A major new study published in Cell has identified distinct gut microbiome patterns associated with several cancers — including some particularly interesting differences in early-onset colorectal and breast cancer.
Researchers from Mayo Clinic studied the gut microbiomes of 1,364 people with cancer, alongside 287 healthy controls, making this a large real-world investigation across multiple cancer types.
Importantly, the researchers didn’t simply ask: “Do people with cancer have different gut bacteria?”
They went further.
Because our microbiome is influenced by so many things — including age, medications, health conditions, diet, lifestyle and our environment — the researchers used a framework designed to separate these broader influences from microbial patterns that appeared more specific to particular cancers.
After accounting for these factors, they identified 341 cancer-associated bacterial species across five cancer groups.
Some of the most interesting findings involved younger adults.
In people aged 50 or younger with colorectal cancer, researchers found:
→ higher levels of lactate in stool
→ increased Veillonella parvula — a bacterium capable of using lactate as an energy source.
This is particularly interesting because tumours can produce high levels of lactate. Whether this creates an environment that encourages certain bacteria to grow — or whether those bacteria have any role in the disease itself — is something researchers now need to investigate.
In early-onset breast cancer, researchers found differences across 64 bacterial species along with changes involving bile acid metabolism, including lower levels of fecal bile acids and Clostridium scindens, a bacterium involved in bile acid and steroid metabolism.
The microbiome may also influence how medications are processed.
One of the most fascinating findings involved the chemotherapy drug 5-fluorouracil (5-FU).
Patients who went on to develop diarrhoea following 5-FU treatment had lower levels of bacterial genes involved in breaking down the drug.
Much of this activity was associated with the gut bacterium Anaerostipes hadrus.
The same relationship wasn’t seen with another chemotherapy drug, carboplatin — suggesting this wasn’t simply a general marker of chemotherapy-related diarrhoea.
Researchers also identified gut bacteria associated with survival outcomes in several cancers.
So, does this mean gut bacteria cause cancer? No.
This study identifies associations — not proof of cause and effect. But it does highlight something increasingly difficult to ignore:
Our gut microbiome is part of a much larger biological system.
It interacts with our metabolism, immune system, inflammatory signalling, bile acids, microbial metabolites and even some medications.
And rather than looking simply at whether we have “good” or “bad” bacteria, research is increasingly exploring what those microorganisms are actually doing.
The future of microbiome research may therefore extend far beyond digestive symptoms.
One day, microbial patterns may potentially help researchers understand disease mechanisms, identify people more likely to experience particular treatment side effects and develop more personalised approaches to care.
We aren’t there yet.
But studies like this are helping us understand just how interconnected the gut microbiome, metabolism and human health really are.
Clinical Nutrition • Gut & Metabolic Health • Personalised Nutrition
This post is for education only. Microbiome testing is not currently a diagnostic test for cancer and should not replace recommended cancer screening, medical investigation or treatment.
Research: Yang L, et al. Microbiome signatures linked to cancer and treatment adverse events in a real-world cohort. Cell. 2026.
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