Gut Health Affects Cancer Treatment
Gut health affects cancer treatment, not just "chemo brain", cognitive decline from chemotherapy. Chemo brain affects up to 75% of women receiving chemotherapy for breast cancer. Chemo brain can manifest in symptoms including difficulty thinking clearly, memory issues, lack of focus, difficulty finishing tasks, and/or challenges in learning. This has been shown to occur due to disruptions in the gut-microbiome-brain axis from dysbiosis, an imbalance in the diversity of gut microbes and, it is independent of chemotherapy-induced markers of inflammation.
Dysbiosis, particularly during cancer treatment, can cause harm beyond chemo brain, however.
Cancer is, of course, a complicated disease, usually with many contributors. Importantly, there is an association between the initiation of cancer and intestinal dysbiosis, and it is known that shifts in the microbiome and gastrointestinal tract health may be caused by the cancers themselves.
As this webpage will show, cancer treatment impacts the gut and its microbiome, and alarmingly, gut health affects cancer treatment effectiveness. Research shows that there is an intricate relationship between the gut microbiome and anticancer drugs via different mechanisms that are explored in this webpage.
Below is a sample of available research. It is summary of research on two common anticancer treatment drugs and their interactions with the gut microbiome.
Trastuzumab
Trastuzumab (Herceptin and others) is a monoclonal antibody used to treat primarily HER2-positive breast and stomach cancers. In a 2021 study on trastuzumab in HER2-positive breast cancer (BC), both preclinical models in mice bearing HER2-positive BC and clinical outcomes in 24 patients with primary HER2-positive BC were investigated. In mice, the antitumor activity of trastuzumab was decreased when the BC mice were given antibiotics or administered fecal microbiota transplantation (FMT) from antibiotic-treated mice. When fecal microbiota from responsive or non-responsive HER2-positive BC cancer patients were put into BC mice, the mice with the responsive FMT experienced the best response from trastuzumab.
In the patients, twenty patients received 4 cycles of adriamycin plus cyclophosphamide, followed by four to six cycles of taxane and trastuzumab as therapy, while four patients received only taxane plus trastuzumab. Stool samples were analyzed prior to trastuzumab therapy. Patients who were nonresponsive had lower gut alpha diversity, a measure of the different types and numbers of species in a sample, as well as lower abundance of several families of commonly commensal or probiotic bacteria. These bacteria included Lachnospiraceae, Turicibacteraceae, Bifidobacteriaceae, and Prevotellaceae.
These results demonstrated that the gut microbiota was directly involved in the efficacy of trastuzumab, and that the noted bacterial families are important for trastuzumab efficacy independent of tumor subtype. The researchers suggested that increasing dietary fiber to support the noted bacterial families may boost the immune-mediated trastuzumab activity.
Cyclophosphamide
Cyclophosphamide (Frindovyx, Procytox) is an alkylating agent that that inhibits protein synthesis through DNA and RNA crosslinking and is primarily used to treat breast cancer, lung cancer, leukemia, lymphoma, myeloma, ovarian cancer and sarcoma. It is also used for rheumatic conditions to suppress the immune system (Cytoxan).
In cancerous tumor-bearing mice, cyclophosphamide altered the microbial composition in the small intestine and permitted translocation of selected species of Gram-positive bacteria into secondary lymphoid tissues. In those tissues, the bacteria stimulated the immune system to produce potent molecules to fight cancer cells. In germ-free or antibiotic-treated tumor-bearing mice, however, the potent immune molecules were reduced and the tumors were resistant to cyclophosphamide. When the germ-free or antibiotic-treated tumor-bearing mice were given the potent immune molecules, the efficacy of cyclophosphamide was partially restored.
This study showed importantly how the body has inherent wisdom to know which bacteria to allow to translocate out of the intestine when the intestinal lining still has integrity, and also how a robust gut microbiota is required for cyclophosphamide to work. Again, gut health affects cancer treatment.
How Gut Health Affects Cancer Treatment
There are many pathways through which cancer treatments can cause dysbiosis. Dysbiosis can occur due to direct cytotoxic effects on gut and microbial cells, immune dysregulation, disruption of normal microbial metabolic processes, and/or alterations in gut lining integrity.
Obvious symptoms of dysbiosis are diarrhea, nausea, vomiting, painful ulcers, and other gastrointestinal distress. More subtle symptoms encompass a myriad of other side effects of anticancer treatments such as increased risk of infections, immune-related adverse effects, autoimmune disorders, and malabsorption of nutrients.
