The Gut-Brain Axis

A Look At How Your Gut Microbiome Affects Cognitive Function

If we were writing just a few years ago, this would all have been dismissed as pseudoscience. But today, it is becoming increasingly obvious that there is a strong link between the gastrointestinal tract and the brain. 

Researchers are now convinced that bidirectional signalling takes place between the gut and the brain. We are seeing a growing body of evidence that the brain and the gut are, in a way, dependent on one another. Each has an immediate and profound impact on the performance of the other. 

This relationship is often called the ‘Gut-Brain Axis’, or the ‘Mind-Gut Connection’. 

Although we are only just beginning to understand the ways in which the gastrointestinal tract and the brain are connected, it is already becoming a hot topic of conversation online. 

Bloggers, vlogger and health experts have been churning out content on the gut-mind axis like there’s no tomorrow. If you look on YouTube, for example, you’ll find dozens of videos explaining how your gut microbiome influences your cognition, causes depression, and prevents you from sleeping. 

But you’ll also find plenty of skeptics. After all, there seems to be a new health fad every 2 years. Is the obsession with the gut microbiome just the next diet fashion?

Is there really a strong connection between the gut and the brain?

How does the gut microbiome influence cognition?

What does that mean for you? 

Can you manipulate your gut bacteria to enhance cognition?

In the following article, we’re going to explain what the gut-brain axis is, how it affects cognition, and how it can be manipulated to your benefit. We’ll also recommend some dietary changes that you might want to make to enhance mental performance, as well as some supplements that might benefit you if you really need to give your gut microbiome a boost!

If you have any questions, or if you want to share your opinions on this topic, please don’t hesitate to post in the comments section at the end. 

Can Your Gut Communicate With Your Brain?

Before we get into any discussions of how you might alter your gut microbiome to enhance mental performance, or in what ways it can even make a difference, we first need to establish whether there is a connection at all!

Obviously, we believe that there is a definite link between the gut and the brain.

But lots of people find the concept very counter-intuitive. They’ve probably never thought for a second that their gut bacteria could be influencing their mental energy levels, their ability to focus, or their mood.

For most people, the gut is the gut and the brain is the brain. That’s that.

Well, they’re wrong. What we now know about the gut microbiota makes this assumption completely untenable.

Let’s look at the data that convinced us of the importance of the gut-brain axis.

First of all, we should establish that your gut bacteria can physically communicate with the brain.

Few people know that we actually have a huge concentration of neurons embedded in the lining of the gastrointestinal tract. The enteric nervous system (ENS) is composed of over 500 million neurons. That is only about 0.5% of the neurons in the human brain, but it is 100% larger than the number of neurons found in the spinal cord, and it is 66% of the number of neurons in a cat’s entire nervous system (brain included). It begins in the oesophagus and ends…right at the end!

It is mainly responsible for reporting on conditions within the gut; chemical composition, physical state, etc. The motor neurons in the ENS control peristalsis, which is where the gut contracts and expands to move contents down the tract.

The ENS operates independently of the brain and spinal cord. It is a sub-division of the autonomic nervous system, which is responsible for keeping all of your internal organs running smoothly without you having to think about them.

However, the ENS can and does communicate with the central nervous system (CNS) via the vagus nerve and the prevertebral ganglia.

The ENS will continue to function if these channels are severed, but while they are open, the ENS and CNS can and do communicate.

The ENS uses over 20 neurotransmitters. In fact, some of the best-known neurotransmitters are concentrated in the gut rather than the brain. More than 90% of the body’s serotonin is found in the gut, as well as sizable quantities of dopamine, acetylcholine, and others.

It shouldn’t come as a big surprise to learn that the ENS is sometimes referred to as a “second brain”. Although it has a tiny fraction of the neurons in the human brain, it is as complex as the entire nervous systems of some mammals.

With that information in mind, it is a lot easier to grasp the idea that the brain and the gut are in constant communication, and that one affects the other.

How Your Gut Affects Your Brain – Mechanisms of Action

The gut has been observed as having an effect on cognition in numerous different ways: focus, memory, mood, and mental health.

