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

Background: Cancer is the second leading cause of premature death globally. A dietary pattern centered around whole plant foods, such as fruits and vegetables, whole grains, legumes and nuts and with little red meat and no processed meat has been recommended by the World Cancer Research Fund for the prevention of cancer. However, there has been relatively limited data on plant-based dietary patterns such as vegetarian and vegan diets and cancer risk. The available studies have not been fully consistent and they may have had too low statistical power to detect significant associations for some cancers. In this meta-analysis we will examine the association between vegetarian and vegan dietary patterns and risk of cancer overall as well as specific cancers. Aim: We will conduct a meta-analysis of prospective cohort studies on the associations between vegetarian and vegan diets and cancer risk overall as well as specific cancer sites. Materials and methods: We will search PubMed and Embase databases for studies on vegetarian and vegan diets and cancer overall and specific cancers. We will estimate summary relative risks (RRs) and 95% confidence intervals (95% CIs) for the association between vegetarian and vegan diets and total cancer and specific cancers by using a random effects model. Heterogeneity will be examined using I2 and Q-tests and Egger's test and inspection of the funnel plots will be used to examine publication bias. We will conduct subgroup analyses by sex, geographic location, adjustment for confounding factors such as age, smoking, alcohol consumption, and physical activity. We will also examine whether adjustment for BMI has an impact on the observed summary RRs as there is some data suggesting vegetarians and vegans have a lower prevalence of overweight and obesity. Project participants: Dagfinn Aune, Department of Epidemiology and Biostatistics, Imperial College London, UK. Sabrina Schlesinger, Institute for Biometrics and Epidemiology, German Diabetes Center, Leibniz Center for Diabetes Research at Heinrich Heine University, Dusseldorf, Germany. Jakub G. Sobiecki, Department of Epidemiology and Biostatistics, Imperial College London, UK.