Part II. Microbiome and Mental Health: The Gut – Brain Axis

By Theophila Lee

This is the second of a three-part installment on the microbiome on the microbiome and mental health. For an introductory overview, see here , and for a discussion on the microbiome and maintaining skin health, see here.

Have you ever had a gut feeling? Felt butterflies in your stomach when nervous? Gotten sick to your stomach when stressed? A growing body of research is showing that our gut and our brain are in fact inextricably linked. This communication occurs via the gut- brain axis, a bidirectional pathway that links the cognitive and emotional centers of the brain with the intestinal functions of the gut. Furthermore, research is increasingly demonstrating that maintaining a healthy, diverse environment for your gut bacteria has wide ranging implications on your mental health.  

Bacteria and Behavior: Neurotransmitters in your gut 

What do eating that chocolate bar you’ve craved or experiencing a winning gambling streak have in common? They release dopamine, the “feel-good” neurotransmitter that contributes to pleasure and satisfaction in our reward system. Similarly, eating foods with high levels of tryptophan or sweet and starchy carbohydrates boosts serotonin, the “contentment” neurotransmitter that curbs cravings, suppresses appetite, and leaves you feeling satisfied and full.  Close to 60% and 90% of all your dopamine and serotonin respectively is produced in your gut, and microbes regulate levels of these mood-regulating neurotransmitter production.

Because of this gut-brain axis, neurotransmitter levels in the gut and brain mirror one another. A low level of neurotransmitters in the gut leads to constipation and indigestion, while a low level in the brain can manifest in depression. Conversely, an abundance of neurotransmitters in the gut leads to cramping and diarrhea, while an abundance in the brain can manifest in anxiety

Microbiome and mental health: Research experiments  

Researchers have used mice models to investigate the link between the microbiome and mental health in several fascinating research studies.  They found that stressed mice, such as those that had an aggressive cage mate or restricted access to food and water, had elevated levels of unhealthy bacterial strains and depressed levels of healthy bacterial strains in their gut. Gut bacteria from these stressed mice were transplanted into another strain of calmer mice, and, as a result, these calmer animals appeared to become more anxious. Conversely, mice injected with healthy bacterial strains were shown to have reduced levels of stress-related hormones and increased perseverance when subjected to a series of stressful situations, including a test measuring how long they continued to swim in a tank of water. These mice also had higher levels of a protein associated with learning, memory, and higher order thinking and exhibited more curiosity in exploring their surroundings. Researchers hypothesized that these healthy bacterial strains helped mice to more effectively manage and regulate their stress levels. Several studies in human have also demonstrated the impact of microbes on brain chemistry. A study of 77 toddlers found that children with the most diverse types of gut bacteria more frequently exhibited behaviors associated with curiosity, sociability, positive mood, and impulsivity. Toddlers with less diverse microbes exhibited more fear and self-restraint. In another study, 25 healthy women were recruited for four weeks; 12 ate a cup of commercially available Greek yogurt containing strains of four probiotic live bacteria, while the rest didn’t. Participants were given brain scans to gauge their response to a series of facial expression images- happiness, sadness, anger, etc. The results were stark – the yogurt eaters reacted more calmly to the images than did the control group. Researchers hypothesized that probiotics in the yogurt altered the subject’s gut microbiome, which modified production levels of mood-regulating neurotransmitters.  

Mealtime and mental health: Importance of food 

We saw in the this article how important eating a wide range of plant-based, probiotic, and prebiotic foods were to maintaining a healthy microbiome (among others). Given the microbiome’s impact on mental health, diet can play a role for those struggling with depression or anxiety. 

Consider breakfast, the most important meal of the day. For individuals with anxiety (i.e., high levels of neurotransmitters in the brain, also associated with cramping and diarrhea), a high protein breakfast like eggs or sausage can reduce neurotransmitter levels by 35%. For individuals with depression (i.e., low levels of neurotransmitters in the brain, also associated with constipation or indigestion), a starchy, fibrous, and well- balanced breakfast of sweet potatoes, steel cut oats, or quinoa can raise serotonin levels by 10%. For this individual, a high protein breakfast could actually bring serotonin or dopamine levels down even further, exacerbating depression and triggering cravings later in the day. Play around and experiments with different diets, fruits, and vegetables. Your body might just thank you for making small, easy dietary changes by making you feel better! 

Understanding how something as simple as food can affect our mental health can be a catalyst for self-care. As humans, we are wired for connection. Mental health illnesses like anxiety and depression spur disconnection. By taking better care of ourselves, we’re able to build the foundations of a stronger community that will enable us to reach out and take care of others as well. 


  1. Kilgore, Lisa. “Microbes, Mental Wellness & Mealtime.” YouTube, Ted Talks, 2015,
  2. Gordon, Olivia. “Your Microbiome and Your Brain.” YouTube, SciShow, 2017, 
  3. Robertson, Ruairi. “Food for Thought: How You Belly Controls Your Brain” YouTube, Ted Talks, 2015, 
  4. Kohn, David. “When Gut Bacteria Hijack the Brain.” The Atlantic, Atlantic Media Company, 3 Aug. 2017,
  5. “Toddler Temperament Could Be Influenced by Different Types of Gut Bacteria.” ScienceDaily, ScienceDaily, 27 May 2015,

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