Your Gut Bacteria May Predict Chemo Side Effects: What a Landmark Mayo Clinic Study Found
Quick Answers
What happened? Researchers at Mayo Clinic studied stool samples from 1,364 people with cancer and 287 people without cancer. They found gut bacteria patterns linked to five groups of cancers, published in the journal Cell on September 29, 2026.
What is the gut microbiome? The trillions of bacteria and other microbes that live in your digestive tract. They help with digestion, the immune system, and breaking down some medicines.
What did they find? After accounting for other health conditions, 341 bacterial species were linked to five cancer groups. Gut bacteria were also linked to survival and to treatment side effects.
What is the biggest takeaway for chemo patients? People who later developed diarrhea on the chemotherapy drug 5-FU had lower levels of a gut bacterium called Anaerostipes hadrus, which helps break the drug down. One day, a stool test before treatment might help predict side effects — but that test is not ready yet.
What Happened in Cancer Microbiome News?
Two patients can get the same chemotherapy, at the same dose, and have very different experiences. One feels mostly fine. The other ends up in the bathroom all day with diarrhea, cramps, and weakness.
Doctors have long wondered why. Part of the answer may be living in the gut.
On September 29, 2026, researchers at Mayo Clinic published a large study in Cell, one of the world's top science journals. The study is called "Microbiome signatures linked to cancer and treatment adverse events in a real-world cohort".
The team analyzed stool samples from 1,364 people with cancer and 287 healthy people without cancer. The patients came from Mayo Clinic sites in Arizona, Florida, and Minnesota, and they represented 40 states. Samples were collected before patients started a new round of treatment, then linked to detailed medical records on side effects and outcomes.
The result is one of the clearest maps yet of how gut bacteria relate to cancer — and to what treatment feels like.
The trillions of microbes in your gut may quietly shape both cancer itself and how hard chemotherapy hits you.
What Is the Gut Microbiome?
Your digestive tract is home to trillions of microbes — bacteria, viruses, and fungi. Together, they are called the gut microbiome.
Think of your gut like a busy city. The microbes are its residents. Most are helpful workers. They:
Help digest food and make certain vitamins
Train and calm the immune system
Protect the lining of the gut
Break down medicines and other chemicals
Make substances that affect the whole body, such as bile acids and lactate
When the mix of microbes gets out of balance, it can affect far more than digestion. Research has linked the microbiome to weight, mood, immune diseases, and now — more and more — to cancer.
This does not mean bacteria cause every cancer, or that fixing your gut fixes cancer. It means the gut is part of the environment your body and your treatment work in. And environments matter.
Why Many Earlier Microbiome Headlines Were Misleading
You may have seen headlines before claiming "this bacteria causes that cancer." Many of those studies had a hidden problem.
People with cancer often have other health conditions too — diabetes, high blood pressure, lung disease, liver problems, and more. Those conditions also change gut bacteria. So do age, diet, and medicines.
If a study does not account for those other conditions, it can blame the cancer for changes that were really caused by something else.
It is like blaming a city's traffic on one new building, when road construction, weather, and a ball game were happening the same day.
The Mayo Clinic team built a special framework to sort this out. They adjusted for comorbidities (other health conditions), plus age, sex, and clinical details, to separate changes that were truly linked to cancer from changes that were just along for the ride.
What they found was humbling for the field: many of the microbiome changes shared across cancers were largely driven by those other health conditions — not by the cancer itself.
That may sound like bad news. It is actually good science. It means the signals that survived this strict filtering are the ones most worth trusting.
What Did the Mayo Clinic Study Find?
The researchers first compared people with cancer to the 287 people without cancer. Then they compared cancers against each other to find patterns specific to each type.
The key results
After accounting for other health conditions, the team identified 341 bacterial species linked to five cancer groups.
Neuroendocrine tumors showed a broad loss of common, health-associated gut bacteria.
Liver and intrahepatic bile duct cancers had higher levels of several bacteria, including Enterococcus faecalis.
Esophageal cancer had higher levels of six species, including several Streptococcus bacteria.
Distinct patterns were also found in lymphoid leukemia and in multiple myeloma and related plasma cell cancers.
