A Western diet could be feeding colon cancer through gut bacteria, new study reveals

For years, scientists have known that diets high in red meat and saturated fat are linked to a higher risk of colorectal cancer. What has remained unclear is exactly how those foods increase the risk. Now, a new study published in the journal Gut has uncovered a key piece of the puzzle, pointing to specific gut bacteria that transform bile acids into compounds capable of promoting tumour growth.

The research, conducted by an international team of scientists, provides some of the strongest evidence yet that the combination of a Western-style diet and certain gut microbes can directly contribute to the development of colorectal cancer.

It's not just what you eat—it's what your gut bacteria do with it

When people eat a high-fat diet, the liver produces more bile acids to help digest the fat. Once these bile acids reach the large intestine, some gut bacteria convert them into secondary bile acids.


One of these compounds, called deoxycholic acid (DCA), has long been linked with colorectal cancer in observational studies. Until now, however, researchers had not been able to prove that gut bacteria producing DCA were actually driving cancer development.

The new study changes that.

"Our work provides functional evidence for the causal role of secondary bile acids produced by gut bacteria in colorectal cancer under adverse dietary conditions," the researchers wrote.

Western diet made cancer worse in pig model

To investigate the connection, scientists used genetically engineered pigs carrying an APC gene mutation, one of the most common genetic changes associated with colorectal cancer in humans.

Unlike many laboratory mouse models, these pigs develop tumours in the large intestine—the same location where colorectal cancer usually develops in people.

The animals were fed either a standard diet or a Western-style diet enriched with red meat and lard for three months.

The results were striking.

Compared with pigs on the standard diet, those eating the Western-style diet developed:

• More colon polyps
• Larger tumours
• Faster growth of cells lining the colon
• Higher levels of the secondary bile acid deoxycholic acid (DCA)

Researchers also found that giving the pigs colestyramine, a drug that binds bile acids, reduced the excessive cell growth in the colon, suggesting that bile acids were playing a direct role in tumour development.

Human cancer patients showed similar bacterial patterns

The team then examined stool samples collected from multiple groups of colorectal cancer patients.

They found that people with colorectal cancer were more likely to carry gut bacteria containing bai genes, which enable bacteria to convert primary bile acids into cancer-associated secondary bile acids.

Among the bacteria identified were Clostridium scindens and closely related species known for producing deoxycholic acid.

This finding strengthened the link between gut bacterial metabolism and colorectal cancer observed in the animal experiments.

Scientists proved the bacteria were responsible

The researchers went a step further to determine whether these microbes were merely associated with cancer—or actually causing it.

Using germ-free mice, they introduced bacterial communities that either contained or lacked bile acid-producing bacteria.

Mice carrying bacteria such as Clostridium scindens and Extibacter muris produced higher levels of deoxycholic acid and developed significantly more colon tumours than mice without those bacteria.

In another experiment, researchers genetically modified one bacterial species so it could no longer produce secondary bile acids.

Mice colonised with the modified bacteria developed fewer tumours, while laboratory-grown human colon tissues exposed to the modified strain showed much lower levels of abnormal cell proliferation.

Together, these experiments provided direct evidence that the bacteria—and the bile acids they produce—actively contribute to tumour formation.

Why this discovery matters

Colorectal cancer is the third most commonly diagnosed cancer worldwide, accounting for around 1.9 million new cases and nearly 900,000 deaths in 2022, according to figures cited in the study.

Although inherited genetic mutations increase risk for some people, most colorectal cancers are influenced by environmental and lifestyle factors, particularly diet.

The new findings suggest that the danger may not come solely from eating more red meat or fatty foods. Instead, those foods may encourage gut bacteria to produce harmful secondary bile acids that stimulate excessive cell growth in the colon, creating conditions that favour cancer development.

The researchers believe this opens the door to future prevention strategies aimed at altering gut bacterial activity, blocking secondary bile acid production or reducing their effects in the intestine.

While the findings are based on animal models supported by analyses of human patient samples, they provide compelling evidence that the interaction between diet and the gut microbiome plays a central role in colorectal cancer development. Future clinical studies will determine whether targeting these bacterial pathways could become a new strategy for preventing one of the world's most common cancers.