Some classes of cancer treatments and their influences on dysbiosis are presented below.
- Chemotherapy drugs such as cyclophosphamide, 5-fluorouracil (5-FU), and doxorubicin have direct cytotoxicity effects in that they kill both cancer cells and gut lining and gut microbe cells resulting in a loss of beneficial microbes and microbial diversity. 5-FU (Adrucil and others) is an antimetabolite drug that interrupts cancer cell division that is commonly used for colorectal cancer. Doxorubicin (Adriamycin) is an anthracycline that blocks topoisomerase 2 that cancer cells need to divide and grow.
- Cyclophosphamide and 5-FU also disrupt short-chain fatty acid (SCFA) production. SCFA play many beneficial roles in the body including modulating immune function and maintaining gut health.
- Immune modulators, such as the immune checkpoint inhibitors pembrolizumab and nivolumab, alter immune responses that can promote damaging inflammation resulting in increased gut inflammation and disruption of microbial balance.
- Antibiotics used to disrupt metabolism of microbes in cancers alter diversity and inhibit drug metabolism, contribute to drug resistance, and cause loss of beneficial bacteria.
- Platinum-based antioxidant activity interference agents such as cisplatin and carboplatin alter oxidative stress pathways promoting disturbances in microbial communities and decreased numbers of beneficial bacteria.
- Targeted therapies, such as tyrosine kinase inhibitors like imatinib, disrupt nutrient availability, which favors pathogenic or opportunistic bacteria to proliferate.
- Chemotherapy and radiation damage the gut lining causing increased gut permeability, translocation of microbes from the gut into systemic circulation, and proliferation of pathogenic microbes.
Cancer treatments then, in general, can reduce gut microbiome diversity and alter microbiome composition and function resulting in dysbiosis. This, in turn, influences drug metabolism, modulation of the immune response, and treatment side effects, which have the potential to affect patient treatment outcomes.
Dysbiosis affects cancer drug metabolism by disrupting gut microbial enzyme activity, which results in unpredictable outcomes. For example, some cancer drugs, like the previously-mentioned cyclophosphamide, require microbial activation for immune-enhancing effects.
For other cancer drugs, particularly with pathogenic/opportunistic E. coli and Gammaproteobacteria influences, dysbiosis can promote tumor resistance to anticancer drugs and increase tumor toxicity, as well as degrade active drugs like gemcitabine prematurely, thus reducing its effects against tumors. Dysbiosis may also affect detoxification of the cancer drugs resulting in drug accumulation or insufficient drug clearance, with the end result of drug toxicity and nasty side effects.
Absorption, distribution, metabolism, and excretion rates of cancer drugs in a particular individual also affect how much of the medication is available to that individual, and this also must be taken into consideration in addition to dysbiosis. However, these rates may all be affected by dysbiosis in addition to the functioning of specific organs such as the liver and kidneys. Oral medication efficacy, in particular, is dependent on an individual's gastrointestinal tract integrity and function, but even intravenous medication efficacy is influenced by the state of the gut microbiome.
Conclusions and Final Thoughts
The studies and other presented information in this article show that patient response to the aforementioned cancer drugs is influenced by the status of a patient's gut microbiome, and cancer therapies themselves can cause dysbiosis. Gut health affects cancer treatment, and vice versa.
One last thing that must be considered when discussing microbiomes is the cancerous tumor microenvironment or oncomicrobiome. This oncomicrobiome plays a role in tumor initiation, progression, and drug resistance.
Bacteria, fungi, viruses, archaea, and parasites may be part of this microenvironment, may be causes of or enablers of cancer, and may be surrounding and/or inside cancerous tumors. These invaders are thought to be spread from the gut and oral cavity, organs, and immune/lymphatic system to adjacent tissues or through circulation to targeted tissues.
For example, it has been shown that in a leaky gut, gut microbiota can translocate out of the gut into the bloodstream to pancreatic tumors and affect the immune system in favor of tumors.
There is not one standard tumor microenvironment signature per type of cancer, and in fact, microbial variation can occur in subtypes of the same type or stage of cancer. Usually, however, there are known predominant organisms that are enriched in certain types of cancers. Examples include Streptococcus and Veillonella in lung cancer, E. coli and S. epidermidis in breast cancer, and P. gingivalis, F. nucleatum and Malassezia fungi in pancreatic cancer. Some of these organisms may serve as both biomarkers and therapeutic targets.