Some links are much stronger and more well established than others, but there is good evidence on all fronts. We’ll focus on the three most important mechanisms of action.

1. Neurotransmitter Synthesis

One of the most important ways in which the gut influences cognition is through the synthesis of important neurotransmitters.

Serotonin

Over 90% of the body’s serotonin is found in the enteric nervous system (ENS), where it is primarily used to regulate intestinal movements. The other 10% is synthesized in the CNS, where it is used for all of the functions we typically associate with serotonin: feelings of well-being, happiness, contentment, etc.

However, the two systems are not entirely separate. We already know that drugs intended to manipulate serotonin levels in the brain often have quite pronounced gastrointestinal side effects. The relationship seems to work in the opposite direction too.

“A big part of our emotions are probably influenced by the nerves in our gut,” says Emeran Mayer in an article in Scientific American.

This is no doubt why so many studies have found that introducing certain germs to mice can bring about drastic increases in anxiety levels. According to the authors of one trial, published in 2011: “We conclude that the presence or absence of conventional intestinal microbiota influences the development of behavior, and is accompanied by neurochemical changes in the brain.” 

The specific changes in neurochemistry and behavior point to serotonin in particular as the locus of change.

GABA

GABA is an anxiolytic neurotransmitter. It is incredibly important; GABA is actually the chief inhibitory neurotransmitter in all mammalian brains. Its release brings about feelings of calmness, a lowering of inhibitions, and rapid reductions in anxiety.

We now know that specific strains of bacteria found in the human gut produce large quantities of GABA. Lactobacillus and Bifidobacterium seem to be the strains primarily involved in GABA synthesis. Research is on-going, but early findings suggest that GABA plays a key role in “the modulation of immune cell activity associated with different systemic and enteric inflammatory conditions”. It plays other roles in the enteric nervous system too.

Amazingly, some studies have found that the ingestion of Lactobacillus bacteria by mice led to dramatic changes in GABA brain activity. The mice in question showed changes in how they responded to stress, as well as their subjective well being.

Dopamine

Dopamine is an immensely important neurotransmitter. We have all experienced the effects of dopamine personally; it is the neurotransmitter responsible for feelings of motivation and reward when we do a behavior that was deemed to be worthwhile. It’s easy to grasp why this neurotransmitter is so important for productivity, good cognitive function, not to mention overall good mental health and happiness.

Irregularities in dopamine levels are associated with anxiety, depression, and various forms of addiction (particularly with drugs like alcohol and cocaine). Well, like serotonin and GABA, a substantial amount of Dopamine is produced in the human gastrointestinal tract. In fact, it seems that more than half of all the dopamine in the human body is produced by bacteria in the digestive system.

This is not fringe science any more.

The idea of the gut-brain axis was taken as quite outlandish a few decades ago. The idea that the enteric nervous system could communicate with the central nervous system, via neurotransmitters produced in both the brain and the gut, would have been seen as ridiculous.

Now, many researchers take it as fact. Studies such as this one published in Biological Psychiatry in 2013, or this literature review published in the Journal of Cell Physiology in 2018 are all coming to the same conclusions.

To quote the researchers from the latter cited study: “Furthermore, research shows that this relationship is bi-directional, suggesting that changes in the GI system and enteric nervous system can affect the central nervous system, which is evident in human diseases such as Parkinson’s disease. Neurotransmitters namely epinephrine, norepinephrine, serotonin and dopamine, have shown that they play a major role in controlling and maintaining homeostasis within the gut system in terms of nutrient absorption, blood flow, gut microbiome, local immune system, and overall gut motility.”

2. Neurotrophic Factor Synthesis

Perhaps a more powerful way in which your gut bacteria affect your mental health, performance and development is through the synthesis of various neurotrophic factors.

One of the most important of these is Brain Derived Neurotrophic Factor (BDNF). This is a protein responsible for regulating the growth, proliferation and maintenance of brain cells.

