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Specialist Advice — 10 minutes

Colorectal cancer and the microbiome: Some interesting links!

March 28, 2024

Raymond Lepage, PhD, Doctor in Biochemistry
Raymond Lepage, PhD, Doctor in Biochemistry
Science popularizer

The Canadian Cancer Society is currently running a major campaign called “Together, Let’s Dethrone Colon Cancer.”[1] Indeed, colorectal cancer (CRC) is the second most deadly cancer in Quebec. Thanks to a recent decision by the Ministère de la Santé et des Services sociaux (MSSS), a test for blood in the stool is now available without a medical prescription. This provides us with a valuable screening tool that could help dethrone CRC from its position as runner-up.

What are the risk factors for colon cancer?

Over the years, researchers have identified the major risk factors for developing CRC. Apart from genetic considerations, a number of dietary and lifestyle factors[2] are also involved:

  • Sedentary lifestyle, lack of exercise, excess weight and obesity
  • Consuming alcohol and tobacco
  • A diet rich in red and processed meats and/or low in fibre
  • Eating foods cooked at a high temperature
  • Inflammatory bowel disease (such as Crohn’s disease or ulcerative colitis)
  • Diabetes

The word “microbiome” does not appear in this list. However, over the past decade, numerous studies have identified close links between many of these risk factors and the health of our microbiome, and consequently, the development of colorectal cancer.

Micro what? Microbiome or microbiota!

The microbiome, also known as the microbiota, refers to all the microorganisms, including bacteria, viruses, moulds and their genetic material that normally live in our bodies, especially in the digestive tract. In the human body, there are just as many microbial organisms as there are human cells, underscoring the crucial importance of the microbiome to our health. Its diversity is remarkable: For bacteria alone, there are more than 1,000 different species in the body of a normal human, not counting viruses, moulds and all the interactions between these various microorganisms.[3] The composition of the intestinal microbiome forms progressively in the newborn, first through contact with the mother’s vaginal and fecal flora during vaginal delivery, or with microorganisms in the atmosphere during a caesarean delivery. It then develops through the influence of diet, genetics, hygiene, the environment and certain treatments such as antibiotics.[4] Although many types of microorganisms are found in most human beings, the differences between individuals are so great that the microbiome is sometimes compared to a fingerprint.[4]

Microbiome, diet and lifestyle habits

Among the many factors known to alter the microbiome (genetic, dietary and others), several are also found in the list of risk factors for colorectal cancer.

  • Obesity can alter the composition of the microbiome.
  • Inflammatory bowel disease can lead to an inadequate activation of the immune response in the gut.
  • Physical exercise can increase the diversity and proportion of good bacteria.[5]
  • Eating red meats can lead to an increase in harmful bacteria and the secretion of toxins.[6]
  • Alcohol consumption can alter the composition of the microbiome and lead to the secretion of endotoxins.[7]

As such, it is not surprising that changes in the microbiome (dysbiosis) are now considered a significant, even pivotal, risk factor for the development of colorectal cancer.[8]

What does the microbiome tell us?

The relationship between microbiome composition and CRC risk is so close that some researchers believe an analysis of microbiome composition could predict the risk of developing CRC with good certainty.[9,10] However, this complex analysis is not yet sufficiently developed for use in medicine.

What role does the microbiome play in recovering from intestinal surgery?

Last year, a group of researchers at the CHUM published a study. In mice, altering the microbiome before intestinal surgery was found to reduce post-operative complications in patients with CRC.[11] Accordingly, by modifying the mouse’s microbiome to reduce the amount of harmful bacteria and increase the amount of useful bacteria, the researchers observed an improvement in intestinal healing. However, in reality, almost one in three patients undergoing CRC surgery develops serious post-operative complications linked to leakage caused by poor healing of the intestinal barrier. These complications take the form of inflammation, severe infection or recurrence of the cancer. The researchers were able to identify more specifically a strain of bacteria responsible for these leaks, as well as another strain of bacteria which, on the contrary, promotes healing.

These results are promising in a field where research is currently scarce.[12] The technique used in mice involved “transplanting” feces from an animal that had healed well after surgery to another mouse prior to its own surgery. This fecal transplant (FMT) approach is less straightforward in humans, although the technique is already used to treat recurrent C. difficile infections.[13] It is easier to find a human donor uninfected with C. difficile than to find a donor with the right proportion of beneficial and harmful bacteria. Moreover, a great deal of research is still needed to determine the right mix of prebiotics and probiotics that will be effective in the majority of patients.