Gut bacteria were linked to survival in colorectal, liver and bile duct, melanoma, ovarian, and prostate cancers. In liver and bile duct cancer, for example, Bifidobacterium longum was linked to longer survival, while Blautia A massiliensis was linked to shorter survival.
The microbiome measured before treatment was linked to side effects during treatment — including diarrhea from a widely used chemotherapy drug.
“We can now narrow the search to those microbial changes that are most specific to individual cancers,” said Dr. Ruben Mars, a Mayo Clinic microbiome researcher and co-corresponding author of the study. “Those are the signals we need to understand first if we want to determine whether the microbiome plays a causal role in cancer and ultimately develop targeted interventions.”
A plain-language translation: instead of a blurry photo of the gut, researchers now have a sharper one. The next step is finding out which of these bacteria are drivers, and which are just passengers.
It is important to be clear about what this study does not show. It shows links — associations. It does not prove that any bacterium causes cancer, prevents cancer, or changes survival. The patients all came from one health system, and the findings need to be confirmed in other groups of patients.
Early-Onset Cancers Show Distinct Gut Patterns
One of the most striking parts of the study looked at early-onset cancers — cancers diagnosed at age 50 or younger.
This matters because these cancers are becoming more common. According to the American Cancer Society, colorectal cancer rates are rising about 3% a year in adults ages 20 to 49, and breast cancer rates are rising about 1.4% a year in women under 50.
The Mayo team found that younger patients had different gut patterns than patients diagnosed later in life:
Early-onset colorectal cancer: higher levels of lactate, and more of a bacterium called Veillonella parvula. This bacterium feeds on lactate — and tumors can produce a lot of lactate. Whether that connection helps drive early-onset disease is not yet known.
Early-onset breast cancer: changes across 64 bacterial species, and lower levels of fecal bile acids. One of the species that differed was Clostridium scindens (also written as C. scindens), a bacterium involved in processing bile acids and steroids.
Early-onset brain cancer: no comparable age-related differences were found.
Here is a helpful way to picture it. Lactate is like exhaust from a fast-running engine. Veillonella parvula is a microbe that likes to live on that exhaust. Finding more exhaust and more exhaust-lovers in younger colorectal cancer patients raises real questions — but questions are not answers. The researchers are careful to say these differences do not prove that bacteria or lactate cause early-onset cancer.
A Gut Bacterium That May Protect Against 5-FU Diarrhea
For many patients, this is the finding that hits closest to home.
5-fluorouracil, called 5-FU for short, is a chemotherapy drug used to treat many cancers, including colorectal cancer. It is part of common regimens like FOLFOX. One of its hardest side effects is diarrhea, which can lead to dehydration, electrolyte loss, hospital visits, and even delays or dose cuts in treatment.
Your body uses an enzyme called dihydropyrimidine dehydrogenase — DPD for short — to break down 5-FU. People whose bodies break down 5-FU slowly can get very sick from it.
The Mayo team discovered something surprising: some gut bacteria carry DPD too.
A common gut bacterium called Anaerostipes hadrus makes a bacterial version of the DPD enzyme.
In the study, patients on 5-FU who had more of this drug-breaking-down bacterial function were protected against 5-FU diarrhea. Patients who went on to develop diarrhea had lower levels of the bacterial genes that break the drug down — much of that function came from Anaerostipes hadrus.
In other words, the gut may act like a backup liver for this drug — helping process some of the 5-FU before it can harm the gut lining.
Two details make this finding stronger:
The same signal was not linked to diarrhea in patients receiving carboplatin, a different chemotherapy drug. That suggests the effect is specific to 5-FU, not just a general sign of a “weak gut.”
The samples were collected before treatment, so the signal was there before the diarrhea happened. That is what makes it a possible predictive signal, not just a result of being sick.
“The microbiome is not the sole driver of cancer or treatment outcomes, but it is an underappreciated component that has not traditionally been considered in therapeutic approaches,” said Dr. Purna Kashyap, senior author of the study and director of the Mayo Clinic Microbiome Program. “This gives us a proof of concept that we can begin to understand why some patients experience a particular side effect and identify a target that could potentially be acted upon.”
