Vegetarian diets and the risk of gastrointestinal cancers: a meta-analysis of observational studies

The systematic review aimed to assess the association between vegetarian diet and the risk of gastrointestinal tumorigenesis. PubMed, Embase, Cochrane Library, and Web of Science were searched from inception to August 2022 for observational studies on vegetarian diets and the risk of gastrointestinal tumorigenesis. The primary outcome was morbidity due to gastrointestinal cancer. The Newcastle-Ottawa Scale was used to assess the quality of included studies. Pooled effects were analyzed using a random-effects model. The study protocol was registered in PROSPERO (no. CRD42022310187). Eight original studies (seven cohorts and one case-control), involving 686 691 participants, were included. Meta-analysis showed a negative correlation between vegetarian diets and gastrointestinal tumorigenesis risk [relative risk (RR) equals 0.77, 95% confidence interval (CI) is (0.65-0.90)], compared with non-vegetarian diets. Subgroup analysis indicated that vegetarian diets were negatively correlated with the risks of gastric cancer [RR = 0.41, 95% CI (0.28-0.61)] and colorectal cancer [RR = 0.85, 95% CI (0.76-0.95)], but not with that of upper gastrointestinal cancer (excluding stomach) [RR = 0.93, 95% CI (0.61-1.42)]. Vegetarian diets were negatively correlated with the risk of gastrointestinal tumorigenesis in men [RR = 0.57, 95% CI (0.36-0.91)], but were uncorrelated in women [RR = 0.89, 95% CI (0.71-1.11)]. Vegetarian diets were negatively correlated with the risk of gastrointestinal tumorigenesis in North American [RR = 0.76, 95% CI (0.61-0.95)] and Asian populations [RR = 0.43, 95% CI (0.26-0.72)] and were uncorrelated in the European population [RR = 0.83, 95% CI (0.68-1.01)]. Adhering to vegetarian diets reduces the risk of gastrointestinal tumorigenesis. More data from well-conducted cohort and other studies are needed.