“I have been eating yoghurt every day since my diagnosis because someone told me it helps. Does it actually make a difference?”
“My son finished leukaemia treatment last year, but his stomach has never quite been the same. Could chemotherapy have changed something?”
These are the kinds of questions patients ask quietly in clinic, often almost as an afterthought. For a long time, most haematologists would not have had much to say. We were trained to focus on the bone marrow, the blood counts, the biopsy, and the chemotherapy plan.
That is changing.
In recent years, researchers have been paying much closer attention to the gut microbiome, the enormous community of bacteria and other microorganisms that live in our intestines. What once felt like a side topic in gastroenterology is now becoming part of a much bigger conversation in cancer care, including haematology. The idea is not that gut bacteria cause or cure blood cancer in any simple way. It is that they may influence inflammation, immune balance, treatment tolerance, and perhaps even response to certain therapies.
What is the gut microbiome?
The gut microbiome refers to the trillions of microorganisms living in the digestive tract. They help break down food, produce useful compounds, support the gut barrier, and interact constantly with the immune system. When this ecosystem is balanced and diverse, it tends to support gut health and immune stability. When it becomes disrupted, a state often called dysbiosis, the effects may extend far beyond the intestine.
This matters in blood cancers because the immune system is deeply involved in both the disease itself and the treatments we use. A disturbed microbiome may contribute to more inflammation, weaker barrier protection in the gut, and less favourable immune signalling. That does not mean every digestive symptom is a microbiome problem, and it does not mean we can reduce cancer to gut bacteria. But it does mean the gut may be part of the bigger picture more often than we once appreciated.
What happens during treatment?
Chemotherapy does not act only on cancer cells. It can also damage the lining of the gut and reduce the diversity of beneficial bacteria. Patients often experience this in very real ways: nausea, diarrhoea, bloating, mucositis, poor appetite, and infections during vulnerable periods of treatment. Broad-spectrum antibiotics, which are sometimes necessary and life-saving, can add another layer of microbiome disruption.
There is also growing interest in the reverse question: does the microbiome influence how treatment works? Early evidence suggests that it might. In some studies, patients with more diverse gut microbiota have had better outcomes or fewer complications, particularly in transplant settings. This is still an evolving area, but it is one of the reasons the field has moved from curiosity to serious research.
Why is this especially important in stem cell transplantation?
If there is one area where the microbiome has become especially relevant, it is allogeneic stem cell transplantation.
After transplant, the gut is one of the organs most vulnerable to injury. It is also one of the main sites involved in graft-versus-host disease, or GVHD, where donor immune cells attack the recipient’s tissues. Several studies have shown that lower gut microbiome diversity around the time of transplant is associated with poorer survival and a higher risk of transplant-related complications. Low-diversity patterns can also become dominated by bacteria such as Enterococcus, which is generally not a good sign in this setting.
This does not mean a stool test can yet tell us everything, but it does mean the microbiome is no longer an abstract idea in transplant medicine. It is increasingly being studied as a real clinical factor.
What about faecal microbiota transplantation?
One of the most talked-about developments in this field is faecal microbiota transplantation, often shortened to FMT.
The concept is simple, even if the name makes people pause for dramatic effect. A carefully screened sample of healthy donor microbiota is introduced into the patient’s gut in an attempt to restore microbial balance. In stem cell transplantation, this is being studied as a way to rebuild diversity after heavy treatment and possibly reduce complications such as GVHD.
An important 2025 study from Fred Hutch tested oral FMT early after allogeneic transplant and found that it was able to restore microbial diversity, but with an important twist: donor choice mattered. One donor achieved a mean species-level engraftment rate of 67%, and patients whose microbiomes were more depleted before treatment seemed to engraft donor microbes more successfully. That is a fascinating finding, because it suggests FMT may not be a one-size-fits-all intervention. The donor may matter. The recipient’s starting point may matter. Timing may matter too.
