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How Your Diet & Gut Bacteria Work Together to Reduce Risk of Colon Cancer

We all know you are what you eat—or so the expression goes—but it's good to remember that what you are (at least intestinally) is mainly bacteria. A new study has shown that what you eat, and how your gut microbiome reacts to that food, might be a key player in your risk of developing a certain type of colon cancer—and changing your diet can help decrease your risk.

Only a handful of microbes have been linked to cancer, and most of those microbes were viruses. However, the bacteria Fusobacterium nucleatum was found in abundance in colorectal cancer tumors during a a 2012 study, and a 2016 study found it in 87% of the colorectal cancer tumors tested. In another related study, it was shown that people with F. nucleatum-rich colorectal cancer have about a 25% decreased survival compared to people whose tumors lacked the bacteria.

Whether or not the bacteria actually caused the cancers is the subject of intensive research, but its association with the tumors is indisputable. F. nucleatum is not usually found in the human gut, where the tumors were found, so where did the bacteria come from?

A paper published today in JAMA Oncology found that the amount of F. nucleatum found in some colon cancer tumors could be altered by changing diet, indicating the gut microbiome could have a role in the development of this cancer. And they've suggested dietary guidelines that might cut your risk in half.

Make Friends with Your Gut Microbiome

Diet and colorectal cancer information on 47,449 men and 89,768 women who participated in the decades-long Nurses' Health Study and Health Professionals Follow-Up Study provided the basis for this latest research. Among the participants, there were 1,019 cases of colorectal cancer where F. nucleatum was looked for in the tumor. It was detected in about 12% of the tumors.

The percentage of tumors infected with F. nucleatum in this study (12%) and the number found in the 2016 study (87%) are likely due to the fact that samples in the new JAMA Oncology study, from researchers at Hardvard, MIT, and various Boston-area hospitals, were not chosen randomly. The researchers used preexisting data and information on F. nucleatum that was not available on all tumors.

Study participants who ate a prudent diet, rich in fruits, vegetables, and fiber, had about half the risk of developing colorectal cancer containing F. nucleatum, compared to people who ate a regular Western diet. However, the risk for developing colorectal cancer lacking F. nucleatum was not affected by diet. Of the healthy components of the prudent diet, fiber was associated most strongly with the reduction in risk of F. nucleatum-rich colorectal cancer.

Since the diet was the one factor that differed between the cancers containing or lacking F. nucleatum, that pointed to the diet as a modulator of the gut microbiome—and its role to increase or decrease the F. nucleatum population there.

Study researcher Dr. Andrew Chan, of Massachusetts General Hospital, Brigham and Women's Hospital, and the Broad Institute of MIT and Harvard, said in a press release from Dana Farber Cancer Institute:

These data are among the first in humans that show a connection between long-term dietary intake and the bacteria in tumor tissue. This supports earlier studies that show some gut bacteria can directly cause the development of cancers in animals.

More studies are needed to examine the "complex intersection of diet, the gut microbiome, and carcinogenesis," wrote the study authors.

Cutting Your Risk

Colorectal cancer—cancer of the rectum or colon—hits about about 1 in 21 men and 1 in 23 women during their lifetime, according to the American Cancer Society. It's the second leading cause of death in men and the third in women. Since 1975, the number of newly-diagnosed cases and the deaths per year have gone down and the five-year survival has gone up.

Image by NIH/NCI Surveillance, Epidemiology, and End Results Program

That's all wonderful news, and a five-year survival rate of 67.2% might be good for most cancers, but it's still a scary statistic if you're the one who's facing it.

Treatment has come a long way since 1975. That, combined with early detection and removal of colorectal polyps, are the likely drivers of the five-year survival increase. Surgery may be used to remove the tumor and chemotherapy given to try to kill it, but the newest developments in treatment have come from therapies that target specific genes related to the tumor or its growth, immunotherapies with agents that support and augment the body's immune responses, and cancer vaccines.

But the best "treatment" is still prevention. The trouble is, we don't know what causes colorectal cancer. That said, factors have been identified that we know can increase the risk of developing the disease. The American Cancer Society politely groups these into risks you can change and those you can't change.

Of course, you can't change the increased risk factors of age over 50, history of inflammatory bowel disease, or family history of colorectal cancer, but there are some behaviors you can modify that will decrease your risk of getting colorectal cancer.

These are smoking, drinking more than two drinks a day if you are a man, or one if you are a woman, being overweight, and a diet high in red and processed meats.

Changing those risk factors you can and having regular screenings can lower your risk of getting and dying of colorectal cancer. And thanks to the research work by Chan and his colleagues, we know that paying attention to your gut microbiome by eating a prudent diet can cut our chances of developing F. nucleatum-rich colorectal cancer in half.

Cover image by National Cancer Institute/National Institutes of Health

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