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

OBJECTIVES: To 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). METHODS: Studies 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. RESULTS: Seven 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). CONCLUSIONS: The 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 SOURCES: This work was partially funded by MatPrat Norway. The funders had no input on the analysis or preparation of this abstract.