Specific Gut Bacteria Turn a Western Diet Into Colon Cancer Risk, Study Finds

If a diet heavy in red meat and fried food has you wondering whether it's quietly raising your colon cancer risk, and what your gut bacteria have to do with it, a new multi-model study offers one of the more complete biological explanations yet. Researchers traced a specific chain of events: from a Western-style diet, through a small group of gut bacteria that process bile, to tumor growth in the colon, and then found the same bacteria are more common in people who already have colorectal cancer. The findings don't prove that changing your own gut bacteria will lower your cancer risk, but they map out a mechanism worth understanding if colon health runs in your family or you're already managing a digestive condition.

Key takeaways

• A study in pigs, mice, and over 2,000 human stool samples found that a small set of gut bacteria convert bile acids from a Western-style diet into compounds linked to colon tumor growth.
• The two bacteria singled out, Clostridium scindens and Clostridium hylemonae, were significantly more common in the gut microbiomes of 1,034 colorectal cancer patients than in 1,108 healthy controls.
• A bile-acid-blocking drug stopped the diet-driven tumor growth in pigs, supporting a causal pathway, but the human data is still observational, and no clinical trial has tested whether targeting these bacteria lowers cancer risk in people.

Specific Gut Bacteria Convert Bile Acids Into Tumor-Promoting Compounds

Researchers from the German Institute for Human Nutrition Potsdam-Rehbrücke and the Technical University of Munich, including scientist Sören Ocvirk, identified a group of gut bacteria that carry genes for a chemical process called 7-alpha-dehydroxylation (a reaction bacteria use to strip a molecular group off bile acids after the liver releases them into the gut to help digest fat). This process turns ordinary "primary" bile acids into "secondary" bile acids, including one called deoxycholic acid (DCA) and another called 12-KLCA. In the study, animals fed a Western-style diet high in red meat and lard had higher levels of these secondary bile acids in their feces, and higher rates of colon cell proliferation and tumor growth, than animals on a control diet. Two bacteria species did most of this conversion work: Clostridium scindens and Clostridium hylemonae.

Colon Cancer Risk Ties Diet to a Specific Bacterial Signature

Colorectal cancer is one of the most diet-linked cancers, and readers managing inflammatory bowel disease, a family history of colon cancer, or simply trying to eat around a gut-health concern have long been told that red meat and saturated fat raise risk, without a clear biological story for why. This study adds a concrete link: bacteria carrying 7-alpha-dehydroxylation genes were significantly more frequent in the gut microbiomes of people with colorectal cancer than in people without it. That comparison came from a metagenomic analysis (genetic sequencing that identifies which microbe species are present in a stool sample) of 1,034 colorectal cancer patients against 1,108 controls, pooling 2,142 stool samples from multiple published research cohorts. A Cell Host & Microbe review of the broader field notes that bacteria-modified bile acids can also influence gut inflammation and immune signaling, adding another route by which they could shape cancer risk.

Pigs, Gnotobiotic Mice, and Human Stool Samples Built the Case

To test whether the bacteria-bile-acid link is more than a coincidence, the team used three connected experiments. First, they fed genetically engineered pigs carrying a mutation in the APC gene (the same gene commonly mutated in human colon cancer, chosen because pig digestive anatomy closely resembles a human's) either a Western-style diet or a control diet, across two trials of 8 to 9 and 4 to 5 pigs per group. Second, they used gnotobiotic mice (mice raised germ-free, then deliberately colonized with a defined set of bacteria) and colonized groups of 5 to 10 animals with the DCA-producing bacteria to see if the bacteria alone, without the full Western diet, could drive tumor growth. Third, they compared bacterial gene signatures across the 2,142 human stool samples described above. A meta-analysis of 23 separate studies independently found that elevated fecal secondary bile acids, including deoxycholic acid, track with higher colorectal cancer risk across human populations, a pattern consistent with what this study found at the bacterial level.

