed baseline, or MB).MethodsStudies examining processed meat consumption and incidence of T2DM were selected from references of the Global Burden of Disease (GBD) project's 2020 systematic review. For studies meeting the inclusion criteria, a meta-analysis regression was fit to the RR of the lowest exposure group relative to the reference. Both fixed and random effect models and modifiers including grams/day and the type of baseline (ZB or MB) were tested for significance.ResultsSeven of the 19 cohorts that met the inclusion criteria had ZB. The MB model, using all 15 studies, was best fit by a random-effect model modified with both grams/day and the use of ZB as reference. The effect size for additional 10 g/day was 1.07 (95% CI 1.02–1.12). The coefficient for use of a ZB suggests those studies have a lower RR than MB studies when also controlling for differences in the gram/day consumed in non-reference group. The best fitting model for the ZB studies (no consumers in the baseline) was the fixed effect model without addition of any covariates. The effect for 10 g/day estimated with this meta-analysis was 1.07 (1.03–1.12).ConclusionsThe identification of zero-baseline study groups as a covariate of the meta-analysis had a significant impact on the model. In this model, both the addition of grams per day and of an identifying variable of a zero-baseline group had a significant effect, indicating that estimate of effect based on a zero-consumption reference group can be more precise than reference groups with consumers. However, the expected impact of additional precision in the exposures would be a larger effect size estimate. The overall lack of change in adjusted relative risks from the meta-analysis combined with this result suggest evidence of effect of processed meat and T2DM at low consumption levels might not be consistent, and possibly biased by residual confounding.Funding SourcesThis work was partially funded by MatPrat Norway. The funders had no input on the analysis or preparation of this abstract.