We already know the headline villain:
Western diets = higher colon cancer risk.

High fat. Lots of red and processed meat. Low fiber.

But why that happens has always been a bit fuzzy.

This new research pins down a key suspect — and it’s not just the food.
It’s what your gut bacteria do with that food.

The Big Picture

Colorectal cancer (CRC) is one of the world’s deadliest cancers:

  • ~1.9 million new cases
  • ~900,000 deaths in 2022 alone

Genes matter, but for most people, diet and lifestyle are the real drivers.

And this study shows how a Western diet quietly rewires your gut in ways that actively promote tumor growth.

Meet the Middlemen: Gut Bacteria

Your gut bacteria aren’t passive passengers. They’re chemical factories.

When you eat a high-fat diet, your liver makes more bile acids to digest that fat.

Those bile acids travel to your colon — where certain gut bacteria transform them into secondary bile acids.

One of them is especially nasty:

Deoxycholic acid (DCA)

Think of DCA as a biological troublemaker with a résumé that includes:

  • DNA damage
  • Increased cell division
  • Inflammation
  • Cancer-friendly signaling

What This Study Did Differently

Previous studies just dumped DCA directly into lab mice (not very realistic).

This one went bigger and better:

  • Genetically engineered pigs
  • Human stool samples
  • Special “germ-free” mice
  • Human colon organoids (mini colons grown in the lab)
  • Multi-omics analysis (genes, metabolites, microbes)

Translation:
They followed the whole chain — diet → bacteria → bile acids → tumors.

The Smoking Gun

Here’s what they found:

1️⃣ Western diets change gut bacteria

High-fat diets encourage bacteria that specialize in converting bile acids into DCA.

2️⃣ Those bacteria promote tumors

Specific microbes (like Clostridium scindens) ramp up production of DCA.

When these bacteria were added to mice:

  • Tumor numbers went up
  • Cell proliferation increased
  • Cancer-related genes switched on

3️⃣ Block the bile acids, reduce the damage

When bile acids were trapped and excreted instead of recycled:

  • Colon cell overgrowth dropped
  • Cancer-related signals decreased

This suggests reducing bile acid exposure may lower cancer risk.

Why DCA Is So Dangerous

DCA doesn’t cause cancer by itself — it sets the stage.

It:

  • Pushes intestinal stem cells to divide more
  • Increases DNA damage
  • Disrupts protective signaling in colon cells
  • Amplifies inflammation

Basically, it turns the colon into fertile soil for tumors.

A Twist: Sex Differences

Female mice had higher DCA levels than males — yet both sexes developed more tumors when exposed.

Why?

  • Hormones influence bile acid metabolism
  • Estrogen and testosterone affect cancer risk differently

Bottom line: both men and women are vulnerable, even if the biology looks slightly different.

It’s Not Just What’s Added — It’s What’s Missing

Western diets don’t just add harmful factors. They remove protective ones.

Low fiber = less:

  • Butyrate (a cancer-protective compound)
  • Gut barrier strength
  • Anti-inflammatory signaling

So the risk comes from a double hit:

  1. More tumor-promoting bile acids
  2. Fewer tumor-suppressing metabolites

What This Could Mean for Prevention

This research points to real, practical strategies:

  • Increase dietary fiber → changes gut bacteria behavior
  • Reduce excess fat intake → less bile acid overload
  • Target bile acid metabolism with medications or supplements
  • Avoid extreme Western eating patterns long-term

Interestingly, completely eliminating bile acids isn’t the goal — you still need them.
The sweet spot is balance, not zero.

The Takeaway

A Western diet doesn’t just increase colon cancer risk directly — it recruits your gut bacteria to help do the damage.

Your food choices shape your microbiome.
Your microbiome shapes your bile acids.
And those bile acids can quietly push colon cells toward cancer.

The good news?
Diet is modifiable.
Microbiome behavior is flexible.

Which means colon cancer risk isn’t just written in your genes — it’s influenced by what’s on your plate.

Osswald A, Wortmann E, Wylensek D, Kuhls S, Coleman OI, Peuker K, Strigli A, Ducarmon QR, Larralde M, Liang W, Treichel NS, Schumacher F, Volet C, Matysik S, Kleigrewe K, Gigl M, Rohn S, Guo CJ, Kleuser B, Liebisch G, Schnieke A, Ridlon JM, Bernier-Latmani R, Zeller G, Zeissig S, Haller D, Flisikowski K, Clavel T, Ocvirk S. Secondary bile acid production by gut bacteria promotes Western diet-associated colorectal cancer. Gut. 2025 Dec 18:gutjnl-2024-332243. doi: 10.1136/gutjnl-2024-332243. Epub ahead of print. PMID: 41412727.