Gut Bites MD

Christopher Damman, Associate Professor of Gastroenterology, School of Medicine, University of Washington. Editor-in-Chief of Gut Bites MD.

Colorectal cancer is steadily rising. Learn how diet & the gut microbiome affect risk, and which dietary practices & tools might reduce it.

Chances are you know someone impacted by colorectal cancer. The disease is on the rise, especially in young adults, with rates increasing 50% over the past 30 years. In another 5 years, it is projected to become the leading cause of cancer deaths in adults under 50. 

Contributing to this rise, more than 1 in 4 people ages 45-50 already have precancerous polyps, a figure that climbs to over 1 in 3 people for people after age 50. That is why 45 is the new 50 for starting colon cancer screening, to ideally catch the polyps early before they become cancer. But what’s behind this alarming trend? 

I’m Chris Damman, a gastroenterologist at the University of Washington, where I perform colorectal cancer screening colonoscopies, and study the microbiome’s impact on gut health. Over my 15 years in practice, I’ve personally witnessed an unsettling rise in early colorectal cancer.  Growing evidence suggests that the microbes in your gut may play a key role.

Colon Cancer‘s Microbial Suspects

Modern changes in diet and lifestyle have been repeatedly linked to rising rates of colorectal cancer. Factors include ultra-processed diets, reduced physical activity, obesity, increased stress, as well as exposure to frequent, broad-spectrum antibiotics. 

One lesser-known, but increasingly studied factor is our gut microbiome–30 trillion plus bacteria, viruses and fungi that live in our colon and help regulate immunity and metabolism as well as protection from bad acting microorganisms. 

While a diverse microbiome can protect our health and may help prevent the development of colon cancer, some unsavery components of the microbiome can attach to the colon lining, cause inflammation, and even alter DNA, transforming healthy colon cells into cells that may become cancerous. 

The same lifestyle factors that influence colon cancer risk also disrupt the microbiome: processed foods, sedentary lifestyles, obesity, stress, and antibiotics. They reduce protective butyrate producers and promote inflammatory species.

These data raise the intriguing question of how much well-known lifestyle factors act directly on the colon versus indirectly through their effects on the microbiome.

Bad Actors at Play in the Colon

Research shows that certain strains of bacteria—with well-known names like E. coli and lesser-known ones like Fusobacterium nucleatum and Bacteroides fragilis—produce toxins and other factors that over time disrupt normal cell function and promote uncontrolled growth. 

Combinations of tests to detect these various strains are being used to develop new colorectal cancer diagnostics and prognostics. Ongoing research is also exploring whether these bacteria are merely associated bystanders or active instigators in cancer development.

Findings from animal model studies suggest the later may be true. In these experiments, researchers introduce specific microbes into germ-free or antibiotic-treated mice and directly observe whether tumors develop—offering the experimental control needed to demonstrate cause and effect in ways that human studies cannot.

Some human clinical studies suggest that these bacteria can also promote cancer cells spreading to other parts of the body, and may even hitch a ride with cancer cells during metastasis.

The Many Biotics Being Studied

This naturally raises the question: can colorectal cancer be treated or prevented by targeting bacteria?

There is precedent for targeting bacteria to treat GI cancers. Dr. Barry Marshall famously proved that H. pylori causes stomach ulcers by infecting himself and then curing the infection with antibiotics, a discovery that later helped establish H. pylori as a cause of certain stomach cancers and earned him the 2005 Nobel Prize.

The colon, however, is far more complex, containing thousands of bacterial species. Proving that any one type or collection of unhealthy bacteria causes cancer (rather than simply coexisting with it) is more challenging. Still, early trials are testing whether selective antibiotics, combined with standard chemotherapy treatments, might improve colorectal cancer treatment outcomes.

Other studies are exploring probiotics (beneficial bacteria) and prebiotics (the food that nourishes them) to reduce inflammation and potentially lower cancer risk, though data remains too preliminary for any formal recommendations.

