The Cancer-Microbiome Link

How Gut Health Shapes Cancer Risk and Treatment Outcomes

When I think about cancer, I think about genetics, tumor biology, and the immune system. That is what I was trained to focus on, and for most of my career, that is exactly what I did. But over time, I have realized there is another major factor that deserves more attention—the trillions of microbes living in our gut.

The microbiome plays a direct role in inflammation, immune function, and metabolism. These three things influence not only how cancer develops but also how we prevent and treat it. The more I learn, the more I see just how much we have been overlooking.

I see it in my patients. Some respond incredibly well to treatment, while others who should have had the same chance do not. Some people develop cancer despite having no obvious risk factors, while others avoid it even with a strong family history. It makes me wonder how much of this comes down to what is happening inside the gut. Could the microbiome be one of the missing links in understanding cancer risk and outcomes?

We are just starting to grasp how diet, lifestyle, and even medications shape the microbiome in ways that could tip the scales toward or away from cancer. And while there is still a lot to learn, one thing is clear. The microbiome is not just a background player. It is part of the bigger picture, and it is time we start paying closer attention.

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Photo by julien Tromeur

The Microbiome and Cancer Risk

When it is in balance, the microbiome supports the immune system, maintains gut health, regulates inflammation, and processes food into beneficial compounds. But when this balance is disrupted, a condition known as dysbiosis, it can fuel chronic inflammation, cause DNA damage, and weaken immune defenses, increasing the risk of cancer.

We see this most clearly in cancers linked to specific bacterial infections:

• Helicobacter pylori and gastric cancer¹

• Fusobacterium nucleatum and colorectal cancer²

• Porphyromonas gingivalis and oral and pancreatic cancers³

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Leaky Gut, Microbiome, and Cancer

I often see "leaky gut" mentioned in wellness articles as a catch-all explanation for a wide range of health issues, from fatigue and bloating to autoimmune disease and even cancer. While the term is sometimes used loosely, the underlying concept of intestinal permeability is real and well-studied in immunology and oncology.

The gut lining serves as a critical barrier, allowing nutrients to be absorbed while keeping harmful bacterial byproducts, like lipopolysaccharides, from entering the bloodstream. When this barrier is compromised, the immune system responds with inflammation, which, over time, can become chronic and contribute to disease.⁴

This does not mean that gut permeability directly causes cancer, but it does create an inflammatory environment that can fuel tumor growth. This is particularly relevant in cancers of the liver, pancreas, and colon, where chronic inflammation is a known risk factor. As we learn more about how gut health influences cancer risk, it is becoming clear that maintaining a strong gut barrier and a balanced microbiome plays a vital role in long-term health.

If the microbiome is so influential in cancer, can we modify it to reduce risk or improve treatment outcomes? Evidence suggests we can.

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The Microbiome and Cancer Treatment

One of the most groundbreaking discoveries in recent years is the microbiome’s influence on cancer therapy response, particularly immunotherapy.

Checkpoint inhibitors, such as PD-1 inhibitors like nivolumab and pembrolizumab, have revolutionized cancer care but do not work for everyone. Recent studies suggest that gut microbiome composition predicts which patients will respond to these therapies.

A landmark 2018 study found that patients with a higher abundance of Akkermansia muciniphila, a beneficial gut bacterium, had significantly better responses to PD-1 inhibitors.⁵ Other research suggests that patients with more diverse microbiomes fare better with immunotherapy.

This has led to experimental fecal microbiota transplants (FMT) in melanoma patients, where gut bacteria from responders are transferred to non-responders. Early trials suggest that this approach can enhance responses to PD-1 inhibitors in patients who were previously resistant, paving the way for a new frontier in precision medicine.

Diet, Antibiotics, and Lifestyle Impact

Photo by Jez Timms

If the microbiome is so influential in cancer, can we modify it to reduce risk or improve treatment outcomes? Evidence suggests we can.

Diet and the Microbiome

• High-fiber, plant-based diets promote the growth of beneficial bacteria that produce short-chain fatty acids, particularly butyrate, which has anti-inflammatory and anti-cancer properties

• Processed foods and red meat increase levels of harmful metabolites, including secondary bile acids that promote colorectal and liver cancer

• Fermented foods, such as yogurt, kimchi, and sauerkraut, increase microbial diversity and may reduce inflammation

Antibiotics and Cancer Risk

• While antibiotics are essential for treating infections, their overuse can disrupt microbiome diversity and has been linked to an increased risk of colorectal and lung cancer

• Some studies suggest recent antibiotic use may reduce response rates to immunotherapy by depleting gut bacteria that influence immune function

Exercise and Microbiome Health

We’ve already discussed how exercise can influence cancer risk. Emerging research now shows that regular exercise increases gut microbial diversity, enhances short-chain fatty acid production, and improves immune function.⁶ This could explain why physically active individuals tend to have lower cancer risk and better treatment outcomes.

