The gut microbiome refers to the microorganisms found in the intestines. They help break down food, support your immune system, and affect your hormones. Scientists are still learning all the ways these microbes impact your health, including cancer risk and treatment outcomes.
Recently, HealthTree Foundation hosted a webinar with Francesca Castro, a registered dietitian and researcher who specializes in cancer nutrition. In this article, we will recap the webinar and share additional information about recent research and the gut microbiome’s role in health, disease development, and therapeutic care.
The link between our diet, microbial diversity, and disease management
The body hosts numerous bacteria, fungi, viruses, and other microbes that exist on the skin, in the mouth, and throughout the digestive tract. When balanced, these organisms help break down nutrients, support the immune system, and lower inflammation.
A key factor in gut health is diversity, which means having many different types of helpful microbes. Think of your gut like a forest. A forest with many kinds of trees and plants stays stronger against disease than a field with only one crop.
On the other hand, dysbiosis happens when the gut loses this balance. Eating too much sugar, having high stress, avoiding exercise, taking certain drugs (like antibiotics), or eating low-fiber foods can cause this imbalance.
Why is gut health relevant in cancer?
Healthcare providers advise eating whole, nutrient-dense foods and skipping ultra-processed items to protect gut health. But why is this so important?
Your body turns the fiber you eat into healthy compounds called short-chain fatty acids (SCFAs), such as butyrate. If your gut is out of balance during cancer treatment, two main problems can happen:
Higher body-wide inflammation and a weaker immune system can make it harder for your body to respond to diseases or clear damaged cells.
Treatments may not work as well, as an unhealthy gut can change how your body processes drugs, increase side effects, and lower overall survival rates.
Evidence suggests that diet-driven changes in the gut can affect disease progression. When good bacteria feed on dietary fiber, they make short-chain fatty acids (like butyrate) that lower inflammation and prevent tumor growth.
Gut microbes and cancer development
The International Agency for Research on Cancer (IARC) identifies 11 organisms as direct causes of cancer, including Helicobacter pylori and human papillomaviruses (HPV). But there are many other microbes that may indirectly drive tumor formation. For example, some strains of E. coli produce a toxin called colibactin, which damages DNA and can lead to colon cancer. Another bacteria, Fusobacterium nucleatum, can speed up tumor growth in the colon and pancreas
In clinical pilot studies, such as the NUTRIVENTION trial, adopting a high-fiber, plant-forward diet significantly increased gut microbiome diversity and boosted butyrate-producing bacteria. Furthermore, preliminary preclinical models showed that high-fiber dietary interventions helped delay the progression from precursor conditions (like smoldering myeloma) to active cancer. Conversely, diets high in refined grains, sugary drinks, and ultra-processed foods can promote inflammation and suppress beneficial microbial activity.
Eating patterns and lifestyle habits for optimal gut health
Only 5% of American adults currently get enough daily fiber. But adding more plant foods can quickly improve your gut.
How to improve the gut microbiome
Eat a large variety of plants: Studies show that eating 30 or more different plant foods each week, including fruits, vegetables, whole grains, beans, lentils, nuts, and seeds, builds a much stronger, more diverse gut than eating 10 or fewer.
Incorporate fermented foods: Foods like sauerkraut, kimchi, plain yogurt, kefir, tempeh, and miso add live, beneficial bacteria (probiotics) that work with fiber to keep your gut lining healthy.
Microbes as diagnostic tools and treatment partners
Doctors are now using the microbiome to spot and treat cancer earlier. Because microbial patterns change during illness, simple non-invasive tests using saliva, stool, or blood plasma (liquid biopsies) are being created to catch cancer sooner.
Your gut microbes also affect how well cancer treatments work:
Chemotherapy: Helpful gut bacteria can boost how the immune system responds to drugs like cyclophosphamide. However, some bacteria inside tumors can break down drugs like gemcitabine and stop them from working.
Immunotherapy: Patients with a diverse, healthy gut may respond better to immunotherapy and CAR-T cell therapies. Doctors are even testing stool transplants (also known as a fecal microbiota transplant or FMT) to help patients respond better to these drugs.
Stem cell transplants: For patients getting stem cell transplants for blood cancers, a diverse gut rich in SCFA-producing bacteria helps protect against dangerous graft-versus-host disease (GvHD) and improves survival.
More evidence-based guidelines are needed
Current cancer prevention recommendations highlight filling at least two-thirds of your plate with plant-based foods while limiting red and processed meats, sugar-sweetened beverages, and ultra-processed foods. The American Institute for Cancer Research recommends aiming for at least 30 grams of dietary fiber daily to maintain optimal gut and metabolic function.
Francesca Castro addresses that health recommendations should be based on objective, unbiased, and evidence-based science. Any dietary transition should be personalized, prioritizing sustainable, step-by-step progress over strict perfection.
"My advice is always progress not perfection. Picking one and building upon those things."
-Francesca Castro
When in doubt, talk with your care team about gut health
Dietary choices are a personal path, but for people living with or recovering from cancer, it's important to understand potential digestive side effects and therapeutic interactions. Discussing your nutrition, gut health, and antibiotic use with your healthcare team or a registered dietitian can help you make safe, informed choices tailored to your specific medical needs.
Watch the recording of the community event Impact of GI Microbiome on Cancer Progression.
Keep track of your health with a Personal Health Record
To learn more about lifestyle and nutrition guidance for blood cancer patients, visit the HealthTree website and stay tuned for future updates on evidence-based cancer care and prevention.
Source:
The Human Microbiome and Cancer: A Diagnostic and Therapeutic Perspective (Cancer Biology & Therapy)
The role of the gut microbiota in chemotherapy response, efficacy and toxicity: a systematic review