There are also encouraging reports in patients with difficult gut GVHD. Some early studies suggest that selected patients with steroid-refractory gastrointestinal GVHD may improve after FMT. Still, this remains a specialised and developing area, not something patients should see as routine standard care yet.
Does the microbiome matter for CAR-T therapy too?
Possibly yes, and this is where the future becomes especially interesting.
Research in CAR-T therapy suggests that gut microbiome composition may correlate with both treatment response and the risk of complications such as cytokine release syndrome. This is still early science, but it raises an important possibility: in the future, optimising the microbiome may become part of preparing some patients for high-intensity or immune-based therapies.
Nobody should read this and think, “I just need the right yoghurt before CAR-T.” Sadly, the microbiome is not that easy to negotiate with. But the broader message is important. We may be moving toward a model of care that supports not only the cancer treatment itself, but also the biological environment in which that treatment happens.
So what can patients do now?
This is the part where it is important to stay practical and not drift into internet magic territory.
A few reasonable takeaways stand out:
- Eat as well as you realistically can. During treatment, appetite and digestion are not always cooperative. But when possible, a varied diet with fibre-rich foods can help support microbial diversity. This is not a cure, but it is part of caring for the body as a whole.
- Do not self-prescribe probiotics without asking your team. Some data suggest possible benefit in selected settings, but probiotics are not universally risk-free in immunocompromised patients.
- Talk to your doctors about antibiotics when they are needed. In many situations, antibiotics are essential and should never be avoided out of fear of “hurting the microbiome”. But in transplant medicine, researchers are actively studying whether antibiotic choice and duration can influence gut injury and GVHD risk.
- Do not try FMT outside a proper medical setting. This is not a home experiment. In cancer care, FMT belongs in regulated clinical environments with careful donor screening and specialist oversight.
Where is this field heading?
The most exciting part of this story is not that we have all the answers. We do not.
The exciting part is that haematology is beginning to look beyond the tumour itself and pay attention to the biological environment in which treatment happens. The microbiome may become part of a more personalised model of care, one that considers not just the disease, but also the immune system, the gut, treatment tolerance, and recovery. That is where this field has real room to grow.
What I want you to remember
Your gut is not separate from the rest of your cancer journey.
It is connected to your immune system, affected by treatment, and increasingly recognised as part of the conversation in blood cancers. That does not mean every patient needs supplements, probiotics, or special testing tomorrow morning. It means we are learning that good cancer care may one day involve protecting and understanding the body’s internal ecosystem more carefully than we have in the past.
For now, the microbiome is not a miracle. But it may become an important part of the future of haematology.
And that future, quietly, has already begun.
Disclaimer
This article is for general education only and does not replace medical advice, diagnosis, or treatment from your own clinician. Microbiome research in blood cancers is evolving quickly, and many of the approaches discussed are still being studied in clinical trials. Any decisions about diet, probiotics, antibiotics, or experimental therapies should be made with your haematology or oncology team.
List of References:
- Dumitru IG, et al. Impact of the Gut Microbiome on Cancer Treatment Efficacy in Hematological Malignancies. J Clin Med. 2025;14(9):2982.
- Reddi S, et al. Fecal microbiota transplantation to prevent acute GVHD: interim analysis of donor effect. Nat Commun. 2025;16(1):1034.
- Li M, et al. FMT for refractory acute GI-GVHD after allo-HSCT: five-year retrospective study. Cytotherapy. 2025;28(4):102043.
- Zhang M, et al. Gut microbiota shapes cancer immunotherapy responses. npj Biofilms Microbiomes. 2025;11:86.
- Gargiulo A, et al. Advancing Dysbiosis Treatment in Onco-Hematology with Microbiome-Based Therapeutics. Microorganisms. 2024;12(11):2256.
- van Lier YF, et al. Donor FMT ameliorates intestinal GVHD in allo-HCT recipients. Sci Transl Med. 2020;12(556):eaaz8926.
- Teng H, et al. Gut microbiota as a new target for anticancer therapy. npj Biofilms Microbiomes. 2025;11:23.