People on vegetarian diets that also eat some fish are associated with an overall lower incidence of colorectal cancers and have a much lower risk compared with non-vegetarians, a large U.S. study has found.

The study, published in JAMA Internal Medicine online on March 9, 2015, was led by Dr M.L. Orlich, an assistant professor in medicine and public health at Loma Linda University in Southern California.

The objective of the study was to evaluate the association between vegetarian dietary patterns and the incidence of colorectal cancer.

“We were surprised to find that pescovegetarians had a significantly lower risk of colorectal cancers than people on other vegetarian diets,” said Dr Michael Orlich.

Dr Orlich and colleagues used dietary questionnaires, medical records, and cancer registries to examine the link between eating habits and cancer prevalence in a nationwide sample of 77,659 Seventh-Day Adventists, a religion that encourages a healthy lifestyle and abstinence from smoking and drinking.

After an average follow-up of 7.3 years, there were 380 cases of colon cancer and 110 cases of rectal cancer. Overall, compared to regular meat eaters, the vegetarians were 22% less likely to have colorectal malignancies, the study found.

Pescovegetarians, which the researchers defined as people who ate fish at least once a month and meat less than once a month, had the biggest risk reduction at 43%.

For lacto-ovo vegetarians, who consumed eggs and dairy while limiting fish and meat to less than once a month, the risk reduction was 18%, while Vegans, who ate eggs, dairy, fish, and meat less than once a month, had a 16% risk reduction.

Even limiting fish and meat to once a week had some benefit with semi-vegetarians having an 8% risk reduction.

According to the researchers, vegetarians might be protected from colorectal cancer not only because they eat less meat, but because they eat more plants. “Diets high in fibre are linked with decreased risk, and fibre comes from whole plant foods,” Dr Orlich said.

The vegetarian groups also ate fewer fatty foods and snacks in general than the non-vegetarians, which helps reduce excess levels of insulin in the blood, which has been linked to elevated risk for colorectal cancers.

The benefits of eating fish are more difficult to interpret, since the pescovegetarians ate about as much fish as did the non-vegetarians. Dr Orlich said that pescovegetarians’ extra protection against cancer may not be only from the fish itself, but perhaps from a combination of fish and increased consumption of plants.