The Gut-Brain Axis

Part 1: The journey from food to feelings


In some way or another, we have likely all experienced the effects of direct communication between the brain and the digestive system. And while it may be more immediately obvious how the brain can impact the digestive system, it is crucial to understand that this communication represents a two-way street.


The vagus nerve is the largest in the body and is a superhighway of communication between the gut and the brain. What may be surprising is that a majority of this is coming from the gut to the brain.


The gut is essentially the mediator between the microbiome — our internal community of bacteria, viruses, yeast, and fungi — and the brain, witnessing the processing of the food and reporting relevant information from that process to the brain via the vagus nerve. As the site of food digestion, the gut has immediate knowledge of what is being consumed and information about nutritional and energy content is transmitted via the vagus nerve to ensure the brain stays up-to-date on this sensory information, like hunger cues and feelings of fullness. This knowledge is important for the brain, so it can determine:

  • How to drive related impulses (e.g., telling your brain your gut is full and therefore you should stop eating).

  • How to shift your mood (e.g., if you’re hungry, your mood can become irritable).

  • Where it is best to send energy (e.g., when you are cold, energy is sent to warm your most vital organs).


Additionally, the production of our major neurotransmitters like GABA, serotonin, and dopamine begins with the work being done by our gut's microbial residents.

As a complex and sensitive circuit, issues of function with either our own digestive system, the balance of the microbiome, or the brain itself can cause problems across the entire system. Take a look back at our article on digestive difficulties for more.


IBS and the Gut-Brain Axis


In response to a stressful stimulus, the body's sympathetic nervous system becomes actively engaged, resulting in the 'fight-or-flight'response, Historically, this was an important survival mechanism, allowing us to flee from animals who might've seen a person as their next meal. However, thanks to the marvels of modern life, this exact same response in the body can be triggered by thinking of your next meeting, test, or a phone call from an unknown number.





This state happens to be the opposite end of the physiological spectrum compared to the 'rest and digest' state governed by the parasympathetic nervous system. When under stress, the digestive process is stopped in its tracks


Irritable Bowel Syndrome (IBS) is marked by gas, bloating, abdominal pain, and altered bowel habits; all of which can be worsened when the brain is in a state of stress. The gut can also become overactive in sending a pain response to the brain, resulting in visceral hypersensitivity.


IBS and the Gut Microbiome


The relationship between IBS and dysbiosis - alteration of the balance of normal gut flora - is complicated. Condiotins like IBS and Small Instestine Bacterial Overgrowth (SIBO) are becoming virtually interchangeable as the symptoms of one condition can be the ideal set of circumstances for developing the other.



Stress can not only exacerbate symptoms of IBS but can also create change in the makeup in the microbiome, such as reducing the levels of helpful Lactobacillus species.



Depression and Anxiety


Dietary changes over the last century — including industrial farming, the use of pesticides and herbicides, and the degradation of nutrients in foods — are the primary forces behind growing mental health issues like depression. Low nutrient availability, inflammation and oxidative stress affect the neurotransmitters dopamine, norepinephrine and serotonin, which control your moods, ease tension and raise alertness. It’s also a two-way street when it comes to your gut and mood. Additionally, a healthy microbiome works to boost mood in a few more important ways: by generating healthy levels of brain-derived neurotrophic factor (BDNF) and of the neurotransmitter gamma-aminobutyric acid (GABA) along with its receptors. GABA is a calming amino acid, known to reduce activity in areas of the brain that are overactive in ADHD.



Research has also shown that hypothalamic-pituitary-adrenal (HPA) axis dysregulation can be reversed by treatment with Lactobacillus and Bifidobacterium. This research verifies the 2-way communication between the digestive tract and the brain, and reveals how the HPA axis is modulated by the enteric microbiota.


Human studies are limited in number, but the ones available corroborate findings that replenishing a healthy microbiome can certainly confer psychological benefit. In one such study, a 30-day administration of Lactobacillus and Bifidobacterium lowered psychological distress and depression, decreased anger and hostility, lessened anxiety, and improved problem-solving, compared with the placebo group.


In another double-blind, placebo-controlled trial, healthy subjects were fed either a probiotic-containing milk drink or placebo for 3 weeks, and mood and cognition were assessed before treatment and after 10 and 20 days of consumption. Subjects who initially scored in the lowest third for depressed mood showed significant improvement in symptoms after probiotic treatment.