A vegetarian diet, especially one that includes fish, significantly reduces the risk of colorectal cancer, a large new study reports.

Researchers recruited 77,659 men and women from Seventh-day Adventist churches nationwide. Adventists were chosen because they abstain from smoking and drinking, and are encouraged to eat a vegetarian diet. All filled out well-validated questionnaires that included more than 200 food items.

Meat intake in the population was very low: about two ounces a day. During an average of seven years of follow-up, the scientists found 490 cases of colorectal cancer. Over all, after adjusting for many health and behavioral variables, vegetarians had a 21 percent reduced risk  compared with nonvegetarians. The results are in JAMA Internal Medicine.

But some vegetarian diets were better than others. People who modified a vegetarian diet with eating meat or fish up to four times a month derived little benefit. But “pescovegetarians,” who ate fish one or more times a month and other meats less than once a month, reduced their risk by 42 percent compared with nonvegetarians.

“We’re looking at the low end of the meat consumption spectrum,” said the lead author, Dr. Michael J. Orlich, an assistant professor of epidemiology at the Loma Linda University School of Public Health, a Seventh-day Adventist institution, “but even compared to a moderate intake of meat, a zero intake looks better, with or without fish.”

A version of this article appears in print on 03/17/2015, on page D6 of the NewYork edition with the headline: Nutrition: Diet May Cut Colon Cancer Risk.