Where does colorectal cancer screening fit in?

The “diagnosis” of CRC is officially established in the report of the pathologist who examines tissue removed by biopsy (during colonoscopy or surgery) under the microscope. Screening for CRC, on the other hand, aims to identify, within a large population, people who are at increased risk of developing CRC before the onset of specific symptoms, thereby justifying a colonoscopy. To be effective, a screening program must be based on the use of a simple, yet sensitive, inexpensive and easily available technique. Only people who are at high risk, or who have had a positive result in a screening test, are referred for more invasive, costly and less readily available examinations.

What is meant by “occult blood” in the stool?

The larger the intestinal polyps, the more likely they are to be associated with tiny losses of blood in the stool that are invisible to the naked eye. For this reason, these losses are referred to as occult.

For the past decade, the search for occult blood in the stool has been based on the use of an antibody that detects human hemoglobin in a small stool sample. This method has eliminated the need for a special diet, including abstention from red meat for several days, which was required with the guaiac test, an earlier detection method. As a result, fecal occult blood testing by immunological technique (iFOBT) has all the qualities of a screening technique: It is simple, sensitive and inexpensive. Until recently, the main drawback was limited access, as it required a medical prescription. However, this barrier has now been lifted for people between the ages of 50 and 74, who are considered most at risk of developing CRC.[14] They can now benefit from screening every two years. These tests are available at many clinics in the healthcare network, as well as private laboratories such as Biron Medical Laboratory.[15]

When colorectal cancer is diagnosed, the patient meets with the care team to discuss the treatment. This may involve surgery, chemotherapy or a combination of both, depending on the location of the tumour, where the cancer has reappeared (in the event of recurrence), the stage of the cancer, as well as the patient’s overall health condition and choice.

Are you concerned about colorectal cancer? Please fill out our brief questionnaire to confirm your eligibility for the screening program and order your kit.

  1. Canadian Cancer Society. “Together, Let’s Dethrone Colon Cancer,”
  2. Biron. “Neat Little Guide – Colorectal Cancer,”
  3. National Institute of Environmental Health Sciences (NIH). “Microbiome,ʺ
  4. INSERM. “Microbiote intestinal (flore intestinale),”
  5. V. Monda V, I. Villano I, A. Messina, A. Valenzano et al. “Exercise Modifies the Gut Microbiota with Positive Health Effects,” Oxid Med Cell Longev, 2017;2017:3831972,
  6. C. Lee, J. Lee, J.K. Eor, M.J. Kwak et al. “Effect of Consumption of Animal Products on the Gut Microbiome Composition and Gut Health,” Food Sci Anim Resour, Sep. 2023, 43(5):723-750,
  7. F. Bishehsari, E. Magno, G. Swanson, V. Desai et al. “Alcohol and Gut-Derived Inflammation,” Alcohol Research, Vol. 38, #2, pp. e1-e9,
  8. M. Rebersek. “Gut microbiome and its role in colorectal cancer,” BMC Cancer 21, 1325 (2021),
  9. E. Bailey. “Gut Microbiome variation could predict colon cancer risk, new study finds”.
  10. R. Hajar, E. Gonzalez, G. Fragoso, M. Oliero et al. “Gut microbiota influence anastomotic healing in colorectal cancer surgery through modulation of mucosal proinflammatory cytokines,” Gut, Vol. 72, #6,
  11. CRCHUM. “Colorectal cancer surgery: gut microbiota helps healing,” January 31, 2023,
  12. Cleveland Clinic. “Fecal Transplant,”
  13. Le Devoir. “Cancer colorectal: nouvel outil de dépistage,” February 5, 2024,
  14. Biron. “Terms and conditions – Colorectal cancer screening,”
Raymond Lepage, PhD, Doctor in Biochemistry
Raymond Lepage, PhD, Doctor in Biochemistry
Science popularizer
For about 50 years, Raymond Lepage worked as a clinical biochemist in charge of public and private laboratories. An associate clinical professor at the Faculty of Medicine of the Université de Montréal and an associate professor at the Université de Sherbrooke, he has also been a consultant, researcher, legal expert and conference speaker. He has authored or co-authored more than 100 publications for scientific conferences and journals, and now devotes part of his semi-retirement to popularizing science.