It might be trendy to cut carbs out of your diet, but doing so could carry significant implications for your health.
New research from University of Toronto scientists suggests that a diet low in carbohydrates and fibre could heighten one’s risk of colorectal cancer — and it all has to do with bacteria in the gut.
The study, published Monday morning in Nature Microbiology, sheds new light on a type of cancer that is surging among young people across developed countries worldwide. Colorectal cancer is the fourth most commonly diagnosed cancer in Canada today, and made up 10 per cent of all new cancer cases and 11 per cent of all cancer deaths in 2024.
The findings “raise awareness of the potential risks of a long-term, low carb diet and highlights the importance of fibre in the diet,” explained Alberto Martin, a professor of immunology at U of T who led the study. “It could also lead to targeted prevention strategies for high risk individuals, such as those with Lynch syndrome or inflammatory bowel disease.”
One unaffiliated expert called the results “very validating” amidst an ongoing increase in colon cancer among Canadians under 50 — especially given the popularity of low-carb diets, like the keto diet, for weight loss.
“This really does help tie in the environmental and dietary exposure side (of colon cancer) that a lot of us have theorized and some data has proposed,” continued Dr. Sami Chadi, the Wise Family Chair in Colorectal Surgical Innovation with the University Health Network.
But because the study was mostly performed on mouse models, Chadi noted that further research is required before the findings could be fully extrapolated onto humans.
Low carb, low fibre diets linked to colorectal cancer
The human gut is home to an estimated 38 trillion bacterial cells, slightly eclipsing the total number of human cells in the average adult body.
Within this gut “microbiome” there can exist numerous bacterial species capable of damaging our DNA — and potentially contributing to cancer initiation and progression — by producing a toxin called colibactin.
These include certain strains of E. coli, which are present in about a fifth of the total human population but have been detected in roughly 60 per cent of all colorectal cancer cases and 40 per cent of people with irritable bowel disease.
After feeding mice a diet low in carbs and fibre, Martin’s team found the animals developed more polyps, which can eventually lead to cancer, compared to mice that were fed regular chow. Even more polyps were detected in mice infected with a strain of colibactin-producing E. coli, their paper reads.
Further investigation suggested a low-carb diet “reduced the mucus layer that separates our colon cell wall from the trillions of microbes that exist in our gut,” Martin said. It allowed bacteria to come in close contact with surface cells in the colon and cause inflammation — “and this inflammation is what allows this specific E. coli strain to bloom, to grow,” he explained.
At the same time, the reduced protection of the colon cells may allow colibactin-producing microbes to snuggle up close to the walls of our colon, allowing the toxin to “more easily damage our DNA,” Martin continued.
The team went further and replicated their experiment in mice with mutations similar to Lynch syndrome, an inherited condition that greatly elevates one’s risk of colon cancer. They found these mice were especially susceptible to colibactin-producing E. coli, which may partly explain why up to 80 per cent of people with Lynch syndrome end up developing colon cancer in their lives.
On the flipside, the team found a diet high in fibre — specifically a prebiotic fibre called inulin — may be able to reverse some of these effects.
“What it did specifically is that it reduced the abundance of this specific E. coli strain… and it reduced colon cancer development,” Martin expanded. His lab is now investigating whether inulin can produce the same results in humans as it does in mice.
A ‘Western-style diet’ didn’t have the same effects
Numerous studies have shown that a diet high in sugar and fat — a so-called “Western-style diet” — is linked to colorectal cancer, as well as obesity and chronic diseases like inflammatory bowel syndrome.
But to the team’s surprise, mice fed this diet were no more prone to cancer than normal. Martin mused that, because his study focused specifically on gut bacteria, it’s possible this diet could still contribute to colon cancer through a different mechanism.
“We were a bit surprised because the Western-style diet also has low fibre, but not as low as the low-carb, low-fibre diet,” he added.
Study limitations
According to Chadi, “the obvious limitation is the fact that, as all mouse studies are, they’re limiting in how you can extrapolate the results to the human environment.”
Martin’s team did perform limited research on humans, which suggested that a low-carb diet promoted gut inflammation and increased E. coli abundance in people. This helped provide some “human validation for the associations that they’re proposing from the mouse model,” Chadi added, “which is very insightful.”
But he and Martin noted that more human studies are needed to solidify the link between carbohydrates, fibre and cancer.
“These findings could help inform dietary recommendations, probiotic safety guidelines and targeted prevention strategies for high risk individuals,” Martin said. “But at this point, I would agree it would be premature to recommend specific diets to lower the risk of colon cancer.”
