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Articles July 2015 Issue

Lowest Colorectal Cancer Risk: Vegetarian Diet Plus Fish

Pescovegetarians at even lower risk in new study than vegans.

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Eating a more plant-based diet could reduce your risk of colorectal cancer—and especially so if it includes regular consumption of fish. Surprisingly, the latest findings from the Adventist Health Study-2 report that pescovegetarians—people who are otherwise vegetarians but also eat fish at least once a month—were at the lowest risk of such cancers. Adding fish to a vegetarian diet was associated with less risk than any other type of vegetarian diet, including a vegan regimen.

“This is a well-constructed study that confirms the idea that a plant-based diet conveys significant protection against colorectal cancers,” says Joel B. Mason, MD, Tufts professor of medicine and nutrition. “Prior studies had also suggested that consumption of fish did not increase the risk for the cancer, but this study goes one step further and suggests fish consumption might convey additional protection up and above what a plant-based diet alone provides.”

PLANT POWER: Dubbed the Adventist Health Study because it focuses on members of the Seventh-Day Adventists, the ongoing research project at Loma Linda University in California has previously reported that vegetarians tend to be at lower risk for not only certain cancers but also heart disease, diabetes, obesity and hypertension. Because the Adventist religion has historically advocated vegetarianism, the study offers an unusual opportunity to compare the health effects of meat avoidance in a large population.

The latest findings, published in JAMA Internal Medicine, examined diet and colorectal cancer risk among 77,659 participants over an average of 7.3 years. During that span, researchers recorded 490 cases of colorectal cancers. Overall, vegetarians of all types were at 22% lower risk than non-vegetarians.

But the scientists said they were surprised at the variation among vegetarian lifestyles. Compared to non-vegetarian participants, pescovegetarians were 43% less likely to develop such cancer. That risk was significantly lower even than vegans (16% lower than non-vegetarians), lacto-ovo vegetarians (eating dairy and eggs, 18% lower), and semi-vegetarians, defined as eating meat or fish once a week (8% lower).

LESS MEAT: The apparent benefits of a vegetarian and pescovegetarian diet are probably understated by the findings, according to lead author Michael J. Orlich, MD, PhD. “It’s worth pointing out that our non-vegetarians are still a relatively low meat-consuming group,” he said. “They average about two ounces of meat a day, so we are comparing vegetarians with a pretty low meat-consuming group, and a relatively healthy group overall.”

In fact, Dr. Orlich added, after adjusting for age, race and sex, the rate of colorectal cancer among the nonvegetarians was 27% lower than you would expect for an age-, sex- and race-matched population in the US. “If we were to compare our vegetarians with a more average population, the effects might even look stronger.”

But be aware that avoiding meat is neither necessary nor a guarantee of a healthy diet. “Fries and a Coke are vegetarian,” says Dariush Mozaffarian, MD, DrPH, dean of Tufts’ Friedman School. “More important than what you avoid is what you actually eat. The healthiest diets are rich in fruits, nuts, fish, vegetables, yogurt, beans, vegetable oils and whole grains. Being or not being a vegetarian does not add much to that.”

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