Scientists Studied 1,300 Cancer Patients, Then Found a Pattern in the Gut - Newsweek

After studying stool samples from more than 1,300 cancer patients, scientists uncovered distinct gut microbiome patterns tied to different cancers.

Researchers from Mayo Clinic analyzed samples from 1,364 cancer patients across a range of cancer types, stages and treatments as part of the Mayo Clinic Cancer Microbiome Cohort study, published in Cell.

Scientists found unique microbiome signatures associated with five major cancer groups, while also identifying notable differences in patients diagnosed at younger ages.

One of the most striking findings involved early-onset colorectal and breast cancers, both of which have been increasing among younger adults in recent years.

Patients aged 50 and under with colorectal cancer had higher levels of lactate and greater amounts of a bacterium called Veillonella parvula, which feeds on lactate.

Meanwhile, younger breast cancer patients showed changes across 64 bacterial species and lower levels of primary bile acids.

“We observed distinct changes in microbes and metabolites in early-onset breast and colorectal cancers, but not in brain cancer,” Purna C. Kashyap, M.B.B.S., a gastroenterologist at Mayo Clinic, told Newsweek.

At this stage, Kashyap said the findings do not yet explain why breast and colorectal cancers are becoming more common among younger adults.

“However, they do provide potential biomarkers and mechanistic leads that can now be tested in independent clinical cohorts and in preclinical models,” he said.

In colorectal cancer, for example, tumors can produce high levels of lactate, potentially creating an environment that supports the growth of V. parvula.

Whether that relationship contributes to cancer development remains unclear.

Beyond age-related differences, the team identified 341 bacterial species associated with five cancer groups after accounting for other health conditions.

Neuroendocrine tumors were linked to a loss of beneficial gut bacteria, while liver and intrahepatic bile duct cancers showed higher levels of several microbes, including Enterococcus faecalis.

Distinct microbial patterns were also observed in esophageal cancer, lymphoid leukemia and multiple myeloma.

The study also suggested the microbiome could help predict which patients are more likely to experience treatment side effects.

Researchers found that among patients receiving the chemotherapy drug 5-fluorouracil (5-FU), those who later developed diarrhea tended to have lower levels of bacterial genes capable of breaking down the medication before treatment began.

The association was not found among patients receiving a different chemotherapy drug, carboplatin.

Ruben Mars, Ph.D., an assistant professor of medicine at Mayo Clinic, said the findings raise the possibility that microbiome-based tests could one day help identify patients at greater risk of side effects from certain treatments.

“Importantly, we found a microbiome connection with side effects from one chemotherapy drug—intravenous 5-fluorouracil—but not from all chemotherapy drugs,” Mars told Newsweek.

While the findings could eventually support the development of a clinical test, he said that researchers still need to validate the results in larger and more diverse patient groups.

“Ultimately, prospective clinical studies will be necessary to demonstrate that incorporating microbiome information into clinical decision making actually improves patient outcomes,” Mars said. “Despite this, I am cautiously optimistic that we will see microbiome-based testing begin to enter the clinic in the near future.”

Researchers additionally identified gut bacteria associated with survival outcomes in colorectal, liver and intrahepatic bile duct, ovarian and prostate cancers, as well as melanoma.

Raymond Chang, medical director at Meridian Medical and former physician at Memorial Sloan-Kettering Cancer Center, said the research highlights the growing importance of the gut microbiome in cancer care.

Chang, who was not involved in the study, said two findings stood out in particular: the link between specific bacterial species and patient survival, and the association between gut microbiome composition and treatment side effects such as diarrhea caused by 5-FU chemotherapy.

“These findings not only highlight the importance of the gut microbiome in cancer but may lead to further research in which manipulating the microbiome may lead to better survival as well as reducing treatment toxicities,” Chang said.

Yang, L., Singh, V., Gawey, B.J., Sinnwell, J.P., Johnson, S., Billings, E.C., Van Gorp, T.M., Harrington, J.J., Slama, M.Q., Till, L.M., Singh, M., Samineni, T.R., Zhu, M., Kalari, K.R., Farrugia, G., Chen, J., Mars, R.A.T. and Kashyap, P.C. (2026). Microbiome signatures linked to cancer and treatment adverse events in a real-world cohort. Cell. [online] doi:10.1016/j.cell.2026.09.009.