A Bile-Acid-Blocking Drug Offers a Clue, Not a Treatment

The strongest evidence for cause and effect came from a drug experiment: giving the pigs colestyramine, a bile-acid sequestrant, blocked the Western-diet-driven increase in colon cell proliferation. That result supports the idea that the bile acids themselves, not just something else about the Western diet, were driving the early cell changes that precede tumors. For a reader managing IBD, a family history of colorectal cancer, or general concern about a red-meat-heavy diet, this is not a signal to start taking a bile-acid-binding drug on your own. It has not been tested as a cancer-prevention strategy in humans, and colestyramine carries its own side effects and interactions. What it is worth doing is raising the general pattern, a diet high in red meat and saturated fat, altered gut bacteria, and bile acid metabolism, with a doctor or gastroenterologist if colon cancer risk is already a concern.

The Human Evidence Is Still Observational, Not Proof

The causal piece of this story, that the bacteria and their bile acids actually cause tumors rather than simply accompanying them, comes entirely from the pig and mouse experiments. The 2,142-sample human dataset shows that 7-alpha-dehydroxylation bacteria are more common in people who already have colorectal cancer, but an association like that cannot show whether the bacteria helped cause the cancer, appeared because of it, or reflect some third factor, like diet itself, that drives both. No clinical trial has yet tested whether deliberately reducing these bacteria, or the bile acids they produce, lowers colorectal cancer risk or recurrence in people. A PMC-hosted review of gut microbiota and colorectal cancer research makes the same point about the field broadly: most of the mechanistic evidence linking specific bacteria to human colon cancer still comes from animal models rather than intervention trials in people. The mouse experiments also turned up an unexplained wrinkle: female mice had higher DCA levels than male mice, a sex difference the researchers flagged but did not explain.

Human Intervention Trials Are the Next Step

The logical next step, and the one this study cannot answer on its own, is a human trial that actually intervenes, either by shifting diet, targeting the specific bacteria, or blocking bile acid signaling, and then tracks whether colorectal cancer incidence or precancerous polyp formation changes. The researchers also note that the tumor increase they saw in colonized mice was concentrated in small, early-stage tumors, which points toward these bacteria acting early in tumor formation ("initiation") rather than driving already-established tumors to grow further ("progression"). Confirming that distinction, and working out why DCA levels differ by sex, are among the open questions a review of intestinal dysbiosis and colorectal cancer also flags as priorities for the field. No timeline for a human trial has been announced.

Common Questions

Does eating red meat cause colon cancer through gut bacteria? This study proposes one possible pathway, specific gut bacteria converting bile acids from a high red meat and fat diet into compounds linked to tumor growth in animal models, but it does not prove this pathway causes colon cancer in people.

Can I get tested for these cancer-linked gut bacteria? Clinical testing for Clostridium scindens or Clostridium hylemonae specifically is not standard medical practice, and there is currently no established action to take based on the result even if you could.

Is this study based on human data or animal data? Both. The causal evidence that bile acids drive tumor growth comes from pig and mouse experiments; the finding that these bacteria are more common in people with colorectal cancer comes from a human stool-sample comparison, which shows association, not proof of cause.

What is a secondary bile acid? It's a bile acid that gut bacteria have chemically altered after the liver first releases it into the intestine to help digest fat; this study focused on two secondary bile acids, deoxycholic acid (DCA) and 12-KLCA.

References

1. Journey L. Wise, Bethany P. Cummings (2023). The 7-α-dehydroxylation pathway: An integral component of gut bacterial bile acid metabolism and potential therapeutic target. Frontiers in Microbiology. https://doi.org/10.3389/fmicb.2022.1093420
2. The effect of fecal bile acids on colorectal cancer risk: meta-analysis. Nature. https://www.nature.com/articles/s41598-024-84801-6
3. The Gut Microbiota and Colorectal Cancer. NIH. https://pmc.ncbi.nlm.nih.gov/articles/PMC11939563/
4. Intestinal dysbiosis and colorectal cancer. NIH. https://pmc.ncbi.nlm.nih.gov/articles/PMC12150945/