People often discover they have colorectal cancer when it is too late; that's why colorectal cancer screening is so important — it can help doctors find potentially cancerous polyps before they become threats.

As important as screening is, prevention is the first line of defense against colorectal cancer. And when it comes to preventing colorectal cancer, diet is at the top of the list. Diet is both a protection and a risk, however. Eating red meat increases the risk of colorectal cancer; a diet rich in foods high in fiber decreases the risk.

Now a new study offers evidence that following a vegetarian, or primarily vegetarian, diet reduces the risk of colorectal cancers. The findings, by a group at Loma Linda University in California, appear to support the idea that mostly plant-based — vegetarian — diets are healthier than diets that include a lot of red meat or processed meat products.

The lowest risk of colorectal cancer was seen among people who ate fish but no meat — pesco-vegetarians.

Researchers studied the eating habits of almost 78,000 men and women. Participants were divided into five dietary groups: non-vegetarian (those who ate red meat), semi-vegetarian (those who ate meat and fish once or less than once per week), vegan (those who ate no meat or animal products), pesco-vegtarian (those who ate fish), and ovo-lacto vegetarian (those who ate dairy and eggs, but not meat or fish) based on a food questionnaire they filled out at the start of the study.

Following up after seven years, patient records indicated 380 cases of colon cancer and 110 cases of rectal cancer among the group. Taken together the vegetarians had a 22 percent lower risk of developing colorectal cancer, a 19 percent lower risk of developing colon cancer, and a 29 percent lower risk for developing rectal cancer compared to meat-eating non-vegetarians.

The lowest risk of colorectal cancer was seen among people who ate fish but no meat — pesco-vegetarians. They had a 43 percent lower risk compared to non-vegetarians. Vegans had a 16 percent lower risk of colorectal cancer, ovo-lacto vegetarians had an 18 percent lower colorectal cancer risk, and semi-vegetarians had an eight percent lower risk.

It was not clear to the researchers why the pesco-vegetarians had a much lower risk of colorectal cancer compared to the non-vegetarians, Michael Orlich, corresponding author on the study, told TheDoctor. The pesco-vegatarians had a similar risk for colorectal cancer as the other vegetarian groups.

In addition to looking more closely at the relationship between diet and colorectal cancer, the team plans to investigate the effect of diet on breast and prostate cancer.

“People who are trying to make changes to reduce colorectal cancer risk, if they eat a substantial amount of meat, should probably reduce red meat and processed meat intake, and introduce a variety of whole plant foods that are high in fiber,” Orlich advised.

The study is published in JAMA Internal Medicine.