The fallout from cancer and its treatment interferes with so many aspects of our lives, with nausea and intestinal motility issues (either too fast or too slow) all too common. And, while we feel these side effects in our guts, because of the strong gut-brain connection, those disturbances also lead to changes in how we feel emotionally, and often linger long past the urge to sip ginger tea.
We’ve long known that there is a strong connection between the gut and the brain. Gut decisions and gut reactions are taking advantage of that connection, as are “butterflies” and the general overactivity in the gut we often feel when under stress. But what exactly is that connection? And how is the developing understanding of the microbiome influencing our thinking about the connection between cancer, its treatment, and the gut? Should we be saying “You feel what you eat” instead of “You are what you eat?” There is a lot to unpack, but the bottom line seems to be, eat well to feel well, physically and emotionally.
The Gut-Brain Connection
There are two ways that the gut and the brain interact. The first is through the network of nerves that connects the two, including 100 million neurons lining the walls of the gut and the vagus nerve, which starts in the brain and ends in the gut, allowing direct communication between the two. Minute changes in the gut are instantly transmitted to the brain creating a complex dialogue to control motility, the mix of digestive acids, feelings of satiety, etc. Given the density of that neural network in the gut, it does seem that the body knows and remembers things at the gut level before those signals actually get processed in the brain.
The second and more complex connection between the gut and the brain is the microbiome: that mass of single-celled organisms that line the gut and play such an important and interactive role in our lives, influencing how our bodies function and how we feel, both physically and, it turns out, emotionally.
Those little critters that make up the microbiome do everything from helping us digest the food we eat and extracting vitamins and nutrients from it, to modulating the immune system, metabolism, and brain function. They even produce and store 95% of the serotonin in our bodies, along with a host of other neurotransmitters that regulate mood and emotions. In fact, recent studies show that changes in the microbiome translate into changes in brain chemistry, influencing anxiety, the perception of pain and response to stress. At the same time, stress influences the makeup of the microbiome, altering the concentration of good and bad bacteria in a giant feedback loop that doesn’t always work to our advantage.
The Cancer-Gut Connection
Our microbiomes, like our fingerprints, are unique—even newborn twins can have differences—but what we eat contributes to the diversity and concentration of various bacteria living in our guts, as does the environment in which we live, aging and a host of other factors, including cancer.
Because the microbiome interacts with the immune system, it contributes to our ability to fight cancer, and plays a role in how we respond to certain cancer treatments. At the same time, the presence of cancer and certain treatments—including antibiotics, surgery, chemotherapy, radiation and immunotherapy—appear to alter the gut microbiome. These changes affect homeostasis, brain function and mood, and can influence the type and severity of adverse effects associated with treatment. Given the role the microbiome plays in mood, it’s no surprise that these cancer-related changes to the gut, including the intestinal distress so many feel after surgery or with chemo and radiation, can contribute to cancer-related changes in our emotional health too.
Eat Your Vegetables!
While the science is still evolving, several recent studies have drawn a connection between eating a Mediterranean style diet—one high in fish, fruits, vegetables, whole grains, nuts and legumes—with an elevation in mood or decrease in depression.[3, 4] It seems this type of diet provides the right mix of foods to support a balanced microbiome and feed the brain a positive mix of neurotransmitters. Indeed, this finding is consistent with other studies suggesting a diet rich in omega-3s, leafy greens and colorful fruits and vegetables is an “anti-cancer” diet.
But there is no magic bullet. Not only do we all have very different microbiomes that are differently affected by diet, cancer and treatment, we don’t yet know enough about the microbiome to be able to determine what is an optimal balance of organisms for a particular individual or prescribe an exact diet or supplement regimen to achieve that or maintain it during cancer treatment. For this reason, while some studies suggest probiotics and prebiotics can be helpful in regulating a healthy microbiome because of their ability to influence the effectiveness of some cancer treatments, talk to your doctor before taking any supplements, especially if you are still in treatment.
But there are some things you can do, if you are well enough. You can snack on nuts and seeds instead of chips and crackers. You can add leafy greens to your diet, sautéed or in a salad or added to soups or smoothies. And you can make your diet colorful by eating berries and peppers and yams and beets. You can add in fermented foods, such as yogurt and kimchi. You also can try to get more of your protein from seafood, add herbs and spices to your diet, and eat whole grains and legumes. All of these help feed the diversity of your microbiome. You can even add dark chocolate to your daily routine, which has been associated with a reduced risk of depression. And, there are high and low-calorie preparations of these foods (think creamed spinach vs steamed spinach), as appropriate to your recovery needs.
There is no overnight cure for the emotional disruption of cancer and its treatment, but making healthy changes to your diet can help you feel better, in your brain and in your gut.
 Bravo JA, Forsythe P, Chew MV, et al. Ingestion of Lactobacillus strain regulates emotional behavior and central GABA receptor expression in a mouse via the vagus nerve. Proc Natl Acad Sci U S A. 2011;108(38):16050-16055. doi:10.1073/pnas.1102999108
 Ma W, Mao Q, Xia W, Dong G, Yu C, Jiang F. Gut Microbiota Shapes the Efficiency of Cancer Therapy. Front Microbiol. 2019;10:1050. Published 2019 Jun 25. doi:10.3389/fmicb.2019.01050
 Firth, J., Marx, W., Dash, S., Carney, R., Teasdale, S. B., Solmi, M., Stubbs, B., Schuch, F. B., Carvalho, A. F., Jacka, F., & Sarris, J. (2019). The Effects of Dietary Improvement on Symptoms of Depression and Anxiety: A Meta-Analysis of Randomized Controlled Trials. Psychosomatic medicine, 81(3), 265–280. doi.org/10.1097/PSY.0000000000000673
 Sánchez-Villegas A, Delgado-Rodríguez M, Alonso A, et al. Association of the Mediterranean Dietary Pattern With the Incidence of Depression: The Seguimiento Universidad de Navarra/University of Navarra Follow-up (SUN) Cohort. Arch Gen Psychiatry. 2009;66(10):1090–1098. doi:10.1001/archgenpsychiatry.2009.129