Liver Cancer

How Bile Acid Chemistry Connects the Gut to Liver Tumours

The animal evidence here is strong and the mechanism is unusually precise, though it hasn't yet been tested in people. Liver cancer is rising in step with obesity and fatty liver disease. The liver receives blood straight from the intestines, so anything gut bacteria produce arrives there first and at full strength. Two separate lines of research have landed on the same bacterial reaction.

A bacterial by-product that damages DNA

Bile acids are made by the liver and released into the gut. Gut bacteria then modify them, converting primary bile acids into secondary ones.

One of the products is deoxycholic acid. A small number of bacterial species, mostly in the genus Clostridium, carry out the reaction that produces it. It's known to cause DNA damage.

Researchers fed mice a high-fat diet alongside a chemical trigger for liver cancer. Obese mice developed tumours. Lean mice given the same trigger largely didn't.

The high-fat diet had changed which bacteria were present, and those bacteria produced far more deoxycholic acid.

The damage showed up in a specific cell type. Hepatic stellate cells, which normally produce scar tissue in response to injury, accumulated DNA damage and entered a state called senescence. Senescent cells stop dividing but keep secreting inflammatory and growth-promoting molecules.

That secretion is what drove the tumours. Reducing deoxycholic acid, either with antibiotics or by blocking its production, prevented the cancer developing.

Source: Obesity-induced gut microbial metabolite promotes liver cancer through senescence secretome

The same chemistry, acting on immune cells

A separate group approached the liver from the immune side and arrived at the same reaction.

The liver contains natural killer T cells, a specialised immune population that patrols for tumour cells. How many accumulate there depends on a signalling molecule displayed by the cells lining liver blood vessels.

Bile acids control that signal, and which bile acids are present depends on gut bacteria.

Giving mice antibiotics reduced secondary bile acids. The signalling molecule increased, more natural killer T cells accumulated in the liver, and both primary liver tumours and liver metastases were suppressed.

Running it the other way confirmed the direction. Colonising mice with a Clostridium species, one of those performing the conversion, reduced the immune cells and increased liver metastases.

The effect was specific to the liver. Tumours elsewhere in the same animals weren't affected, which fits the liver's unique position downstream of the gut.

Source: Gut microbiome-mediated bile acid metabolism regulates liver cancer via NKT cells

Two mechanisms, one bacterial step

Putting those together produces an unusually coherent picture.

The same conversion, primary bile acids into secondary ones, appears twice. In the first study it produces a compound that damages DNA in the cells that go on to drive tumour growth. In the second it suppresses the immune cells that would otherwise kill tumour cells.

So the bacteria performing this reaction affect both how much damage the liver accumulates and how well it defends itself against the consequences.

The amount of that conversion depends on two things. One is how much bile is sent into the gut, which rises with dietary fat. The other is how many bacteria are present to perform the reaction.

What hasn't been shown

The limits here are substantial and worth stating.

All of this is mouse work. No trial has changed gut bacteria in people and measured liver cancer.

Antibiotics prevented tumours in these experiments, but that isn't a treatment anyone is proposing. Wiping out the community carries its own harms and removes beneficial bacteria alongside the target.

The human relevance rests on the observation that liver cancer rates track with obesity and fatty liver disease, and that these mice were modelling exactly that. It's a reasonable inference rather than a demonstrated fact.

Secondary bile acids also aren't simply harmful. They act as signals regulating metabolism and immunity throughout the body, which is why the answer isn't to eliminate them.

What this means for you

Liver cancer risk is driven mainly by conditions that scar the liver, so treating hepatitis B or C, reducing alcohol and addressing fatty liver with a doctor are what most change the outlook. In addition, the research above found the same bacterial bile acid conversion both damaging the cells that drive liver tumours and suppressing the immune cells that fight them. A varied, plant-rich diet supports the same gut bacteria this research ties to a bile acid balance less weighted towards the damaging forms. That's the same idea behind GutLab, everyday gut health starts with regularly feeding the microbiome a broad range of plant-rich ingredients.