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The Microbiome and Early Cancer Detection

One of the most exciting applications of microbiome research is in early cancer detection. Stool-based tests, similar to Cologuard, can be used detect microbial DNA and inflammatory markers associated with colorectal cancer. We expect some of these to be commercially available soon.

New research is taking this further.

• Microbial-based liquid biopsies are being explored as a potential tool for detecting cancer signals in the blood

• AI-driven microbial sequencing may help identify at-risk individuals before tumors develop

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Can We Engineer the Microbiome for Cancer Therapy?

The goal is not just to observe microbial influences but to harness them for treatment. In the future, we may see:

• Microbiome-based probiotics tailored to enhance immune responses

• Fecal microbiota transplants or engineered bacteria used as adjuncts to immunotherapy

• Personalized microbiome profiling to guide cancer treatment decisions

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Five Common Questions on Microbiome and Cancer Wellness

1) Does eating yogurt reduce my cancer risk?

It might help, but the impact depends on the type of yogurt and your overall diet. Yogurt contains probiotics that support gut health and may help reduce inflammation. Some studies suggest that eating yogurt regularly is associated with a slightly lower risk of colorectal cancer, but the effect is small compared to the benefits of a well-rounded diet. I often tell my patients to include yogurt a few times a week as part of a balanced approach to nutrition. I recommend choosing plain, unsweetened varieties with live active cultures over sugary or artificially flavored options. Adding fresh fruit or nuts is a great way to boost its nutritional value.

Recent study: Long-term yogurt intake and colorectal cancer incidence subclassified by Bifidobacterium abundance in tumor

2) Should I take a probiotic daily or when I get sick?

I get asked about probiotic supplements all the time, and my answer is almost always the same. Focus on food first. A diet rich in fermented foods like yogurt, kefir, sauerkraut, and kimchi is a much better way to support your microbiome than taking a daily probiotic pill.

That said, there are times when supplements make sense. If you have antibiotic-associated diarrhea, irritable bowel syndrome, or recurrent infections, certain probiotics can be helpful. But here is the catch. Most commercial probiotic supplements do not contain the strains that have been studied for real health benefits. A lot of what is on the market is unregulated, with little evidence that they actually do anything.

If you are considering a probiotic supplement, look for one with well-researched strains like Lactobacillus rhamnosus GG or Bifidobacterium longum, and make sure it has enough live bacteria to make an impact. But for most people, sticking with a fiber-rich diet and regularly eating fermented foods is the best and simplest way to support gut health.

Comprehensive Review of Probiotics in Cancer (Front. Oncol. 11 March 2021)

3) What foods are best for my microbiome?

• High-fiber foods: Beans, lentils, oats, and vegetables

• Fermented foods: Yogurt, kefir, sauerkraut, kimchi, miso

• Polyphenol-rich foods: Berries, dark chocolate, green tea

• Prebiotic foods: Garlic, onions, leeks, bananas, asparagus

4) Are there any signs of an unhealthy microbiome?

Symptoms of a disrupted microbiome can vary but may include:

• Chronic bloating, gas, or abdominal discomfort – Often a sign of altered gut bacteria or poor digestion of certain foods

• Irregular bowel movements – Persistent diarrhea, constipation, or alternating between the two may indicate dysbiosis

• Frequent acid reflux or heartburn – An altered microbiome can influence stomach acid regulation

• Food intolerances – Sensitivities to foods that were previously well-tolerated can indicate microbiome disturbances

• Unexplained nausea – An unhealthy gut can affect digestion and motility

  *These symptoms alone do not confirm microbiome issues, but persistent gut dysfunction could be a sign that your microbiome needs support. It’s best to discuss any persistent symptoms with your doctor.

5) Should I use a kit to monitor my microbiome health?

I understand the curiosity around microbiome testing kits. I have even looked into them myself. The idea of getting a detailed report on your gut bacteria sounds fascinating, but the reality is that these tests are still more of a novelty than a tool that can truly guide your health.

Most commercial microbiome tests can tell you which bacteria are present in your gut, but they do not provide much useful or actionable information. Just because your microbiome is different from someone else’s does not necessarily mean anything is wrong, and these tests rarely offer insights beyond what we already know. Eating a fiber-rich diet with plenty of whole foods is the best way to support gut health.

If you are curious and want to try one for fun, that is fine, but I would not rely on it to make big health decisions. Instead, focus on the things we know work. A diverse diet, regular exercise, good sleep, and avoiding unnecessary antibiotics will do more for your microbiome than any test can tell you.

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Final Thoughts

We are only scratching the surface of how the microbiome can be leveraged for cancer prevention, detection, and therapy. One thing is clear. Understanding and modulating the microbiome will be an essential part of the future of oncology.

If you or someone you know is undergoing cancer treatment, it may be worth asking whether any microbiome-related trials are available. Research centers are increasingly integrating microbiome studies into standard oncology trials, meaning the data collected today could shape the future of cancer care.

For those interested in staying up to date, organizations like the International Cancer Microbiome Consortium (ICMC) and clinical trial databases like ClinicalTrials.gov provide information on ongoing studies in this space.

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