Your Microbiome

Laura PoleAuthor

Laura Pole, RN, MSN, OCNS, BCCT Senior Researcher

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Nancy Hepp, MS, BCCT Project Manager

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Last updated April 27, 2021.

under constructionMany trillions of microorganisms live in and on us, many of which are in our gut―in fact, non-human microbe cells in our bodies outnumber our human cells. Over thousands of years, our bodies have developed a symbiotic relationship with these organisms, generally supporting each other’s health and well-being. In exchange for food and lodging, these microorganisms:1

The connection between the microbiome and cancer is strong enough that the microbiome has been proposed as a screening tool for early stages of colorectal cancer.

  • Help with digestion and produce essential vitamins and minerals
  • Support intestinal wall integrity
  • Influence our sleep cycles and immune system
  • Signal to each other to affect functions
    • Metabolism
    • Inflammation system
    • Brain and nervous system
    • Immune system

Both direct and indirect effects of an out-of-balance microbiome can make our terrain (body) more hospitable to cancer:2

  • Hyperinsulinemia
  • Obesity
  • Inflammation
  • Excess estrogen
  • Compromised immune function
  • Depression

Highlighted Video 

BCCT advisor Brian Bouch, MD, explains the importance of the microbiome in integrative cancer care.

Some microorganisms may have a more direct effect on cancer. For example, finding certain microorganisms from the mouth in the lungs is associated with advanced stage lung cancer, worse prognosis and tumor spread. The microbes are thought to trigger an immune response.3

The connection between the microbiome and cancer is strong enough that the microbiome has been proposed as a screening tool for early stages of colorectal cancer.4 Early evidence shows that the pancreatic microbiome is substantially different in cancer patients and that antibiotic alteration of the pancreatic microbiome can slow disease progression in mice.5

Microbiome and Cancer Treatments

Your microbiota—your particular collection of microbes—can influence your response across several cancer therapies. Your gut microbiota is particularly influential:6

  • Your gut microbiota is a main player in your response and toxicity to immunotherapy treatments, including immune checkpoint blockade.
  • The condition of the gut microbiota is associated with differences in long-term survival following allogeneic stem cell transplant.
  • The condition of the gut microbiota can influence the toxicity of graft versus host disease (GVHD).
  • Your gut microbiota can shape your response to platinum-based chemotherapies and cyclophosphamide therapy.

Microbiome and Specific Cancer Types

Colorectal Cancer

Microbiome and Gut Surgery

Colorectal surgery is invasive and disrupts the microbial equilibrium in the gut. This disruption can result in reduced gut barrier function and local immune response, promote systemic inflammation, and potentially lead to postoperative infection.7

We know that lifestyle factors and the gut microbiome interact to influence the development and progression of colorectal cancer. We are not yet clear on exactly how this plays out in people or what we can do to manipulate the microbiome favorably. We know that diet influences the microbial community in the gut. Researchers think the interaction between diet and gut microbiota influences colorectal cancer development by changing your  metabolism and immune system.8 Evidence supports these assertions:

  • A high-fat diet is bad news for gut health, as it produces secondary bile acids. These acids change the microbiome, resulting in increased oxidation and inflammation that damage colon cells.9
  • Beneficial bacteria in the gut are needed to process and create essential nutrients through fermenting  dietary fiber and producing butyrate. These microbial processes provide energy to colon cells and promote protective immune system effects. Adequate dietary fiber is thus essential for a healthy interplay between the gut microbiome, colon cells and immunity,10 and lower levels of butyrate-producing bacteria are associated with the presence of colorectal cancer.11

Antibiotic Use and Colorectal Cancer

A matched, case-control study suggests that oral antibiotic use is associated with an increased risk of colon cancer—even with minimal use—but a reduced risk of rectal cancer. The increased risk was strongest with antibiotics with anti-anaerobic effects.12

“When asked about the difference between the apparent impact of antibiotic use on the risk of cancer in the colon when compared to the rectum, [senior author Cynthia] Sears commented, ‘We think these differences highlight the differences in biology and likely the microbiome between these two cancer sites. Hence we hypothesize that antibiotics impact disease at these sites differently.’”13

Impacts of a healthy microbiome with colorectal cancer include these:14

  • A healthy gut microbiome appears to support the anticancer action of the chemotherapy drug oxaliplatin.
  • Bacteria in the genus Bifidobacterium are crucial to optimizing the anticancer action of ligand 1 drugs (PD 1 checkpoint inhibitors), which activate the immune system to attack tumors.
  • Gut microbes can prevent reactivation of drug metabolites that can damage the intestines and cause diarrhea related to drugs such as camptothecin.
  • Microbial species can modulate inflammation,

