By Gilang Cahya 
Researchers at the University of Toronto have unveiled a concerning link between low-carbohydrate diets and an increased risk of colorectal cancer, specifically by exacerbating the DNA-damaging capabilities of certain gut bacteria. This groundbreaking study, published in the esteemed journal Nature Microbiology, sheds new light on the intricate interplay between diet, the gut microbiome, and cancer development, offering crucial insights for prevention and risk management.
Unraveling the Gut-Microbe-Diet Connection in Colorectal Cancer
For decades, colorectal cancer has been understood as a multifactorial disease, influenced by a complex web of elements including dietary habits, the composition of the gut microbiome, environmental exposures, and an individual’s genetic predisposition. However, the precise mechanisms by which these factors interact to initiate and promote cancer have remained a subject of intense scientific inquiry. The University of Toronto team, led by Professor Alberto Martin of the Department of Immunology at U of T’s Temerty Faculty of Medicine, set out to investigate a pivotal question: "Does diet influence the ability of specific bacteria to cause cancer?"
To address this, the research team, with postdoctoral fellow Bhupesh Thakur at the helm, conducted a comprehensive study involving mice. These mice were deliberately colonized with one of three bacterial species previously implicated in colorectal cancer. Subsequently, they were subjected to one of three distinct dietary regimens: a standard diet, a low-carbohydrate diet, or a Western-style diet characterized by high fat and high sugar content. This experimental design allowed researchers to isolate the effects of specific dietary patterns in conjunction with particular gut microbes on cancer development.
The Culprit: A Specific E. coli Strain and a Carb-Deficient Environment
The findings revealed a stark and significant outcome: only the combination of a low-carbohydrate diet and a particular strain of Escherichia coli (E. coli) bacteria led to the development of colorectal cancer in the study subjects. This specific E. coli strain possesses the capability to produce colibactin, a potent genotoxic compound known to induce DNA damage.
"Colorectal cancer has always been thought of as being caused by a number of different factors including diet, gut microbiome, environment and genetics," stated Professor Martin. "Our question was, does diet influence the ability of specific bacteria to cause cancer?" The study unequivocally demonstrated that the answer is yes, particularly when it comes to a low-carbohydrate diet interacting with colibactin-producing E. coli.
Mechanisms of Damage: Weakened Defenses and Microbial Dominance
The research elucidated several key mechanisms by which this dietary pattern promotes cancer. A diet deficient in carbohydrates and, crucially, soluble fiber, was found to instigate inflammation within the gut. This inflammatory state disrupts the delicate balance of the gut microbial community, creating an environment where opportunistic pathogens, such as the colibactin-producing E. coli, can flourish and proliferate.
Furthermore, the study observed a significant thinning of the protective mucus layer that lines the colon. This mucus layer acts as a vital barrier, physically separating the gut microbes from the epithelial cells of the colon wall. In mice fed a low-carbohydrate diet, this protective shield was compromised. This weakened barrier allowed the colibactin produced by E. coli to penetrate more readily and reach the colon cells. Once there, colibactin could wreak havoc by causing genetic damage, which in turn drives the uncontrolled cell growth characteristic of tumor development.
These damaging effects were found to be particularly pronounced in mice possessing genetic mutations in their DNA mismatch repair (MMR) pathway. The MMR pathway is a critical cellular mechanism responsible for correcting errors that occur during DNA replication. When this pathway is defective, the cell’s ability to repair DNA damage is significantly impaired, making it far more susceptible to the mutagenic effects of compounds like colibactin.
Broader Implications for Colorectal Cancer Risk and Prevention
The implications of these findings are substantial, especially considering the prevalence of colorectal cancer. Colorectal cancer is the fourth most commonly diagnosed cancer globally and a significant public health concern. In Canada, for instance, an estimated 15% of colorectal tumors exhibit mutations in MMR genes. These same gene mutations are also central to Lynch syndrome, an inherited genetic disorder that substantially elevates an individual’s risk of developing several types of cancer, including colorectal cancer.
The University of Toronto researchers are optimistic about the potential applications of their work in cancer prevention strategies. "Can we identify which Lynch syndrome patients harbour these colibactin-producing microbes?" Professor Martin posed. He suggests that for individuals identified as carrying both a genetic predisposition (like Lynch syndrome) and these specific bacteria, modifying their diet to avoid low-carbohydrate intake or considering targeted antibiotic treatments to eradicate the colibactin-producing bacteria could be viable strategies to mitigate their cancer risk.
The study also touched upon the use of probiotics. Notably, a strain of E. coli known as Nissle, which is frequently found in over-the-counter probiotics, also produces colibactin. Professor Martin indicated that ongoing research in his lab is investigating the long-term safety of consuming this probiotic, particularly for individuals with Lynch syndrome or those adhering to a low-carbohydrate diet. This underscores the complexity of microbiome interventions and the need for personalized approaches.
The Protective Power of Fiber: A Ray of Hope
A particularly encouraging aspect of the study was the observation regarding dietary fiber. Bhupesh Thakur highlighted that supplementing the low-carbohydrate diet with soluble fiber led to a marked reduction in the levels of the cancer-causing E. coli, less DNA damage, and a significant decrease in tumor formation. "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 finding points towards fiber as a crucial dietary component in counteracting the negative effects of low-carbohydrate diets in the context of gut health and cancer risk. The research team is now actively pursuing this avenue, collaborating with Heather Armstrong at the University of Alberta. Their joint efforts will focus on testing the efficacy of inulin, a type of soluble fiber, 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.
A Call for Awareness and Dietary Caution
The researchers emphasized the need for further validation of these findings in human populations. However, the current evidence strongly suggests a cautionary approach to popular dietary trends. "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. "More work is needed but we hope that it at least raises awareness."
The study’s findings, which emerged from meticulous research conducted over a period that likely involved experimental design, data collection, and analysis spanning several years, offer a critical insight into the intricate pathways that can lead to colorectal cancer. The initial hypothesis, born from observations of increased colorectal cancer rates and evolving dietary trends, was systematically tested, leading to the identification of a specific dietary-microbial interaction. The results, published in Nature Microbiology in early 2024, have already begun to stimulate discussion within the scientific and medical communities about dietary recommendations and personalized cancer risk assessment.
This research is a significant step forward in understanding the nuanced relationship between what we eat and the microscopic inhabitants of our gut, and how together, they can influence our susceptibility to serious diseases like cancer. It underscores the importance of a balanced diet rich in fiber and calls for a more personalized approach to dietary recommendations, especially for individuals with a genetic predisposition to cancer.
