Vegetarian and vegan diets and cancer incidence: a systematic review and meta-analysis of prospective studies.

Abstract

Several studies have suggested that vegetarian and vegan vs. non-vegetarian diets are associated with lower cancer risk overall, however, results for specific cancer sites have been less consistent. We conducted a systematic review and meta-analysis of prospective studies on vegetarian and vegan diets and cancer incidence to clarify the associations across cancer sites. PubMed and Embase databases were searched for relevant studies up to 5 July 2025. Summary relative risks (RRs) and 95% confidence intervals (95% CIs) were calculated for the association between vegetarian and vegan diets and cancer incidence. World Cancer Research Fund (WCRF) criteria was used to evaluate the strength of the evidence. Seventeen publications (seven prospective studies) were included. The summary RRs (95% CIs) for vegetarians vs. non-vegetarians was 0.87 (0.84-0.91, I² = 0%, n = 4 studies) for total cancer incidence, 0.55 (0.36-0.86, I² = 32%, n = 4) for stomach cancer, 0.86 (0.76-0.97, I² = 14%, n = 6) for colorectal cancer, 0.79 (0.67-0.93, I² = 38%, n = 7) for colon cancer, 0.55 (0.31-0.97, I² = 0%, n = 2) for proximal colon cancer, 0.77 (0.62-0.95, I² = 0%, n = 5) for pancreatic cancer, 0.79 (0.66-0.94, I² = 0%, n = 4) for melanoma, 0.92 (0.86-0.99, I² = 0%, n = 7) for breast cancer, 0.81 (0.69-0.95, I² = 0%, n = 3) for postmenopausal breast cancer, 0.78 (0.62-0.98, I² = 0%, n = 5) for bladder cancer, and 0.76 (0.63-0.93, I² = 0%, n = 4) for non-Hodgkin's lymphoma. In addition, non-statistically significant inverse associations were observed for some cancers, with summary RRs of 0.85 (0.70-1.04, I² = 0%, n = 6) for lung cancer, 0.83 (0.68-1.02, I² = 0%, n = 5) for ovarian cancer, and 0.87 (0.75-1.00, I² = 43%, n = 6) for prostate cancer. Results for other cancer sites were imprecise or near the null. The summary RRs (95% CIs) for vegans vs. non-vegetarians were 0.77 (0.70-0.85, I² = 0%, n = 3) for total cancer, 1.02 (0.71-1.48, I² = 42%, n = 3) for colorectal cancer, 0.80 (0.64-1.00, I² = 0%, n = 4) for breast cancer, and 0.87 (0.50-1.49, I² = 49%, n = 3) for prostate cancer. BMI explained a moderate part of the associations. The strength of evidence [judging the likelihood of causality] for vegetarian diets and total, colorectal, colon and breast cancer was judged as probable, and limited suggestive for stomach, pancreatic, and bladder cancers, melanoma and non-Hodgkin's lymphoma, and for vegan diets and total cancer and breast cancer was considered limited-suggestive. Vegetarian diets compared to non-vegetarian diets are associated with reduced risk of total cancer and seven specific cancer types, while vegan diets are associated with reduced risk of total and breast cancer. Although further studies are needed to assess the long-term adherence to vegetarian and vegan diets and cancer incidence and across less investigated cancers, these results support recommendations to adopt much more plant-based diets for cancer prevention.