People with colorectal cancer have less diversity of their gut bacteria, especially lactic acid bacteria such as Lactobacillus. Harmful species also tend to be present in colorectal cancer patients.15 For example, enterotoxigenic Bacteroides fragilis [ETBF], is associated with a greater number of early-stage carcinogenic lesions and increased risk of colorectal cancer.16

The connection between the microbiome and cancer is strong enough that the microbiome has been proposed as a screening tool for early stages of colorectal cancer.17

Probiotics, Prebiotics and Synbiotics

Probiotics are living microorganisms (bacteria and some yeasts) that, when consumed in sufficient numbers, can provide health benefits that go beyond basic nutrition, such as supporting gut and immune health and keeping the gut microbiota in balance. Examples of probiotic foods are yogurt, kefir, sauerkraut, tempeh and kimchi.

Prebiotics are dietary fibers that feed the friendly bacteria in your gut. Most prebiotics are soluble fiber substances like inulin, found in chicory root and foods like bananas, onions, jerusalem artichokes, jicama, garlic and others. Your helpful bacteria turn inulin and other fibers into energy for the colon cells and create protective immunity. Inulin is increasingly being added to a number of processed foods and probiotic supplements. 

Synbiotics simply means consuming both prebiotics and probiotics together.

Use of pre- and probiotics show these effects to date:

  • During chemotherapy and radiation treatment, probiotics may reduce treatment-related diarrhea.18
  • Probiotics may reduce irritable bowel symptoms, and they improved function-related quality of life and cancer-related quality of life scores.19
  • Some evidence shows that perioperative prebiotic or probiotic use may reduce postoperative infections and help maintain the intestinal mucosal barrier, but some studies have shown no effect.20

See also:

Ovarian Cancer

Some initial evidence shows that the microbiome in the cervix and vagina may relate to or indicate ovarian cancer risk. Lower levels of the protective bacteria lactobacillus in the microbiota in the cervix was associated with higher incidence of ovarian cancer or or BRCA1 mutation status, especially in younger women. The research is too preliminary to draw a causal relationship, and no evidence yet shows that restoring the lactobacillus microbiota in the cervix/vagina might lower the risk.21

Pancreatic Cancer

Early evidence shows that the pancreatic microbiome is substantially different in pancreatic cancer patients and that antibiotic alteration of the pancreatic microbiome can slow disease progression in mice.22

Influencing Your Microbiome

"Profound and intensive changes in dietary regimens can significantly alter the gut microbiota in a relatively short amount of time."23 Changing your diet can bring rapid and significant changes to your microbiome and your immune response. Increasing dietary fiber and eliminating animal fats are each well known to improve outcomes. Adding food enriched with plant polysaccharide inulin prebiotics, or adding inulin supplements to your diet may also improve the condition of your microbiome. However, much is still unknown about the influence of supplements on cancer outcomes. A study presented at the 2019 annual meeting of the American Association for Cancer Research found that melanoma patients taking over-the-counter probiotic supplements had a 70 percent lower chance of response to cancer immunotherapy treatment with anti-PD-1 checkpoint inhibitors.24  

Diet is important in supporting the health and balance of our microbiome. A Mediterranean diet provides substrates for your microorganisms to produce a number of products including short-chain fatty acids, which are beneficial to your health.25 Microorganisms in a person who eats a Western diet, however, produce end-products from fat and protein breakdown that are associated with arteriosclerosis and colon cancer.26

Changes in microbiota can happen quickly. Shifting from a Mediterranean diet to a standard Western diet alters the microbiota in one day, and a diet high in sugar decreases necessary microbial diversity within one week.27

Foods that nourish a healthy microbiome:

  • Fermented foods such as yogurt, kefir, miso, sauerkraut and kombucha may help replenish the gut with healthy microorganisms, although the evidence is mixed. Eating fermented foods is generally beneficial to health.28
  • Stronger evidence suggests that healthy microorganisms thrive on foods that are part of a healthy diet pattern, such as cruciferous vegetables (broccoli, cauliflower, kale and others) and beans.29
  • Eating walnuts has altered gut bacteria and suppressed colon tumors in mice.30


In her review article, Dr. Tina Kaczor concludes: “We are just beginning to define an optimal microbiome that will positively affect outcomes of chemotherapy or immunotherapies. Given the early data, it appears likely that commensal organisms [microbes] are integrally involved in a tumor’s response to many cancer treatments. In addition, bacterial diversity appears to be associated with better treatment response. . . A healthy omnivorous diet, such as the Mediterranean diet, along with exercise and proper sleep, may be the best, though less-than-precise, prescription to complement conventional therapies.”31

Written by Laura Pole, RN, MSN, OCNS; with review by Nancy Hepp, MS; most recent update on April 27, 2021.

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