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March 23, 2023 | 8 minute read

Diet, the Gut Microbiome and Colorectal Cancer: Connecting the Dots

Is diet’s influence on the gut microbiome a key element in how the AICR Cancer Prevention Recommendations reduce risk of colorectal cancer?

Strong research shows that diet can reduce risk of colorectal cancer in several ways. Let’s explore how research may help to connect the dots between diet, the gut microbiome and colorectal cancer.

Diet and Colorectal Cancer: What Do We Know Already?

Colorectal cancer (CRC) is considered one of the cancers most preventable by diet and lifestyle choices. The AICR Cancer Prevention Recommendations work together toward that goal.

  • Be physically active as part of your everyday life and keep your weight within a range that’s healthy for you. These two steps are strongly linked to lower risk of colorectal cancer.
    • Regular physical activity and avoiding excess body fat reduce CRC risk by fending off chronic, low-grade inflammation and unhealthy levels of insulin and related growth factors that promote cancer development.
  • Boost fiber and more nutrients by amping up the whole grains, vegetables, fruits and beans in your everyday eating. Use AICR’s New American Plate as a guide and aim to include these foods as at least two-thirds of your plate for each meal and snack.
    • Because research is so clear that diets higher in fiber reduce risk of colorectal cancer, AICR recommends a target of at least 30 grams of fiber per day.
    • Research is more limited about how other nutrients in these foods may be protective against CRC specifically. There’s good evidence, though, that the many vitamins, minerals, polyphenols and other natural plant compounds in these foods work together supporting antioxidant and anti-inflammatory defenses.
  • Limit red meat, processed meat and alcohol. Processed meats – like bacon, hot dogs and deli-meats – clearly increase CRC risk so it’s best to avoid eating them regularly. Eating them occasionally is likely OK but if you currently eat them several times a week, choose something else instead.
    • Beef, pork and lamb are the most common red meats in the U.S. If you want to include them in your eating habits, AICR analysis shows that you reduce CRC risk by limiting amounts to no more than 12-18 ounces per week. (Depending on your portion size, that means eating red meat no more than three to six times per week.)
    • If you’ve been swayed by a “health halo” for alcohol, think again. Especially in amounts beyond two standard drinks a day for men and one for women, strong evidence shows that it increases risk of colorectal cancer. And for overall cancer prevention, lowest risk comes from avoiding alcohol altogether.

AICR Recommendations provide a blueprint for creating habits that help in other ways, too. 

  • Make water your “default” drink and avoid sugar-sweetened drinks. This step helps avoid unwanted weight gain that increases cancer risk. Some research suggests that daily consumption of sugar-sweetened drinks could be linked with greater risk of early-onset colorectal cancer even after adjusting for overweight or obesity. But a direct link hasn’t been seen in all studies, so more research is needed to understand this potential association. 
  • What about ultra-processed foods? AICR Recommendations call for limiting fast foods and processed foods that are high in added sugars, fats and refined grains. This Recommendation, too, is to help avoid unwanted weight gain. Some population studies link diets high in ultra-processed foods with higher colorectal cancer risk. Be cautious about how you interpret this. Many ultra-processed foods are concentrated in calories, with unhealthy levels of sugars. But not all foods that get classified as “ultra-processed” are the same, so consider which foods are and are not helping you create a healthy diet. 
  • Dairy products and calcium (from food and supplements) decrease risk of colorectal cancer. But don’t go overboard since amounts beyond current dietary recommendations don’t provide any further safety net and could pose concerns for other aspects of health. 

How Is Diet Related to the Gut Microbiome? 

Differences in what people eat are linked to differences in the amount, diversity and specific types of microbes in their digestive tracts. What’s more, studies have demonstrated that when people change their eating choices, the microbes in their gut change in response. 

Research is on the brink of identifying the specifics of how diet can influence the populations of bacteria and other microbes in the gut but elements of the AICR Cancer Prevention Recommendations have been implicated as playing key roles. 

  • Fermentable forms of dietary fiber nourish bacteria that break it down to produce butyrate and other short-chain fatty acids. These compounds are fuel for healthy colon cells and seem to help reduce inflammation. 
  • Phytochemicals, naturally occurring compounds in plant foods, can also get broken down by gut bacteria to form new compounds that protect gut health. For example, ellagitannin compounds found in walnuts and some berries form urolithins, and carotenoids in vegetables and fruit form other compounds that laboratory studies are investigating. AICR grant funding supported work at the University of Connecticut by Daniel Rosenberg, PhD, studying how anti-inflammatory effects of urolithins from walnuts may protect colon cells. 
  • Western diets — high in red and processed meats, refined grains and added sugars – seem to encourage growth of different bacteria, according to laboratory and limited long-term observational human studies. Particular strains of these bacteria pose risk by promoting inflammation and expression of genes that increase DNA damage in colon cells.  

Fermented foods also seem to increase the diversity of different microbes that live in the gut. More research is needed to identify how this diversity can influence health and whether it lasts. 

How Is the Gut Microbiome Related to Colorectal Cancer? 

Certain strains of bacteria, as well as compounds they produce within the gut, are associated with higher or lower risk of colorectal cancer. However, it’s not yet completely clear whether these differences in the microbiome actually increase development of CRC or if they result from it. 

Some examples of how the microbiome may influence colorectal cancer development: 

  • Expression of tumor-suppressor genes in CRC cells may increase in response to the short-chain fatty acids produced by bacteria in the colon produce from fermentable types of dietary fiber in the colon. 
  • Anti-tumor immune pathways may be supported or inhibited by gut bacteria. Research is underway to understand more.  
  • Formation of cell-damaging compounds seems to be affected by gut microbes. Some gut bacteria increase conversion of bile acids secreted during digestion into compounds that can create cell damage that leads to cancer. More studies are needed, but so far, studies in mice and limited human studies suggest that other gut bacteria produce compounds that inhibit formation of these compounds.  

Composition of the gut microbiome can influence the effectiveness of certain kinds of immunotherapy for cancer, according to several human studies. Research, like that discussed at the most recent AICR Research Conference, is exploring how efforts to change the gut microbiome might improve effectiveness of this cancer treatment. This work is still in early stages, with little human evidence for now — and it holds a lot of promise. 

Actions Now and Future Potential 

This story gets even more complex!  

The same diet seems to influence gut microbes differently in different people. Physical activity and body composition, medications, early life influences on the microbiome and more could all be part of individual variations in how diet, the gut microbiome and colorectal health come together. 

As research progresses, scientists say that a better understanding of diet-microbiome relationships could help us understand why people differ in nutritional needs. Future dietary interventions to reduce cancer risk and to improve effectiveness of cancer treatments may be targeted to match or change microbiome composition. 

This picture of the future potential is exciting. But although we can see possibilities for microbiome-based strategies, scientists doing this research emphasize that right now we’re only seeing the tip of the iceberg regarding the influences of specific strains of bacteria in the colon. 

For now: 

  • The AICR Cancer Prevention Recommendations reduce risk of this all-too-common cancer – with research showing potential for benefits through the gut microbiome and through other pathways.  
  • These Recommendations can also be a helpful blueprint for a healthy lifestyle for people living with and beyond colorectal (and other) cancers. As with all plans for adjusting diet and lifestyle, this warrants discussion with health-care providers for individualized decisions. 

Need help turning Recommendations into action steps you can take one at a time? 

>> Use the New American Plate  model to move closer bit-by-bit to eating habits high in fiber and protective compounds. 

>> Take it one achievable step at a time with AICR’s free Healthy10 Challenge 

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