A new study from the University of Toronto has found that a low-carbohydrate diet may increase the risk of colorectal cancer by making certain harmful gut bacteria more dangerous. Researchers discovered that when a specific strain of E. coli is combined with a low-carb, low-fiber diet, it can damage DNA in the colon, leading to cancerous growths.
This finding adds to the growing evidence that diet plays a major role in cancer risk—not just because of the nutrients it provides, but also because of how it affects the gut microbiome.
The study, published in Nature Microbiology, suggests that avoiding extreme low-carb diets or adding more fiber could help lower colorectal cancer risk, especially for people with genetic vulnerabilities.
How Diet and Gut Bacteria Interact
Colorectal cancer is one of the most common cancers, and scientists have long suspected that a combination of genetics, diet, and gut bacteria contributes to its development. The researchers, led by Professor Alberto Martin and postdoctoral fellow Bhupesh Thakur, wanted to know whether diet could influence the ability of specific bacteria to trigger cancer.
To test this, they conducted an experiment using mice. The mice were given one of three diets:
- A normal diet
- A Western-style diet (high in fat and sugar)
- A low-carb diet (low in fiber and carbohydrates)
The researchers then introduced three different bacterial species that had previously been linked to colorectal cancer. They found that only one combination—a low-carb diet and a specific strain of E. coli—led to the development of colon tumors.
The harmful strain of E. coli produces colibactin, a compound that damages DNA. Normally, the body has defenses against this kind of damage, but the low-carb diet seemed to create conditions that allowed the bacteria to thrive and cause more harm.
Why Low-Carb Diets Make E. coli More Dangerous
The researchers identified several reasons why a low-carb diet could make this harmful strain of E. coli more dangerous:
- Less Fiber, More Inflammation: A diet low in fiber led to increased inflammation in the gut, which changed the balance of microbes and allowed the colibactin-producing E. coli to multiply.
- Weaker Gut Barrier: The low-carb diet reduced the thickness of the protective mucus layer in the colon. This mucus normally acts as a shield between bacteria and colon cells, but when it thins, more colibactin reaches the cells and causes genetic damage.
- Greater Risk in Genetically Vulnerable Individuals: The effects were strongest in mice that had genetic mutations in their DNA repair system. These mutations made it harder for their bodies to fix the damage caused by colibactin, leading to more cancerous growths.
Implications for People at Risk of Colorectal Cancer
Colorectal cancer is the fourth-most common cancer in Canada, and about 15% of cases involve mutations in DNA repair genes. These mutations are also found in Lynch syndrome, a genetic condition that increases the risk of colorectal and other cancers.
Professor Martin suggests that people with Lynch syndrome or other risk factors might benefit from avoiding a low-carb, low-fiber diet or even taking targeted antibiotics to eliminate harmful E. coli from their gut.
Interestingly, E. coli strains that produce colibactin are also found in some probiotics, such as E. coli Nissle, which is commonly used for gut health. Martin’s team is now studying whether these probiotics are safe for people on low-carb diets or those at higher risk of colorectal cancer.
Can Fiber Reduce the Risk?
One promising finding from the study was that adding soluble fiber to the low-carb diet reduced the number of tumorsand decreased the levels of cancer-causing E. coli.
“When we supplemented fiber, we saw that it reduced the effects of the low-carb diet,” said Thakur. “Now we’re trying to find out which fiber sources are most beneficial.”
The researchers are now working with Heather Armstrong from the University of Alberta to test whether a type of soluble fiber called inulin can improve gut health and reduce cancer risk in high-risk individuals, such as those with inflammatory bowel disease (IBD).
Review and Analysis
This study highlights a potential danger of long-term low-carb, low-fiber diets—a popular choice for weight loss. While these diets may have short-term benefits, such as blood sugar control and weight reduction, they could also increase colorectal cancer risk by altering gut bacteria in harmful ways.
The findings raise important questions:
- Should people at higher risk of colorectal cancer avoid low-carb diets?
- Can specific fiber supplements protect against the harmful effects of a low-carb diet?
- Should people with genetic conditions like Lynch syndrome be screened for colibactin-producing E. coli?
While more research is needed, this study suggests that maintaining a balanced diet with sufficient fiber could be key to protecting gut health and reducing cancer risk. Scientists hope that in the future, targeted dietary changes or microbiome-based therapies could help prevent colorectal cancer before it starts.
If you care about cancer, please read studies that low-carb diet could increase overall cancer risk, and new way to increase the longevity of cancer survivors.
For more information about cancer, please see recent studies about how to fight cancer with these anti-cancer superfoods, and results showing daily vitamin D3 supplementation may reduce cancer death risk.
The research findings can be found in Nature Microbiology.
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