By Gilang Cahya 
A groundbreaking study conducted by researchers at the University of Toronto has unveiled a concerning link between low-carbohydrate diets and an increased risk of colorectal cancer, demonstrating how specific gut bacteria can become more potent in their DNA-damaging capabilities under such dietary conditions. The findings, published in the esteemed journal Nature Microbiology, shed new light on the intricate interplay between diet, the gut microbiome, and the development of one of the most prevalent forms of cancer globally.
The Study’s Genesis and Methodology
Colorectal cancer, a disease that affects millions worldwide and is the fourth most commonly diagnosed cancer in Canada, has long been understood to be a multifactorial illness. The conventional wisdom posits that a confluence of genetic predisposition, environmental exposures, and dietary habits contributes to its onset. However, the precise mechanisms by which these factors interact, particularly the role of the gut microbiome, have remained a subject of intense scientific inquiry. It was this very question – whether diet could directly influence the oncogenic potential of specific gut microbes – that propelled the research team, led by Professor Alberto Martin of the University of Toronto’s Temerty Faculty of Medicine and postdoctoral fellow Bhupesh Thakur, to embark on their investigation.
To unravel this complex relationship, the researchers devised an experiment involving laboratory mice. These mice were intentionally colonized with one of three bacterial species previously implicated in the development of colorectal cancer. Subsequently, they were subjected to one of three distinct dietary regimens: a standard control diet, a low-carbohydrate diet, or a Western-style diet characterized by high fat and high sugar content. This controlled approach allowed the scientists to meticulously isolate and observe the effects of each dietary condition on the progression of colorectal cancer in the presence of specific microbial inhabitants.
Unveiling a Sinister Combination: Low-Carb Diet and Cancer-Causing E. coli
The results of the study were striking and pointed towards a particularly concerning dietary and microbial synergy. The researchers discovered that only one specific combination of diet and bacteria led to the significant development of colorectal cancer in the animal models. This potent combination involved a diet notably low in carbohydrates and soluble fiber, when paired with a unique strain of Escherichia coli (E. coli) bacteria known to produce colibactin, a potent genotoxic compound capable of damaging DNA.
This specific strain of E. coli, when deprived of adequate carbohydrates and soluble fiber, exhibited an amplified ability to promote the growth of polyps in the colon. Polyps are abnormal growths that, while often benign, can serve as precursors to cancerous tumors. The study’s findings suggest that the absence of these key dietary components creates an environment within the gut that is conducive to the proliferation and increased virulence of colibactin-producing E. coli.
Professor Martin elaborated on the significance of this finding, stating, "Colorectal cancer has always been thought of as being caused by a number of different factors including diet, gut microbiome, environment and genetics. Our question was, does diet influence the ability of specific bacteria to cause cancer?" The study’s results provide a compelling affirmative answer, highlighting a direct link between dietary choices and the carcinogenic potential of certain gut microbes.
The Gut’s Defenses Compromised: Inflammation and a Weakened Mucosal Barrier
Further investigation into the physiological mechanisms underlying this effect revealed how the low-carbohydrate diet compromised the gut’s natural defenses. The deficiency in fiber, a crucial component of a balanced diet, was found to instigate increased inflammation within the gut. This inflammatory response not only alters the delicate balance of the gut microbiome, known as dysbiosis, but also creates a more hospitable environment for opportunistic pathogens like the colibactin-producing E. coli to flourish.
Perhaps more critically, the study demonstrated that mice on the low-carbohydrate diet exhibited a significantly thinner protective mucus layer lining their colons. This mucus layer is a vital barrier, acting as a physical shield that separates the vast community of gut microbes from the colon’s epithelial cells, the very cells that line the intestinal tract and are susceptible to DNA damage. When this barrier is weakened, the genotoxic compound colibactin produced by the E. coli can more readily reach and interact with the colon cells, initiating genetic mutations that can drive tumor formation.
The impact of this compromised barrier was particularly pronounced in mice possessing specific genetic mutations in their DNA mismatch repair (MMR) pathway. The MMR pathway is a crucial cellular mechanism responsible for correcting errors and damage that occur during DNA replication. When this pathway is impaired, the cells are less capable of repairing DNA damage, making them more vulnerable to the genotoxic effects of compounds like colibactin. It is noteworthy that defects in DNA mismatch repair are frequently observed in human colorectal cancers, with an estimated 15 percent of these tumors exhibiting mutations in MMR genes.
Broader Implications for Cancer Prevention and High-Risk Individuals
While the study was conducted in mice, both Dr. Thakur and Professor Martin emphasized the critical need to validate these findings in human populations. However, they expressed considerable optimism regarding the potential clinical applications of their research in the realm of cancer prevention.
The prevalence of MMR gene mutations in colorectal cancer and their association with hereditary cancer syndromes like Lynch syndrome, which significantly elevates an individual’s risk of developing certain cancers, including colorectal cancer, underscores the relevance of this research. Professor Martin posed a pertinent question: "Can we identify which Lynch syndrome patients harbour these colibactin-producing microbes?" If such an identification is possible, the study’s findings suggest that targeted interventions could be developed. For individuals with Lynch syndrome who harbor these specific microbes, avoiding low-carbohydrate diets or undergoing specific antibiotic treatments to eradicate the colibactin-producing bacteria could potentially mitigate their elevated cancer risk.
Furthermore, the research team is exploring the implications of commonly used probiotics. Professor Martin noted that a strain of E. coli known as Nissle, which is frequently incorporated into probiotic supplements, also produces colibactin. Ongoing research in his laboratory 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.
The Promising Role of Soluble Fiber
A particularly encouraging finding from the study was the observed benefit of supplementing the low-carbohydrate diet with soluble fiber. When soluble fiber was added, the researchers noted a significant reduction in the levels of the cancer-causing E. coli, decreased DNA damage, and fewer tumor formations in the mice. This suggests that soluble fiber plays a protective role, counteracting some of the detrimental effects of a carbohydrate-deficient diet in conjunction with specific gut microbes.
"We supplemented fibre and saw that it reduced the effects of the low-carb diet," stated Dr. Thakur. "Now we are trying to find out which fibre sources are more beneficial, and which are less beneficial." To address this, Dr. Thakur and Professor Martin are collaborating with Heather Armstrong, a researcher at the University of Alberta. Their joint efforts will focus on evaluating whether supplementation with a specific soluble fiber called inulin can effectively reduce colibactin-producing E. coli and improve gut health in individuals at high risk for colorectal cancer, such as those with inflammatory bowel disease.
A Call for Awareness and Further Research
The implications of this research are far-reaching, particularly in the context of popular dietary trends. Low-carbohydrate diets have gained considerable traction as a strategy for weight loss and management. However, Professor Martin cautioned, "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."
While acknowledging that more extensive research is necessary to fully elucidate the complex interactions at play, the researchers hope their findings will contribute to a greater public awareness of the potential risks associated with certain dietary patterns and their impact on the gut microbiome and cancer development. This study serves as a critical reminder that dietary choices, especially when combined with specific microbial compositions, can have profound and lasting effects on our health, underscoring the importance of a balanced and fiber-rich diet for maintaining gut health and potentially reducing cancer risk. The ongoing investigations into specific fiber sources and their effects on colibactin-producing bacteria promise to yield further insights, paving the way for more personalized and effective strategies for cancer prevention.