A stool sample taken before chemo may one day help your team predict — and prepare for — side effects like diarrhea.
To be clear: there is no approved stool test for this today, and no proven probiotic that prevents 5-FU diarrhea. This is an early, exciting step — not a product or a promise.
Why Diarrhea During Chemotherapy Is More Than an Inconvenience
Diarrhea is sometimes treated like a minor side effect. It is not minor.
When diarrhea is frequent or severe, your body loses water and electrolytes — minerals like sodium, potassium, and magnesium that your heart, muscles, and nerves need to work right.
Think of electrolytes like the fluid levels in a car. You can lose a little and still run fine. Lose too much, and systems start to fail.
Ongoing diarrhea during chemotherapy can lead to:
Dehydration, dizziness, and weakness
Muscle cramps and an irregular heartbeat from low potassium or magnesium
Kidney strain
Poor nutrition, because food and fluids pass through too fast
Treatment delays, dose reductions, or hospital stays
This is why your care team takes diarrhea seriously, and why you should report it early — even if it feels embarrassing or “not worth a call.” Tumor biology and gut microbes both shape how treatment goes. Read more in our post: Can Bacteria Inside a Tumor Affect How Well Chemotherapy Works?
What Should Patients and Caregivers Watch For?
During chemotherapy — especially regimens that include 5-FU — contact your care team promptly if you notice:
Diarrhea that lasts more than a day, or is worse than your team said to expect
Signs of dehydration: dry mouth, dark urine, urinating much less than usual, or dizziness when standing
Muscle cramps, weakness, or a racing or irregular heartbeat
Fever, chills, or severe stomach pain
Vomiting, or being unable to keep fluids down
Mouth sores that make it hard to eat or drink
Losing weight without trying, or being unable to eat enough
Confusion or extreme tiredness
Do not try to “push through” severe symptoms at home. Early help — fluids, electrolytes, medicine adjustments — can keep a manageable side effect from becoming an emergency.
Why Supportive Care Matters During Chemotherapy
Cancer treatment is not only about fighting the tumor. Staying strong enough to stay on treatment matters too. Learn more in our post: Cancer Treatment Is More Than Fighting the Tumor: Why Supportive Care Matters
Diarrhea, mouth sores, and poor intake can quickly lead to dehydration and electrolyte loss. Fluorouracil-based chemotherapy can also harm the lining of the gut. When the gut lining is hurt, it is harder to absorb fluids and nutrients — which is exactly the problem the new microbiome research is helping scientists understand.
That is the supportive-care space HuMOLYTE was made for. HuMOLYTE is a medical food for use under medical supervision. It is designed to support hydration and electrolyte balance — including magnesium, which many standard drinks do not replace — and gut health during chemotherapy, with 2'-fucosyllactose (2'-FL), a human milk oligosaccharide (HMO). See the research in our post: 2′-FL & Chemotherapy-Induced Mucositis: HuMOLYTE Study
HuMOLYTE does not treat cancer, and it does not replace chemotherapy or care from your oncology team. Talk with your oncologist before starting any new product during treatment, including HuMOLYTE.
Practical steps that often help patients during chemotherapy include:
Track fluid intake, bathroom trips, weight, and energy each day.
Report diarrhea, mouth sores, or cramps early — do not wait for the next visit.
Ask if you need lab checks for electrolytes, including magnesium and potassium.
Ask for a dietitian referral if eating becomes hard.
Questions to Ask Your Oncologist
Does my treatment plan include 5-FU or a similar drug, like capecitabine?
Should I be tested for DPD deficiency before starting fluorouracil treatment?
What level of diarrhea should I expect, and at what point should I call you?
What is our plan for fluids and electrolytes if I develop diarrhea or vomiting?
Should I keep a daily log of fluid intake, bathroom trips, and weight?
Are there foods, supplements, or probiotics I should take — or avoid — during treatment?
Is there research or a clinical trial on the gut microbiome I could take part in?
If side effects get severe, when would we consider a dose change or a treatment break?
Should I see a dietitian who works with cancer patients?
Who should I call after hours if side effects get severe?