Investigations into a new concept called Gut OncoMicrobiome Signatures (GOMS) are showing that certain gut microbial profiles may serve as biomarkers for predicting the efficacy of oncological treatments. They may also serve as both biomarkers and potential therapeutic targets. GOMS share commonalities across different cancer types and might be a promising, non-invasive, and financially-prudent approach for early detection and diagnosis of various cancers.
Interestingly, patients with cancer share gut microbiome signatures with patients diagnosed with seemingly unrelated disorders like cardiovascular or metabolic conditions. What they share in common, but that are NOT common in healthy people, is an imbalance between health-related and chronic-inflammatory disease-related gut bacteria.
Researchers are defining and refining these GOMS as they obtain more data. At the current time, GOMS appear to be promising predictors of resistance to immune-checkpoint inhibitors.
Cancer really is a complicated disease. However, gut health affects cancer treatment and will continue to play important roles in cancer prevention, diagnosis, and treatment. Nutrition is and will continue to be critical, also. Eating a healthy diet may prevent some cancers, and eating as nutritious a diet as you can handle during anticancer therapy may make a difference in your treatment outcome. Also, please remember the power of thoughts. Thoughts influence the physics and chemistry of our bodies in profound ways.
If you are undergoing cancer treatment, please work with your oncology team, particularly before starting probiotics. Different probiotics activate different aspects of the immune system, and you do not want to work against cancer treatments you are receiving. You may share the findings and references in this article with your oncology team, but always follow the advice of your team.
References:
- A.N. Mafe and D. Busselberg. "Microbiome Integrity Enhances the Efficacy and Safety of Anticancer Drug," Biomedicines 13.422 (2025).
- S. Viaud et al. "The intestinal microbiota modulates the anticancer immune effects of cyclophosphamide," Science 342.6161 (2013).
- M. Di Modica et al. "Gut Microbiota Condition the Therapeutic Efficacy of Trastuzumab in HER2-Positive Breast Cancer," Cancer Research 81.8 (2021).
- S. Fanijavadi and L.H. Jensen. "Dysbiosis–NK Cell Crosstalk in Pancreatic Cancer: Toward a Unified Biomarker Signature for Improved Clinical Outcomes," International Journal of Molecular Sciences 26.2 (2025).
- Y. Ma et al. "The oncomicrobiome: New insights into microorganisms in cancer," Microb Pathog 197 (2024).
- A.M. Thomas et al. "Gut OncoMicrobiome Signatures (GOMS) as next-generation biomarkers for cancer immunotherapy," Nat Rev Clin Oncol 20.9 (2023):583-603.
- S. Zang et al. "Tumor microbiome: roles in tumor initiation, progression, and therapy," Mol Biomed 6.9 (2025).
Return to Health Benefits page.
Return to Homepage.
I research studies and share my clinical experience to write this free site to help you find solutions to your problems. As part of that, I recommend products and services that I genuinely believe will be of help to you. If you click on a link to a product/service, I may receive a small commission to support my efforts if you buy something. The item does not cost you more.
Thanks for visiting this site! If you've enjoyed reading this page or have found the information to be useful to you, please "like", tweet about it, or share it so others can benefit, too. You can leave comments below via Facebook or Disqus.
Comment with Disqus (including as a guest), Twitter or Google accounts:
If you are one of my many readers without a Facebook account, you can still comment.
Disclaimer: Please note: By law, I cannot provide any personalized recommendations for your specific health concern on this site. The information contained in this site is educational in nature and is not intended as diagnosis, treatment, prescription or cure for any physical or mental disease, nor is it intended as a substitute for regular medical care. Consult with your doctor regarding any health or medical concerns you may have.
Subscribe to my monthly newsletter and receive a free copy of "How to Use Probiotics to Lose Weight and Be Healthier".
To comply with the EU's GDPR data privacy regulation, please subscribe here:
Some competitors of SBI (Solo Build It) are posting fake negative reviews of SBI. If you are considering creating your own website business, or if you have a brick-and-mortar business but want an online presence, I highly recommend SBI!
Comment with Facebook!
I'd love to hear your opinion about what you just read. Leave me a comment in the box below! Other commenting options follow the Facebook comments.