Healthful and unhealthful plant-based diets and site-specific cancer risk: a systematic review and meta-analysis of observational studies

PURPOSE: Plant-based diets have been proposed as beneficial for cancer risk reduction, though their impact may depend on their nutritional quality. We conducted a systematic review and meta-analysis of observational studies to examine associations of overall Plant-Based Diet Index (PDI), Pro-Vegetarian Dietary Pattern (PVG), healthful PDI (hPDI), and unhealthful PDI (uPDI), with site-specific cancer risk. METHODS: PubMed and Web of Science were searched (June 2018 – December 2025) for cohort and case-control studies evaluating these associations. Meta-analyses were conducted for cohort studies, when ≥ 3 studies assessed the same cancer site and outcome, and if dietary indices were measured on a comparable scale. Pooled hazard ratios were calculated using random-effects models. Leave-one-out sensitivity analyses were conducted to evaluate the influence of individual studies on the pooled estimates. RESULTS: A total of 19 cohort studies and 13 case-control studies were reviewed. Meta-analysis showed that per 10-unit increase, overall and healthful plant-based dietary indices were associated with lower risks of breast cancer (8 and 6% lower risk, respectively), colorectal cancer (5% lower risk each), and liver cancer (17 and 23% lower risk, respectively). Higher uPDI was associated with 3% higher breast cancer risk. Evidence for other cancer sites was promising yet limited. Findings were generally consistent in direction, though some associations were moderately influenced by individual studies. CONCLUSION: Plant-based diets may reduce cancer risk, particularly when they prioritize healthful, minimally processed plant foods. Incorporating this distinction into clinical and policy recommendations may support practical, scalable strategies to reduce cancer burden.