By Gilang Cahya 
A groundbreaking study from the University of Toronto has illuminated a concerning interplay between diet and gut bacteria, revealing how low-carbohydrate diets can amplify the DNA-damaging capabilities of specific gut microbes, thereby increasing the risk of colorectal cancer. Published in the esteemed journal Nature Microbiology, the research provides critical insights into the complex mechanisms driving one of the most common cancers globally, offering potential avenues for prevention and targeted interventions.
The Complex Nexus of Diet, Microbiome, and Cancer
Colorectal cancer (CRC), a significant public health concern with an estimated 1.9 million new cases worldwide annually and a mortality rate of approximately 50%, has long been understood as a multifactorial disease. This understanding encompasses a broad spectrum of influences, including dietary patterns, the intricate ecosystem of the gut microbiome, environmental exposures, and an individual’s genetic predisposition. For years, researchers have sought to disentangle these threads, aiming to pinpoint specific interactions that tip the scales towards disease development.
"Colorectal cancer has always been thought of as being caused by a number of different factors including diet, gut microbiome, environment and genetics," stated senior author Alberto Martin, a professor of immunology at U of T’s Temerty Faculty of Medicine. "Our question was, does diet influence the ability of specific bacteria to cause cancer?" This fundamental inquiry drove the investigation, seeking to establish a causal link between dietary choices and the pathogenic potential of certain gut inhabitants.
Experimental Design: A Mouse Model for Unraveling Mechanisms
To address this complex question, a meticulously designed study was conducted by researchers led by postdoctoral fellow Bhupesh Thakur. The team utilized a mouse model, a standard and effective tool in preclinical research, to examine the combined effects of different dietary regimens and specific gut bacteria implicated in CRC. The experimental setup involved colonizing mice with one of three bacterial species previously associated with colorectal cancer. These mice were then subjected to one of three distinct dietary conditions: a standard diet, a low-carbohydrate diet, or a Western-style diet characterized by high fat and high sugar content. This multi-pronged approach allowed for the isolation of specific variables and the observation of their synergistic or antagonistic effects on cancer development.
The choice of specific bacterial strains was crucial. The researchers focused on microbes known to possess virulence factors, particularly those capable of producing genotoxic compounds – substances that can damage DNA. Among the bacteria tested, a particular strain of Escherichia coli (E. coli) emerged as a key player when combined with a specific dietary context.
The Unveiling of a Potent Combination: Low-Carb Diet and Colibactin-Producing E. coli
The study’s findings revealed a stark and significant interaction. Only the combination of a low-carbohydrate diet and a specific strain of E. coli – one that produces the DNA-damaging compound colibactin – led to the development of colorectal cancer in the mouse models. This was a pivotal discovery, narrowing down the focus from broad dietary categories to a specific bacterial product and its amplification under particular dietary conditions.
Colibactin, a polyketide toxin, has been identified as a potent genotoxin produced by certain pathogenic strains of E. coli. It is known to induce DNA double-strand breaks and cause specific mutations, including C>A transversions, which are frequently observed in human cancers. The presence of colibactin-producing E. coli has been previously linked to an increased risk of CRC, but the precise mechanisms by which it exerts its influence, and how other factors modulate this risk, remained less clear until this study.
Unpacking the Mechanisms: How the Diet Creates a Permissive Environment
The research team delved deeper to understand why this specific combination proved so detrimental. Their investigations uncovered two primary mechanisms by which a low-carbohydrate diet facilitates the pro-cancerous activity of colibactin-producing E. coli.
Firstly, a diet deficient in soluble fibre significantly impacted the gut environment. The researchers observed that such a diet led to increased inflammation within the gut lining. More critically, it altered the composition and diversity of the resident microbial community, often referred to as the gut microbiome. This dysbiosis, or imbalance in the microbial ecosystem, created an environment that was more conducive to the proliferation and dominance of the colibactin-producing E. coli. In essence, the low-carb, low-fibre diet inadvertently cleared the path for the pathogenic bacteria to flourish.
Secondly, the study revealed a direct physical consequence of the low-carbohydrate diet on the gut barrier. Mice fed this diet exhibited a thinner mucus layer separating the gut microbes from the colon epithelial cells. This mucus layer serves as a vital protective shield, forming a physical and immunological barrier between the lumen of the gut, teeming with bacteria, and the delicate cells lining the colon. When this barrier is compromised, it allows for increased proximity and interaction between the bacteria and the epithelial cells. Consequently, more colibactin could reach the colon cells, initiating and exacerbating genetic damage that drives tumour growth.
