Cancers of the digestive system are among the most common types of cancer. Colorectal cancer is the third most common cancer and it comprises up to 10% of all new cancer cases. Furthermore, stomach cancer is the fifth most common cancer and it comprises up to 5,6% new cancer cases. There is a large difference in the incidence of gastrointestinal cancer in different World regions, and an increasing number of studies are addressing the possible causes of these differences. Previous research data suggests an association between different eating habits and incidence rates of gastrointestinal cancer. Diet is responsible for 20 to 42% of all forms of cancer and 50 to 90% of colon cancer. Developed countries with a Western diet have the highest incidence of gastrointestinal cancer. Their diet is characterized by high-calorie processed foods rich in processed and red meat, dairy products and eggs, and reduced intake of fruits, vegetables, whole grains and other plant-based foods. Processed meat is classified as Group 1 carcinogen while red meat is classified as Group 2A carcinogen. Many molecules and mechanisms have been suggested as an explanation for these claims. In recent years, the role of the intestinal microbiome in the development of cancer and other chronic diseases has been increasingly researched. In many developing countries with low incidence of gastrointestinal cancer, plant-based diets with little food of animal origin are more prevalent. An increasing number of studies suggests that a diet based on whole plant sources could be the most effective way of primary prevention of gastrointestinal cancer.