By Dr. Sarah Zadek ND

The importance of the digestive tract, also called the gastrointestinal tract (GIT), is often underestimated. Its purpose may seem to only digest food, absorb nutrients and then excrete waste; however, the world inside the GIT is actually an entire ecosystem, and one that constantly influences our health. The bacterial colonies within this system uses the food we send down, producing nutrients and neurotransmitters, sending signals, influencing the inflammatory response and the immune system, and interacting with the nervous system.

There are literally trillions of living microorganisms inside the gut with the power of suggestion over our mood. Are you experiencing constant sugar cravings? Feel anxious or have a low mood? Do you tend abdominal cramping and/or bowel urgency when you’re anxious or stressed? These experiences are all guided by the gut-brain-axis.

The gut-brain axis essentially is the connection and all communications between the GIT and the brain. In essence, you can “stress out” your gut, and your gut can also stress you out.

The role of gut bacteria
The bacteria in our GIT play a huge role in how this connection functions. Other processes that are affected include: the inflammatory response, other immune system responses, and the permeability of the gut barrier (also termed “gut barrier integrity”).

The term “gut microbiota” is used to describe the multitude of species and colonies of bacteria and yeast in the GIT. The composition of the gut microbiota is influenced by diet, stress and environmental factors, and it influences the production of metabolites that help to maintain our health.

The gut microbiota is very much influenced by our diet as the bacteria feed on the nutritional contents in our intestines. This drives the fermentation of sugars and releases fatty acids for energy production for other, more specialized bacteria.
How the gut influences mood
Several species of gut bacteria play key roles in the production (and/or consumption) of multiple neurotransmitters that influence our mood such as serotonin, GABA, and dopamine.

Serotonin is a major signaling molecule in the gut-brain axis. Interestingly, the majority of serotonin in our bodies is actually located in the gut, made by specialized cells. Tryptophan, a precursor to serotonin, is found in seeds, soybeans, meat and fish. From our diet, tryptophan is absorbed in the gut and crosses the blood-brain barrier where it is transformed into serotonin. However, gut bacteria can also produce serotonin themselves from tryptophan.

This production of serotonin is used to modulate GIT functions such as secretion, peristalsis (movement of food material), blood flow to the intestines, and the perception of discomfort and nausea. Being a major player of the gut-brain-axis though, it can also influence our mood and cravings.

Another important neurotransmitter generated by our gut bacteria is GABA: an inhibitory neurotransmitter used to “calm down” or “turn off” overexcited responses of the nervous system. Changes in GABA levels have been linked to depression, and for this reason, many researchers are investigating the use of certain probiotics as treatment in mental health disorders.

How stress and our mood influence digestion
The vagus nerve is another component of the gut-brain-axis. It is largely involved in the parasympathetic nervous system which activates the “rest and digest” function. This is the opposite of the sympathetic nervous system which activates “fight or flight” responses. So you can see how activating the vagus nerve helps to promote digestion and a sense of calm.

There are several ways one can tone the vagal nerve. The vagus nerve runs from the brainstem, down through the neck alongside the throat, down into the chest and into the abdomen. It branches into other nerves that serve the heart, the stomach, and the intestines.

Because it innervates muscles of the throat responsible for swallowing and speaking, some find they can achieve vagal tone by humming or singing. Deep yogic breathing, also called Ujjayi breath or “ocean breathing” has also been found to elicit a similar response. In this technique, the goal is to make a “HAAAH” sound, as if attempting to fog up a window, but with the mouth closed. This should result in the production of a deep sound in the back of the throat.

By activating the parasympathetic nervous system, blood vessels and passageways of the lungs open up, and saliva production increases. It also promotes proper digestion of food and absorption of nutrients.

Anxiety, agitation and stress can change all of this. These states are activated with our fight or flight responses – the sympathetic nervous system. By flipping this switch we also flip over what our body should be doing. Heart rate increases and slow digestion ceases. The body redirects blood flow from the intestines to your skeletal muscles (so that you can run or fight). For many people this means a quick run to the bathroom as food material is quickly ushered through the intestines, leaving many who are stressed with IBS, urgency of bowels, and loose stool.

Stress and the gut interconnected
Serotonin itself can also activate the vagus nerve, and as such, the gut microbiome can influence our ability to achieve “rest and digest” processes. In this way it’s easy to see the interplay between mood and the function of the GIT. There are so many factors that influence the vagus nerve, the sympathetic and parasympathetic nervous system, and the gut microbiome, each of which influences the others.