The impact of this weakened barrier was particularly pronounced in mice with pre-existing genetic mutations in their DNA mismatch repair (MMR) pathway. The MMR system is a critical cellular mechanism responsible for correcting errors that occur during DNA replication. Defects in MMR genes, such as those found in Lynch syndrome, significantly impair the cell’s ability to repair DNA damage. In these genetically susceptible mice, the increased exposure to colibactin, coupled with their impaired DNA repair capacity, resulted in a significantly higher rate of genetic mutations and subsequent tumour development.
Implications for Human Health and Cancer Prevention
While the study was conducted in mice, the researchers emphasize the compelling need to validate these findings in human populations. Nevertheless, the implications for human health are substantial and immediate, offering promising avenues for cancer prevention strategies.
Colorectal cancer remains a formidable challenge, ranking as the fourth most commonly diagnosed cancer in Canada, with an estimated 153,100 new cases and 52,700 deaths in 2023 in the United States alone. A significant proportion of CRC cases, approximately 15%, are associated with mutations in MMR genes. These genetic alterations are the hallmark of Lynch syndrome, an inherited condition that dramatically elevates an individual’s lifetime risk of developing certain cancers, including CRC, endometrial cancer, and others.
Professor Martin highlighted the potential for personalized risk assessment and intervention for individuals with Lynch syndrome. "Can we identify which Lynch syndrome patients harbour these colibactin-producing microbes?" he posited. If such individuals are identified, the study’s findings suggest that modifications to their diet, specifically avoiding low-carbohydrate diets, or targeted antibiotic treatments to eradicate colibactin-producing bacteria, could significantly reduce their cancer risk.
Further research is already underway to explore the nuances of probiotic use. Professor Martin noted that a strain of E. coli known as Nissle, commonly found in over-the-counter probiotics, also produces colibactin. His lab is actively investigating whether the long-term consumption of this probiotic is safe for individuals with Lynch syndrome or those adhering to a low-carbohydrate diet. This underscores the complexity of the microbiome and the need for careful consideration of even seemingly beneficial interventions in specific contexts.
The Role of Fibre: A Protective Component
The study also provided encouraging evidence regarding the role of dietary fibre. Postdoctoral fellow Bhupesh Thakur expressed excitement over a key finding: the addition of soluble fibre to the low-carbohydrate diet led to a noticeable reduction in cancer-causing E. coli levels, diminished DNA damage, and fewer tumour formations. "We supplemented fibre and saw that it reduced the effects of the low-carb diet," Thakur stated. "Now we are trying to find out which fibre sources are more beneficial, and which are less beneficial."
This observation suggests that dietary fibre plays a crucial protective role, not only by promoting a healthy gut microbiome but also by counteracting the negative effects of low-carbohydrate diets on gut barrier integrity and microbial composition. Future research, a collaborative effort between Thakur, Martin, and Heather Armstrong at the University of Alberta, aims to investigate the efficacy of specific soluble fibre supplements, such as inulin, in reducing colibactin-producing E. coli and improving gut health in individuals at high risk for CRC, including those with inflammatory bowel disease.
A Call for Awareness and Dietary Prudence
The overarching message from this research is a cautionary one, particularly concerning popular weight-loss diets. "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," Professor Martin concluded. While acknowledging that more extensive research is required to fully translate these findings into clinical practice, he expressed hope that the study will at least raise awareness among the public and healthcare professionals about the intricate relationship between diet, gut microbes, and cancer risk.
The study, spanning from its initial conception to the publication of these significant findings, represents a multi-year effort in understanding the intricate biological processes that underpin colorectal cancer. The timeline of such research typically involves years of laboratory work, including pilot studies, extensive experimentation, data analysis, and rigorous peer review before findings are disseminated. This U of T study, initiated to explore long-standing questions about cancer etiology, has now provided a critical piece of the puzzle, underscoring the need for a holistic approach to health that considers not just macronutrient intake but also the dynamic interplay within our own bodies.
The implications extend beyond individual dietary choices, potentially influencing public health recommendations and the development of novel diagnostic and therapeutic strategies. As research continues, the focus will likely shift towards identifying individuals who are most vulnerable to the effects of colibactin-producing bacteria and developing targeted interventions, ranging from dietary counselling to microbiome-modulating therapies, to mitigate their risk of developing colorectal cancer.
