By Gilang Cahya 
Researchers at the University of Toronto have unveiled a critical link between low-carbohydrate diets and an increased risk of colorectal cancer, demonstrating how this dietary approach can exacerbate the DNA-damaging effects of specific gut microbes. The groundbreaking study, published in the esteemed journal Nature Microbiology, sheds new light on the intricate interplay between diet, the gut microbiome, and cancer development, particularly for individuals with genetic predispositions to DNA repair deficiencies.
Unveiling the Microbiome-Diet Nexus in Cancer Genesis
For decades, the development of colorectal cancer has been understood as a multifactorial disease, influenced by a complex web of dietary habits, the composition of the gut microbiome, environmental exposures, and individual genetic makeup. Senior author Alberto Martin, a professor of immunology at the University of Toronto’s Temerty Faculty of Medicine, articulated the central question driving this research: "Does diet influence the ability of specific bacteria to cause cancer?" This inquiry led to a comprehensive investigation into how different dietary patterns might empower or disempower the carcinogenic potential of certain gut bacteria.
The research team, spearheaded by postdoctoral fellow Bhupesh Thakur, embarked on a meticulous comparative study using a mouse model. They examined the impact of three distinct dietary regimens – a standard diet, a low-carbohydrate diet, and a Western-style diet characterized by high fat and high sugar content – in combination with specific bacterial species previously implicated in colorectal cancer. The objective was to precisely isolate the conditions under which gut microbes could actively contribute to cancer initiation and progression.
The Culprit: A Specific E. coli Strain and Low-Carb Diets
The study’s most striking finding emerged from the combination of a low-carbohydrate diet and a particular strain of Escherichia coli (E. coli) bacteria. This specific E. coli strain is known to produce colibactin, a potent genotoxic compound capable of directly damaging DNA. When this colibactin-producing E. coli was introduced to mice on a low-carbohydrate diet, it significantly promoted the growth of polyps in the colon. Colonic polyps are pre-cancerous lesions, and their accelerated development is a well-established precursor to invasive colorectal cancer.
In stark contrast, other dietary regimens and bacterial combinations did not elicit the same concerning outcome. The Western-style diet, while rich in fat and sugar, did not exhibit the same synergistic effect with the colibactin-producing E. coli in promoting polyp formation and cancer development within the study’s timeframe. This observation underscores the specific and potent interaction between carbohydrate restriction and the genotoxicity of this particular bacterial strain.
Mechanisms of Damage: Inflammation, Mucus Barrier Compromise, and DNA Repair Deficiencies
The researchers delved deeper into the physiological mechanisms underpinning this heightened cancer risk. They discovered that a diet deficient in soluble fibre, a hallmark of many low-carbohydrate approaches, led to increased inflammation within the gut. This inflammation altered the delicate balance of the gut microbial community, creating an environment where the colibactin-producing E. coli could not only survive but also proliferate.
Furthermore, the study revealed a critical impact on the gut’s protective mucus layer. Mice fed a low-carbohydrate diet exhibited a significantly thinner mucus barrier separating the gut microbes from the colon epithelial cells. This mucus layer normally serves as a vital physical shield, preventing direct contact between bacteria and the underlying intestinal lining. With a compromised barrier, colibactin produced by the E. coli had greater access to the colon cells, leading to increased DNA damage. This genetic damage, in turn, was found to drive tumor growth.
The impact of this compromised barrier and increased DNA damage was particularly pronounced in mice possessing genetic mutations in the DNA mismatch repair (MMR) pathway. The MMR pathway is a crucial cellular mechanism responsible for correcting errors that occur during DNA replication. When this pathway is defective, the cell’s ability to fix DNA damage is severely impaired, making it more vulnerable to mutations that can drive cancer development. Colorectal cancers are frequently associated with defects in MMR genes, with an estimated 15% of these tumors exhibiting mutations in these critical genes. Lynch syndrome, a hereditary cancer predisposition syndrome, is characterized by mutations in MMR genes, significantly elevating an individual’s risk for colorectal cancer and other malignancies.
Implications for Human Health and Cancer Prevention Strategies
While the current findings are derived from a mouse model, both Dr. Thakur and Dr. Martin stressed the critical need for validation in human studies. Nevertheless, the implications of their research for cancer prevention are substantial and multifaceted.
The identification of a specific dietary trigger (low-carb diet) that amplifies the carcinogenic potential of a known gut pathogen (colibactin-producing E. coli) opens new avenues for targeted interventions. Dr. Martin raised the pertinent question of whether it is possible to identify individuals, particularly those with Lynch syndrome or other genetic predispositions to DNA repair deficiencies, who harbor these colibactin-producing microbes. For such individuals, the study’s findings suggest that avoiding restrictive low-carbohydrate diets or considering targeted antibiotic treatments to eradicate these specific bacteria could significantly reduce their cancer risk.
The research also touches upon the widespread use of probiotics. Dr. Martin noted that a strain of E. coli known as Nissle, which is frequently incorporated into probiotic formulations, also produces colibactin. Ongoing investigations in his laboratory are exploring the long-term safety of consuming such probiotics, especially for individuals with Lynch syndrome or those adhering to low-carbohydrate diets. This research aims to ascertain whether the potential benefits of probiotics outweigh the risks posed by colibactin production in susceptible populations.
The Role of Fibre: A Promising Protective Factor
A particularly encouraging aspect of the study was the observation regarding the role of soluble fibre. Dr. Thakur highlighted that supplementing the low-carbohydrate diet with soluble fibre led to a significant reduction in the levels of the cancer-causing E. coli, diminished DNA damage, and ultimately, fewer tumor formations. This finding strongly suggests that soluble fibre can act as a potent countermeasure against the adverse effects of low-carbohydrate, low-fibre diets.
"We supplemented fibre and saw that it reduced the effects of the low-carb diet," Dr. Thakur stated. "Now we are trying to find out which fibre sources are more beneficial, and which are less beneficial." This ongoing research is crucial for developing evidence-based dietary recommendations. Dr. Thakur and Dr. Martin are collaborating with Heather Armstrong, a researcher at the University of Alberta, to investigate the efficacy of inulin, a type of soluble fibre, in reducing colibactin-producing E. coli and improving gut health in individuals at high risk for colorectal cancer, such as those with inflammatory bowel disease.
Broader Societal Impact and Future Directions
The University of Toronto study serves as a critical reminder of the potential unintended consequences of popular dietary trends. Low-carbohydrate diets are frequently adopted for weight management and perceived health benefits. However, this research underscores the necessity of a nuanced understanding of their impact on gut health and cancer risk, especially when combined with a lack of adequate fibre intake.
"Our study highlights the potential dangers associated with long-term use of a low-carb, low-fibre diet, which is a common weight-reducing diet," Dr. Martin cautioned. "More work is needed, but we hope that it at least raises awareness."
The findings have far-reaching implications for public health messaging, clinical guidelines, and individual dietary choices. They emphasize the need for personalized dietary advice, particularly for individuals with a history of colorectal cancer, family history of the disease, or known genetic predispositions. Further research is essential to fully elucidate the specific types and amounts of fibre that offer the most protection, and to develop diagnostic tools that can identify individuals at higher risk due to the presence of colibactin-producing bacteria. Ultimately, this line of inquiry promises to refine our understanding of cancer prevention and pave the way for more effective, microbiome-informed health strategies.