In the presence of BDNF, your neurons become stronger, they grow, and new neurons begin to grow where they are needed. Likewise, in the absence of BDNF, neurons don’t grow, and new ones aren’t created.

Several studies have now demonstrated that rodent gut bacteria profiles have a profound impact on BDNF levels.

In the absence of certain gut bacterias, mice show drastically lower levels of BDNF. We have now also proven the opposite; that improving gut bacteria profiles in mice leads to higher levels of BDNF in the brain. The cited study is replicating results also observed in this trial, published two years prior in Gastroenterology. 

The effect that BDNF has on brain function and development cannot be over-stated. Low levels of BDNF have been associated with a wide array of illnesses and conditions, from depression and low mood to memory loss and cognitive decline. 

3. Stress Modulation

Much of the data here comes from rodent studies. But the results are extremely interesting, and in theory, there is no reason why these findings should not give us an insight into how the human body works too (especially as there are some human trials in agreement with the findings).

It seems that the make-up of your gut microbiota could have a direct effect on how you process stress.

As you will see if you look at some of the many trials conducted in this area, removing Bifidobacterium from otherwise healthy mice will significantly exaggerate their physiological response to stress; that is, they show a far larger rise in cortisol levels than mice who haven’t had that specific gut bacteria removed.

Other studies have come at this link from a different angle. A paper published in Biological Psychiatry in 2009 found that early-life stress altered the make-up of the gut microbiota. This had lasting effects on development and emotional well-being.

The authors concluded: “These results show that this form of early life stress results in an altered brain-gut axis and is therefore an important model for investigating potential mechanistic insights into stress-related disorders including depression and IBS.”

It seems that a healthy gut may help make for a calm person. More research is needed here, particularly human trials, but the existing evidence is compelling. Your gut bacteria seem to have a direct influence on stress hormone secretion.

Effects On Cognition – The Evidence

We’re going to skip the rambling and jump straight into some hard science. You can then just look at the same data we have and come to your own conclusions about the importance of the gut microbiome with regards to mental performance and health.

Let’s go through some of the key ways in which we believe the gut microbiome affects cognitive function, emotional well-being, and overall mental health.

Mood

The manipulation of gut bacteria has been found to have a profound impact on mood. You may find it hard to believe just how powerful this effect may be.

This study, published in Nutrition in 2016, looked at how patients with clinical depression responded to probiotic administration. Either Lactobacillus acidophilus, Lactobacillus casei, and Bifidobacterium bifidum or a placebo were given to 40 patients with Major Depressive Disorder (MDD). The authors found that, “after 8 wk of intervention, patients who received probiotic supplements had significantly decreased Beck Depression Inventory total scores (-5.7 ± 6.4 vs. -1.5 ± 4.8, P = 0.001) compared with the placebo.”

Another study from the European Journal of Clinical Nutrition backed up these findings. In that trial, researchers found that “the consumption of a probiotic-containing yoghurt improved the mood of those whose mood was initially poor. This improvement in mood was not, however, associated with an increased frequency of defaecation.”

Most amazing of all, however, was this clinical study, published in Neuroscience in 2010. In this trial, patients suffering with depression were given either a powerful probiotic, Bifidobacterium infantis, or the synthetic anti-depressant drug Citalopram.

The results showed that “Probiotic treatment resulted in normalization of the immune response, reversal of behavioral deficits, and restoration of basal NA concentrations in the brainstem.”

Selected probiotics were found to be MORE EFFECTIVE than Citalopram, a prescription anti-depressant. 

Anxiety

It is pretty widely known that your gut bacteria has a massive effect on your mood and emotional well-being. Fewer people know that your gut microbiota can make an enormous difference to your ability to handle stress and anxiety.

It has been noted that probiotic supplementation can help reduce symptoms of stress and anxiety-related cognitive impairment. One study found that medical students given Lactobacillus casei-rich fermented milk exhibited far fewer physical signs of stress than the control group during exam periods.

Another study on mice found that Bifidobacterium administration helped improve the cognitive performance of chronically anxious mice. The results were strong enough for the researchers to conclude: “Clinical validation of the effects of probiotics on cognition is now warranted.”

