Breast Cancer and the Microbiome - How taking care of your gut bacteria may help prevent breast cancer recurrence

What is the microbiome?

‍The microbiome is the community of microorganisms—mainly bacteria, but also viruses and fungi—that live in and on our bodies. Microbiomes exist on the skin, in the mouth, along the respiratory tract, and especially in the intestinal tract, where trillions of organisms play essential roles in digestion, immunity, and metabolism.In the gut, microbes help break down plant fibers and other foods that our own enzymes cannot digest, influencing how we metabolize fats, carbohydrates, and nutrients. The gut microbiome also communicates with other body sites; for example, gut health has been shown to affect skin conditions

.Dysbiosis: When balance is lost.

‍Dysbiosis is an imbalance of the microbiome—where diversity, abundance, or function of microbes is disrupted. People with dysbiosis may not feel sick, but can develop symptoms such as bloating, diarrhea, heartburn, or early satiety after meals.Common causes include:Overuse of antibiotics

Diets high in processed foods, sugar, and unhealthy fats

Alcohol

Stress and environmental toxins

Diagnosis can be made with stool DNA sequencing tests or measurements of metabolites like short-chain fatty acids (e.g., butyrate). Research using sequencing and metagenomic profiling has shown how profoundly the gut microbiome affects health and disease balance.

‍How gut bacteria connect to breast cancer

‍One of the most important roles of the gut microbiome is regulating estrogen. Certain gut bacteria produce enzymes, such as beta-glucuronidases, that increase free estrogen levels in the body.Since most breast cancers are hormone-driven, high estrogen exposure—particularly in postmenopausal women—has been linked to increased risk. Ratios of estrogen metabolites to parent estrogens also correlate with breast cancer risk.Studies of breast tissue microbiota show that tumor tissues harbor different bacterial communities compared to healthy tissue. For example, healthy women often have higher levels of Prevotella, Lactococcus, and Streptococcus, while breast cancer tissues are enriched with Bacillus, Staphylococcus, and Enterobacteriaceae.

‍Microbiome and cancer treatment

‍Research suggests that the gut microbiome influences:

‍Chemotherapy response: Higher microbial diversity has been associated with better outcomes in solid tumors, including breast cancer.

Treatment side effects: Certain bacteria are linked to neurological side effects and tolerance of chemotherapy.

Disease staging: Differences in microbiome composition correlate with tumor grade and lymph node involvement.

In small breast cancer studies, patients with high microbial diversity responded better to chemotherapy than those with lower diversity.

Can dysbiosis be treated?

‍The short answer: yes. But it takes patience. Diet change is key. Switching suddenly to a high-fiber, plant-rich diet may cause bloating or diarrhea because your gut needs time to adapt. Introduce healthy foods gradually.

Don’t toss every treat. Occasional favorite foods can help with stress reduction and motivation. Think of them as rewards, not staples.

Aim for slow and steady change. Consistency is what allows good bacteria to colonize and thrive.

Adopt a Mediterranean-style diet

Emphasizes vegetables, fruits, legumes, whole grains, and fish.

Minimizes processed foods and added sugars.

Has been shown to increase Lactobacillus and reduce inflammatory bacteria like Ruminococcus.

Add fermented foods

Yogurt, kefir, sauerkraut, kimchi, miso, kombucha.

Provide live beneficial bacteria.

Limit alcohol and sugar

Both negatively affect gut flora.

Consider probiotics when appropriate

May be useful after antibiotics, illness, or surgery.

If your gut is already well balanced, supplements may add little benefit.

Some strains show promise in cancer research (Lactobacillus casei Shirota, Bifidobacterium longum, Akkermansia muciniphila), though evidence in breast cancer is still emerging.

Eat prebiotic foods

Non-digestible fibers that feed good bacteria.

Examples: garlic, onions, bananas, artichokes, leeks, oats, mushrooms, berries, green tea, dark chocolate.

Final thoughts

‍Human diets have transformed our microbiome across millennia. Modern processed foods brought convenience but reduced microbial diversity. Rebuilding gut health requires returning to fresh, unprocessed foods—especially fruits, vegetables, and lean proteins.Caring for your microbiome won’t just support digestion and immunity—it may also be an important ally in reducing the risk of breast cancer recurrence.