The Bottom Line
A landmark Mayo Clinic study of 1,364 cancer patients, published in Cell, shows that the gut microbiome carries real, measurable signals about cancer and treatment:
341 bacterial species were linked to five cancer groups — after carefully removing signals caused by other health conditions.
Many microbiome changes shared across cancers were driven by those other conditions, not the cancer itself — a warning against oversimplified headlines.
Early-onset colorectal and breast cancers showed distinct gut patterns, including higher lactate and Veillonella parvula in younger colorectal patients, and changes across 64 species in younger breast cancer patients.
Gut bacteria were linked to survival across several cancers.
A gut bacterium, Anaerostipes hadrus, that helps break down 5-FU was linked to protection against 5-FU diarrhea.
This does not mean bacteria cause these cancers, that a supplement can change your outcome, or that a predictive stool test is ready today. It does mean your gut is part of your cancer story — and that supportive care for your gut, hydration, and electrolytes deserves a real place in your treatment plan.
Frequently Asked Questions
What is the gut microbiome?
The gut microbiome is the community of trillions of microbes — mostly bacteria — that live in your digestive tract. They help digest food, support the immune system, protect the gut lining, and break down some medicines.
What did the Mayo Clinic microbiome study find?
Studying 1,364 cancer patients and 287 people without cancer, researchers found 341 bacterial species linked to five cancer groups after accounting for other health conditions. They also found gut patterns linked to survival and to side effects during treatment.
Which cancers were linked to gut bacteria patterns?
The clearest cancer-specific signals were in five groups: neuroendocrine tumors, liver and intrahepatic bile duct cancers, esophageal cancer, lymphoid leukemia, and multiple myeloma and related plasma cell cancers. Survival links were also found in colorectal, melanoma, ovarian, and prostate cancers.
Can gut bacteria really affect chemotherapy side effects?
In this study, yes — at least as a signal. Patients who later developed diarrhea on 5-FU had lower levels of bacterial genes that break down the drug, much of them from a bacterium called Anaerostipes hadrus. The same pattern was not seen with carboplatin, which suggests the link is specific to 5-FU.
What is DPD, and why does it matter for 5-FU?
DPD (dihydropyrimidine dehydrogenase) is an enzyme that breaks down 5-FU. People with very low DPD activity can become seriously ill from standard 5-FU doses, so testing may be discussed before treatment. This study found that some gut bacteria make their own version of DPD.
Does this mean probiotics can prevent chemo diarrhea?
No. This study did not test any probiotic or supplement, and none is proven to prevent 5-FU diarrhea. Do not start probiotics or supplements during chemotherapy without asking your oncology team — some can be unsafe when your immune system is low.
Is there a stool test that can predict my side effects now?
Not yet. The researchers describe this as proof of concept. The next steps are confirming the findings in larger groups and learning whether acting on these signals actually helps patients.
Why did the study say many microbiome changes were not really about cancer?
Because other health conditions — like diabetes or liver disease — also change gut bacteria, and many cancer patients have those conditions too. When the researchers accounted for them, many “shared” microbiome changes turned out to be driven by the other conditions. That is why the 341 species that remained are considered stronger signals.
What is early-onset cancer?
Early-onset cancer means a cancer diagnosed at age 50 or younger. Rates of early-onset colorectal and breast cancer are rising. In this study, younger patients had distinct gut patterns — higher lactate and Veillonella parvula in colorectal cancer, and changes across 64 species with lower bile acids in breast cancer.
What should I do if I have diarrhea during chemotherapy?
Call your care team early, especially if diarrhea lasts more than a day, you cannot keep fluids down, or you feel dizzy, weak, or have muscle cramps or a racing heart. Drink fluids as your team advises, and ask about checking electrolytes like potassium and magnesium. Do not wait for your next scheduled visit.
Sources
For more information on how HuMOLYTE can support your gut health during chemotherapy, visit our product page or consult your health care provider.
This blog was reviewed by Dr. Sourabh Kharait.
This blog is for educational purposes only and is not intended as medical advice. Always consult with your healthcare provider before making any changes to your treatment plan, hydration strategies, or diet. The information provided here is based on general insights and may not apply to individual circumstances.
