How the Gut Affects the Human Mind

The gut influences the human psyche. At the beginning of the 21st century, this statement would likely have been met with only a puzzled look and a lack of understanding from many a neurologist. Over the past ten years, however, so much research has emerged confirming this hypothesis that there is no doubt about this peculiar relationship between the gut and the brain. After all, the connection between digestion and the mind is reflected even in everyday language: “I can’t stomach it,” “It’s weighing on my stomach,” or “I’m just….” Although it may seem that the gut and the brain have absolutely nothing in common, this is not the case. The enteric nervous system (ENS)—as the network of nerves in the small and large intestines is called—is the second most complex organ in terms of its tangle of nerve fibers. This dense network has one important task—to closely monitor what is happening in the intestines and, using its own “intelligence,” regulate what happens to the food. It is, therefore, the largest sensory organ in the human body—processing more information than the eyes or ears. The results of these complex calculations are then sent to the brain via the vagus nerve, which connects the ENS to the brain. Thanks to this, the brain receives vital information directly from the “center of the action.”

Experiments

The gut itself is a fascinating organ worthy of study. However, research also pays special attention to the bacteria that inhabit the walls of the ENS. It has been found that although these are organisms that have nothing in common with the human body and merely use it as a host, they can have a significant influence on an individual’s behavior. The following studies serve as examples:

1. Research from 2011 examined how gut bacteria alter the behavior of mice in a life-threatening situation. The researchers placed the mice in a container of water where they could not reach the bottom. They therefore had to swim for as long as their strength held out. Half of them were fed the bacterium Lactobacillus rhamnosus, which has a positive effect on the gut microbiota. These mice then swam significantly longer and exhibited lower levels of stress hormones than mice without this bacterium. However, when the researchers severed the vagus nerve, the difference between the mice completely disappeared.

2. Another study (2011) also examined how changes in gut microbiota affect mouse behavior. They selected two inbred mouse strains—one characterized by shyness and the other by boldness and curiosity. First, they completely wiped out the gut microbiota in both groups using antibiotics. Then, they introduced bacteria typical of the opposite group of mice into the digestive tracts of these “sterile” individuals. And the result? The group of timid mice suddenly became curious, while the originally bold group became shy. This was a truly groundbreaking finding, as it demonstrates just how much the tiny inhabitants of the gut are responsible for an individual’s behavior.

3. In 2013, scientists conducted a bacterial study directly on humans. The participants were tasked with consuming a specific mixture of bacteria for four weeks, which is sufficient time for the gut microbiota to undergo a fundamental change. After this period, their brains were subjected to scientific examination. The results were surprising. Certain areas of the brain underwent significant changes, such as the areas responsible for processing emotions or pain.

The Effect of Stress on the Gut

When we experience a stressful situation, the brain sends a clear signal to the gut: the energy it would normally use for digestion must be redirected to the body. This message travels from the brain via sympathetic nerve fibers. The ENS responds to this message by slowing down its activity. This is not a problem when it is an isolated and short-lived event. However, if the body is under stress too often (or constantly), it significantly affects digestion. The person then feels lethargic and suffers from loss of appetite, nausea, or diarrhea. This is the gut’s desperate cry for better treatment. If we ignore this signal, reduced secretion in the organ’s walls leads to overstimulation of immune cells—the gut thus becomes increasingly hypersensitive, and even conditions it once handled with ease can throw it off balance.

Stress also has a significant impact on changes in the gut microbiota. Only certain types of microbes can withstand prolonged adverse conditions, while the more delicate ones perish, which only encourages further proliferation of the survivors. The resulting imbalance can then be responsible for negative psychological changes. The bad news is that this condition does not subside once the stressful situation has passed. Repairing this damage takes much longer. Moreover, judging by the aforementioned studies, these tiny bacteria have a significant influence on how we feel, our self-confidence, and how we perceive pain.

We should therefore not forget that when we experience stress, we are, in a sense, running a deficit. We should then make up for this with well-deserved rest so that our gut is happy again. We should also pay special attention to the times when we eat. We should make sure that we do not experience negative emotions while eating, and that we are not stressed or in a hurry.

Sources:

• Enders, G. (2015). The Gut Is Not a Taboo. Prague: Ikar.

• Bercik, P. et al.: The Intestinal Microbiota Affects Central Levels of Brain-Derived Neurotrophic Factor and Behavior in Mice. In: Gastroenterology. 2011; 141 (2), pp. 599–609. Available here: https://www.ncbi.nlm.nih.gov/pubmed/21683077

• Bravo, J. A. et al.: Ingestion of Lactobacillus Strain Regulates Emotional Behavior and Central GABA Receptor Expression in a Mouse via the Vagus Nerve. In: Proc Natl Acad Sci. USA. 2011; 108 (38), pp. 16050–16055. Available here: https://www.ncbi.nlm.nih.gov/pubmed/21876150

• Tillisch, K. et al.: Consumption of Fermented Milk Products With Probiotics Modulates Brain Activity. In: Gastroenterology. 2013; 144 (7), pp. 1394–1401. Available here: https://www.ncbi.nlm.nih.gov/pubmed/23474283