A vegetarian diet significantly reduces the risk of colorectal cancer, especially if some fish is included.
Among vegetarians, who are supposed to eat no meat, we must distinguish between lacto-ovo vegetarians who allow themselves dairy products and eggs, vegans who exclude all animal products (neither milk nor eggs) and pesco-vegetarians who eat fish and seafood.
Some less drastic people will be considered "semi-vegetarians" because they eat meat from time to time.
Overall, all vegetarian diets appear to be beneficial to health.
According to a large North American study published in JAMA Internal Medicine, the diet combining vegetarianism and fish would be the most protective against colorectal cancer.
An article reports on the study conducted on the association between vegetarian diets and the risk of colorectal cancer. To do this, the researchers used the Adventist Health Study 2 (AHS-2) cohort, which includes 96,354 Seventh-day Adventist men and women, all recruited between January 1, 2002 and December 31, 2007. Of these participants, the study included a sample of 77,659 people. After answering a questionnaire about their diet, we know that this population eats only 50 to 60 g of meat per day.
During the follow-up period (approximately 7.3 years) of this sample population, 380 cases of colon cancer and 110 cases of rectal cancer occurred. Compared with non-vegetarians, this constitutes a cancer colorectal risk reduced on average by 22% (19% for colon cancer and 29% for rectal cancer).
The reduction in cancer colorectal risk was 16% for vegans, 18% for lacto-ovo vegetarians, 43% for pesco-vegetarians, and only 8% for semi-vegetarians. The researchers found no difference between men and women.
In conclusion, a vegetarian diet contributes to reducing the risk of colorectal cancer, but some vegetarian diets seem to be more protective than others: pesco-vegetarians, i.e. those who ate fish at least once a month and sometimes meat less than once a month, showed a significantly lower risk of contracting cancer
The conclusion of the lead author of this study, Michael Orlich (Loma Linda University School of Public Health, an Adventist institution), is that low meat consumption reduces the incidence of colorectal cancer, but total elimination of meat intake (with or without fish) is significantly more protective.
Sources:
Orlich MJ, Singh PN, Sabaté J, Fan J, Sveen L, Bennett H, Knutsen SF, Beeson WL, Jaceldo-Siegl K, Butler TL, Herring RP, Fraser GE. Vegetarian Dietary Patterns and the Risk of Colorectal Cancers. JAMA Intern Med. 2015 Mar 9. doi: 10.1001/jamainternmed.2015.59.