The Gut-Brain Axis and Depression

Depression is one of the most common mood disorders nowadays, and it’s about so much more than just being sad or being in a bad mood.

It encompasses a wide range of emotions that negatively impact the individual and cause a ripple effect, making their daily life a struggle.

The most commonly prescribed antidepressants are somewhat beneficial for regulating the mood, however, despite their rapid-onset effects, a significant part of the people who take them can’t tolerate them very well, and often experience side-effects.

Therefore, a lot of research has been done on the connection between the gut and the brain in relation to mood and depression because it has been increasingly shown that gut health affects the overall health of a person.

What Does This Mean?

A hundred years ago, the Nobel laureate Elie Metchnikoff proposed that lactic bacteria are actually beneficial to human health.

This proposition sparked some serious research into the issue, and it was concluded that, in fact, not only does the gut microbiota play a role in regulating the gastrointestinal (GI) function, but it also helps to regulate mood and cognition.

Since then, increasing evidence has pointed towards the possibility that the brain may influence the gut flora, which may, in turn, have an effect on the brain.

Learning how exactly this connection works may play an important part in finding out how depression can be better managed.

For these purposes, a group of scientists from China have compiled their findings in a review which aims to clarify the effects of gut flora on the brain, as well as discuss the role of the gut-brain axis in the development and management of depression.

The Findings

The gut microbiota constitutes the complete genetic material of the human gut cells.

It represents all microorganisms living in the human body, and all of them communicate with each other and with the host.

These microbes also interact with the central nervous system through a complicated web of signaling mechanisms, and every change to the gut could also impart a change to the brain, like age, diet habits, certain medication, chronic conditions, and the like.

The scientists mention that in previous human and animal studies, good bacteria were more abundant in the gut of healthy people, whereas bad bacteria were more prevalent in patients with depression. 

They also discuss some mouse model trials that were conducted using a number of tests.

The purpose of these tests was to induce the main symptoms of depression, like the inability to feel pleasure, despair, and stress, and monitor the mice’s behavior and neural changes.

The findings revealed that targeted treatment of microbial deficiencies, like the use of prebiotics and probiotics, had a beneficial effect.

In addition, diet was also a major factor in gut health and mood.

They found that high-fat food increased the bad bacteria in the mice, as well as depression-like behavior. They also found some indications that a magnesium-deficient diet alters the gut flora and leads to a change of behavior.

How It Actually Works

The mechanism behind the gut-brain axis hasn’t been completely figured out just yet, however, more and more evidence seems to point towards the importance of neural, immune, and metabolic pathways, and each of them is involved in the processes connected to depression.

  1. The monoamine transmitters regulate the strengthening of neural circuits involved in mood and behavior. They are serotonin, dopamine, and GABA, and their deficiency in the central nervous system is one of the greatest risks for depression. This is why many current antidepressants aim to increase their level. The expression of the monoamine neurotransmitters is largely regulated in the gut flora, with more than 90% of the body’s serotonin being synthesized in the gut.
  2. Inflammation and impairment of the immune system have been strongly linked with depression, because of its undeniable connection to the gut microbiota. Inflammatory cytokines (proteins excreted in response to inflammation) disrupt the synthesis of neurotransmitters, and in recent studies, higher levels of cytokines have been observed in depressed subjects.
  3. Brain-derived neurotrophic factor (BDNF) is a protein that’s involved in several processes, like neuronal survival and growth, and it participates in neuroplasticity, which is the ability of the nerve cells to re-organize themselves. BDNF insufficiency is considered to be a risk factor for developing depression because neuroplasticity becomes impaired. Numerous studies have reported decreased levels of BDNF in patients with depression, in comparison to healthy subjects.

What’s the Conclusion?

Depression is a mood disorder whose treatment should be taken very seriously, and the connection between the gut and the brain shows some very promising outcomes as a basis for future, more concrete studies.

However, even though there are some findings about how exactly the gut-brain axis works, more analytical research is needed to better understand the underlying mechanism of this connection in order to implement the information into a usable treatment.

 

Resource: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7276434/

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