By Gilang Cahya 
Researchers at the University of Toronto have unearthed a concerning link between low-carbohydrate diets, specific gut bacteria, and the progression of colorectal cancer. Their groundbreaking study, published in the esteemed journal Nature Microbiology, reveals how a diet lacking in carbohydrates and soluble fiber can amplify the DNA-damaging capabilities of certain intestinal microbes, potentially creating a fertile ground for tumor development. This research sheds new light on the intricate interplay between diet, the gut microbiome, and the etiology of one of the most prevalent cancers worldwide.
Unraveling the Microbial-Dietary Nexus in Colorectal Cancer
Colorectal cancer, a disease that affects millions globally and ranks as the fourth most commonly diagnosed cancer in Canada, has long been understood to be a multifactorial illness. Traditionally, its development has been attributed to a complex interplay of 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 remain an active area of 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 and postdoctoral fellow Bhupesh Thakur, embarked on a mission to dissect this complex relationship. Their central hypothesis was to investigate whether dietary choices could directly influence the carcinogenic potential of specific gut bacteria. To address this, they designed a comprehensive study using a murine model, meticulously comparing the effects of three distinct dietary regimens on colorectal cancer development.
The diets employed in the study were designed to represent common eating patterns: a standard, balanced diet; a diet deliberately restricted in carbohydrates; and a Western-style diet characterized by high levels of fat and sugar. These dietary interventions were then administered to mice that had been colonized with one of three specific bacterial species previously implicated in the development of colorectal cancer. This experimental design allowed the researchers to isolate the impact of diet in the presence of known carcinogenic microbes.
The Culprit: A Specific Strain of E. coli and a Low-Carb Diet
The findings from this rigorous investigation were striking and pointed towards a specific culprit. The researchers observed that a unique strain of Escherichia coli (E. coli) bacteria, when co-administered with a diet severely restricted in carbohydrates and soluble fiber, significantly promoted the growth of polyps in the colon. Colonic polyps are often considered precancerous lesions, meaning they have the potential to develop into cancer over time.
Crucially, the study pinpointed that only the combination of a low-carbohydrate diet and a specific strain of E. coli known to produce colibactin—a potent genotoxic compound that directly damages DNA—led to the development of colorectal cancer in the experimental mice. This finding underscores the critical role of both the specific microbial strain and the dietary environment in driving oncogenesis.
Professor Martin elaborated on the significance of their question, 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 results of their study suggest a resounding affirmative.
The Mechanisms of Damage: Impaired Gut Barrier and Inflammation
The research delved deeper into the physiological changes induced by the low-carbohydrate diet that facilitated the carcinogenic process. The researchers discovered that a diet deficient in fiber led to increased inflammation within the gut. This inflammatory state not only altered the delicate balance of the gut microbial community but also created an environment conducive to the proliferation of colibactin-producing E. coli.
Furthermore, a critical observation was the thinning of the mucus layer that naturally lines the colon. This mucus layer serves as a vital protective barrier, physically separating the gut microbes from the epithelial cells of the colon wall. In mice fed a low-carbohydrate diet, this barrier was compromised, allowing for increased proximity and interaction between the colibactin-producing bacteria and the colon cells. Consequently, more colibactin was able to reach and damage the DNA of these cells, a process that can initiate and drive tumor formation.
These detrimental effects were found to be particularly pronounced in mice that possessed genetic mutations in their DNA mismatch repair (MMR) pathway. The MMR pathway is a crucial cellular mechanism responsible for correcting errors that occur during DNA replication and repair. When this pathway is defective, damaged DNA accumulates, significantly increasing the risk of cancer. The study revealed that in mice with compromised MMR systems, the DNA damage inflicted by colibactin was amplified, accelerating the progression towards colorectal cancer.
It is noteworthy that defects in DNA mismatch repair are frequently observed in human colorectal cancers, accounting for an estimated 15% of these tumors. These mutations are also central to Lynch syndrome, a hereditary condition that substantially elevates an individual’s risk of developing several cancers, including colorectal cancer.
Implications for Cancer Prevention and Treatment
While the researchers emphasize that these findings are from animal studies and require validation in human populations, they 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, highlighting a key direction for future research. For individuals with Lynch syndrome or those with a high genetic predisposition to colorectal cancer, the study’s findings suggest that avoiding low-carbohydrate diets or exploring targeted antibiotic treatments to eradicate colibactin-producing bacteria could be potential avenues to mitigate their elevated risk.
The research also brings to light an interesting nuance regarding probiotics. Professor Martin pointed out that a strain of E. coli known as Nissle, often found in probiotic formulations, also produces colibactin. This raises important questions about the long-term safety of such probiotics for individuals with Lynch syndrome or those adhering to low-carbohydrate diets. Ongoing research in Professor Martin’s lab is actively investigating this matter, aiming to clarify whether prolonged use of Nissle probiotics poses any risks in these specific populations.
The Protective Power of Fiber
The study also yielded an encouraging insight regarding the role of dietary fiber. Dr. Bhupesh Thakur expressed enthusiasm for a key finding demonstrating that supplementing the low-carbohydrate diet with soluble fiber led to a significant reduction in the levels of cancer-causing E. coli, diminished DNA damage, and fewer tumor formations.
"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 suggests that the type and quantity of fiber consumed could play a pivotal role in modulating the gut microbiome’s impact on cancer risk.
To further explore this promising avenue, Dr. Thakur and Professor Martin are collaborating with Heather Armstrong, a researcher at the University of Alberta. Their joint effort will involve investigating 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, including those with inflammatory bowel disease.
A Cautionary Note on Popular Diets
Professor Martin concluded with a cautionary remark, stating, "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." The popularity of low-carbohydrate diets for weight management and perceived health benefits warrants a closer examination of their potential long-term consequences, particularly concerning gut health and cancer risk.
The research, which originated from studies initiated in the last five years as the scientific community increasingly focused on the gut microbiome’s role in health and disease, serves as a crucial reminder of the intricate biological pathways at play. While more research is undoubtedly needed to translate these findings into definitive clinical recommendations for the general public, this study has undoubtedly raised significant awareness about the potential risks associated with certain dietary patterns and their interaction with our resident gut microbes. The implications for personalized nutrition and targeted interventions for individuals at high risk of colorectal cancer are profound and warrant continued scientific exploration. The journey from understanding microbial metabolites to developing novel preventive strategies has taken a significant step forward with this vital research.
