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We aimed to assess the effect of a high-quality diet on the risk of upper gastrointestinal cancer and to evaluate the overall quality of our findings by searching PubMed, EMBASE, Web of Science, Cochrane, and the references of related articles to February 2020. Two reviewers independently retrieved the data and performed the quality assessments. We defined the highest-quality diet as that with the lowest Diet Inflammatory Index category and the highest Mediterranean Diet Score category. Overall odds ratios and 95% confidence intervals were estimated for upper gastrointestinal cancer risk comparing the highest- versus lowest-diet quality. A random-effects meta-analysis was then applied with Review Manager, and the quality of the overall findings was evaluated with the Grading of Recommendations Assessment, Development, and Evaluation approach. The highest-quality diets were significantly associated with reduced risk of upper gastrointestinal cancers, achieving odds ratios of 0.59 (95% confidence interval: 0.48-0.72) for the Diet Inflammatory Index, pooling the findings from nine studies, and 0.72 (95% confidence interval: 0.61-0.88) for the Mediterranean Diet Score, pooling the findings from 11 studies. We observed a minimum of 69% heterogeneity in the pooled results. The pooled results were graded as low quality of evidence. Although it may be possible to offer evidence-based general dietary advice for the prevention of upper gastrointestinal cancers, the evidence is currently of insufficient quality to develop dietary recommendations.

The quality of existing evidence about the impact of diet quality on colorectal cancer (CRC) risk has only rarely been assessed. In the current review, we searched PubMed, EMBASE, Web of Science, Cochrane, and the resulting references (up to January 2020) for studies that evaluated the role of high diet quality by extreme dietary index categorization and the risk of CRC. Two researchers independently performed the study selection, data extraction, and quality assessment. We then applied a random-effects meta-analysis to estimate the pooled odds ratios (ORs) and 95% confidence intervals (CIs) for CRC at the extremes of each dietary index, and we assessed the quality of the pooled results using the Grading of Recommendations Assessment, Development and Evaluation approach. A high diet quality was significantly associated with reduced CRC risk when patients had a low Diet Inflammatory Index score (OR, 0.66; 95%CI, 0.56–0.78), a high Mediterranean Diet Score (OR, 0.84; 95%CI, 0.78–0.90), high Dietary Approaches to Stop Hypertension adherence (OR, 0.83; 95%CI, 0.78–0.89), and a high Healthy Eating Index score (OR, 0.72; 95%CI, 0.64–0.80). The pooled results for all dietary indices were rated as being of low quality due to concerns over inconsistency or imprecision. We conclude that, despite a high diet quality appearing to have a preventive role in CRC, the evidence is currently of insufficient quality to develop dietary recommendations. [ABSTRACT FROM AUTHOR]