4.4 Colorectal cancer

Colorectal cancer (CRC – cancer of the colon and/or rectum) is the third most common cancer worldwide and is often associated with dysbiosis of not just the gut bacteria, but also other microorganisms found in the gut microbiome, such as viruses, fungi, and Archaea (World Health Organization, 2023).

SCFAs produced by the gut microbiome are also protective against colorectal cancer cells, helping to destroy them.

A study found that in colorectal cancer, the more fibre (particularly from cereal fibre and whole grains) consumed, the lower the estimated risk of developing colorectal cancer (Aune et al., 2011). This can be seen in Figure 28. As fibre consumption increases, the risk of developing colorectal cancer decreases.

Figure 28 Colorectal cancer risk decreases as the consumption of dietary fibre in grams per day (g day⁻¹) increases

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A line graph illustrates a relationship between the decreased risk of colorectal cancer and increased consumption of dietary fibre. The horizontal axis is labelled ‘dietary fibre’ (in grams per day) and ranges from 0 to 40 in increments of 10 units. A rightward arrow labelled ‘increased fibre intake’ is drawn along the horizontal axis. The vertical axis is labelled ‘estimated relative risk’ and ranges from 0.00 to 1.00 in increments of 0.20 units. A kink is present between 0.00 and 0.60. A downward arrow labelled ‘decreased risk’ is drawn along the vertical axis. The line graph shows a line with a negative slope trending downward from upper left to lower right. A line starts from 3.85 grams per day of dietary fibre with an estimated relative risk of 1.00. The line drops gradually and passes through the following points: fibre intake of 10 grams per day decreases risk to 0.92; fibre intake of 20 grams per day decreases risk to 0.83; fibre intake of 30 grams per day decreases risk to 0.78; fibre intake of 40 grams per day decreases risk to 0.66. All values are approximated.

Figure 28 Colorectal cancer risk decreases as the consumption of dietary fibre in grams per day (g day⁻¹) increases

A diet high in fibre helps to ensure that the Firmicutes bacteria survive and thrive to produce lots of beneficial SCFAs, but is also thought to be beneficial in reducing CRC risk in other ways:

• Fibre increases the bulk of the stool, so it travels through the gut more rapidly, reducing contact of the gut wall with cancer-causing substances (carcinogens).
• Fibre increases the feelings of fullness which helps to prevent overeating. Obesity is also linked to colorectal cancer, as well as 12 other types of cancer (Cancer Research UK, 2022).

As you may recall from earlier, the gut microbiome also affects how individuals respond to medications and recreational drugs. This includes chemotherapeutic agents and immunotherapy often used in the treatment of colorectal and other cancers. Therefore, the gut microbiome may have an impact on both the risk of developing CRC and how well an individual responds to the treatment (Wong, 2023).