A Healthy Diet for Good Microbes

In the absence of strong evidence supporting various biotics for colorectal cancer prevention, the best approach is to focus on proven lifestyle measures for general health: eat well, exercise regularly, and manage stress. Recent research adds an important dimension to this advice—nurturing a healthy gut microbiome. 

The microbiome transforms components of food we can’t digest, such as fiber (which gives plant foods their structure) and polyphenols (which give them their color), into beneficial metabolites like butyrate and microbiome-modified polyphenols. These compounds can help reduce unhealthy bacteria and inflammation in the colon and may play a key role in preventing colorectal cancer.

Recent research also supports the use of calcium-rich dairy products and/or fermented dairy products, as well as increased omega-3 rich fish consumption as potentially protective in colon cancer and supportive of the microbiome.

Microbiome-supportive eating largely boils down to choosing more whole plants, dairy, and fish, but we all have our sweet and salty packaged food vices. Some packaged foods contain more fiber and polyphenols, less sugar and salt, and are more microbiome-friendly than others. I developed this public service smart app to help you find them.

Hazards For Microbial Dysbiosis

And what not to do? Smoking and most alcohol, especially hard alcohol and beer, disrupt the microbiome and are strongly associated with higher colorectal cancer risk (Group 1, definite carcinogens).

Current medical guidelines recommend avoiding both tobacco & alcohol altogether, but if you are going to consume alcohol, an occasional glass of wine may be the least associated with colorectal cancer risk.

Likewise, it is best to avoid processed meats—like hot dogs, bacon, sausage, deli meats, jerky (Group 1 carcinogens)—and limit red meats like beef, pork, lamb, and goat, (Group 2A carcinogens). The heme iron that gives red meat its color disrupts the microbiome and can promote the formation of precancerous polyps.

Meats charred at high temperatures during grilling also contain polycyclic aromatic hydrocarbons and heterocyclic amines, compounds that damage DNA, disrupt the microbiome, and raise colon cancer risk.

45 is the New 50 for Screening

And if lifestyle measures aren’t adequate, screening helps catch colorectal cancer early—or ideally prevent it altogether. Most people should start at age 45, or possibly earlier if there’s a family history of cancer or large polyps.

If you notice blood in your stool, change in your bowel habits, or unexplained abdominal pain or weight loss, check in with your primary doctor to see if further testing is needed.

A colonoscopy is the gold standard test, done about every 10 years if normal, or sooner if precancerous polyps are found. It involves drinking a prep solution the day before to clean the colon, sedating medications during the procedure, and usually a quick recovery by the next day.

Stool-based tests are noninvasive and done at home. They check for hidden blood or abnormal DNA—annually for FIT and gFOBT, and every three years for stool DNA tests (like Cologuard). If any test is positive, a colonoscopy is needed to follow up.

A Prescription for Prevention

In short, consume more plants, dairy, and fish and less processed/red meat, alcohol, and tobacco to support a healthy microbiome & colon. As for all the packaged items at the store, tools like this app with barcode scanner can help you choose the most microbiome-supportive foods. Beyond diet, exercise regularly, manage stress, and remember, 45 is the new 50 for screening. Here is a short list of the do’s and don’ts:

Dietary Do’s:

• Increase whole grains, beans, nuts, seeds, fruits and vegetables
• Increase fermented and unfermented dairy products & fish
• Try a scanner to explore the more microbiome-friendly foods

Dietary Dont’s:

• Avoid processed meat: hot dogs, bacon, sausage, deli meats, jerky
• Limit red meat, especially grilled beef, pork, lamb, and goat
• Avoid tobacco & alcohol, especially hard alcohol and beer

Additional Do’s:

• Get daily physical activity-at least 30 minutes of moderate/day.
• Manage stress through meditation, mindfulness, and quality sleep
• Discuss colon cancer screening strategy with your primary doctor

Christopher Damman, Associate Professor of Gastroenterology, School of Medicine, University of Washington

MD-authored gut health literature digests and first-in-class food quality app to power your microbiome.