Focus & Memory

By modulating BDNF expression in the brain, your gut bacteria can have a powerful effect on cognitive function.

Higher BDNF levels have been associated with better memory function and enhanced attentional focus. Lower levels, on the other hand, have been associated with accelerated cognitive decline and poor memory function.

As we explained above, the presence of certain bacterial strains in the gut are linked to healthy BDNF levels in the brain. Optimizing the balance of these different bacterial strains in the gut is a good way to promote optimal BDNF levels, which is in turn a good way to promote optimal brain function.

Prebiotic & Probiotic Supplements – Are They Necessary?

So, it seems that your gut can have an incredible impact on your cognitive performance. The make-up of your gut bacteria clearly affects multiple different aspects of your brain function; mood, anxiety, focus, memory, and potentially the on-set of cognitive decline.

But does that mean that you need to be using prebiotic or probiotic supplements?

Not necessarily!

That very much depends on whether or not your diet is already optimized to promote good gut microbiota health.

Whether or not you need to be using supplements depends entirely on what it is that you’re trying to accomplish, how prepared you are to augment your diet, and how knowledgeable you are on the foods you need to eat to maximize gut microbial health.

For a lot of people, prebiotics wont be necessary.

For many more people, they wouldn’t even be desirable – they just care all that much about optimizing their life to promote good health and mental performance.

But a lot of people can really benefit from pro and prebiotics.

If you have a busy schedule, or you want to take your self-optimization a step further than simply making dietary changes, then supplements might be the way to go.

In our opinion, you should start by looking for a high quality prebiotic supplement.

Many people’s first move is to look for a probiotic. These are supplements that deliver live cultures to your gastrointestinal tract.

Prebiotics, on the other hand, selectively feed ‘good’ strains of your existing gut bacteria. So instead of trying to re-balance your gut bacteria by introducing new, exogenous strains, you just create an environment where your own naturally-occurring ‘good’ bacteria thrive and the less helpful ones starve. 

Ideally we think you should look for a supplement that primarily targets Bifidobacterium. This is the strain of gut bacteria that seems to have the most wide-ranging and profound impact on your overall health, well-being and mental function.

Of course, you can also make big improvements through dietary changes. 

Check out our article on prebiotics to find out how certain foods help your ‘good’ gut bacteria thrive. In case you don’t have time to read the full article, here is a quick run-down of the best foods to eat for a healthy gut microbiome.

Prebiotic Foods

• Onions
• Asparagus
• Artichokes
• Bananas
• Leeks
• Wheat bran
• Chicory root

Adding a few of these foods to your diet each and every week will help your good gut bacteria thrive. This small change could have huge implications for your focus, memory, mood, and mental health.

Probiotic Foods

• Kimchi
• Tempeh
• Sauerkraut
• Miso
• Yoghurt
• Kefir
• Pickles

These foods have been fermented, meaning that they contain live bacterial cultures. These bacteria strains can help re-balance your gut microbiome to be more conducive to good health and mental function.

Further Research

If you’re interested in learning more about the gut-brain axis, then there are some excellent videos on YouTube.

Here are a couple of our favorite lectures, interviews, and video essays discussing the mind-gut axis, how your gut influences your cognitive performance, and how you can use that information to your benefit:

These videos will collectively tell you everything you need to know about the gut microbiota, and how it relates to health and performance.

Of course, you could go even deeper. But together, this article and these videos will give you a good, well-rounded, basic understanding of how your gut bacteria influence the rest of your body.

If you have any videos that you think should be added to this list, get in touch!

Luke Mercia

Luke is our Editor in Chief. He is the main driving force behind NaturalNootropic.com, and he creates most of our most important content. He is extremely passionate about enhancing human cognition; he has experimented with many different nootropic substances over the years, sometimes with negative results. He wants to help people get more out of performance-enhancing supplements, and he is fascinated by recent advances in longevity research. He can be contacted by email at editor@naturalnootropic.com.