The brain-gut microbiota axis: Impact on mental health & potential treatment avenues

Gut-microbiota has been found in recent years to have a significant impact on both physical and mental health. Ted Dinan, the Medical Director at Atlantia Clinical Trials, was interviewed about his work at Atlantia in investigating the impact of gut-microbiota composition and how it might be targeted to improve mental health in human beings.

Drawing on the analysis of rodents and other mammals, Dinan details the routes by which gut-microbiota can influence brain function, exploring how impactful gut-microbiota can be when it comes to illnesses like anxiety, depression, and irritable bowel syndrome, as well as detailing potential treatment avenues deploying these newfound links. 

Please could you introduce yourself and outline your role at Atlantia Clinical Trials?

My name is Ted Dinan, and I am the Medical Director at Atlantia Clinical Trials.

What research has Atlantia undertaken in the field of microbiota as linked to mental health?

Over the past few years, Atlantia has undertaken a significant amount of research into how the gut-microbiota influences brain function and how we might target the gut-microbiota to improve mental health. The average adult gut contains over a kilo of bacteria. The main phyla are bacteroidetes and firmicutes. There are over 400 species of bacteria within the intestine. The actual mass of bacteria within the large intestine weighs approximately the same as the human brain.

Image Credit:Shutterstock/Pikovit

What are the factors that determine the composition of the gut microbiota?

It is important to look at what determines the composition of gut microbiota in humans, as this composition varies from one person to another. Gut composition is arguably as unique as our DNA profile or our fingerprint.

There are many different factors that determine our gut-microbiota. Our genetics play a significant role because genetics will determine what bacteria can actually colonize the colon, but in terms of the initial microbiota, the main determinant is how we were born. If a baby is born per vaginum, then the initial bacteria are acquired from the vagina of the birthing parent, which are called lactobacilli.

A baby born per vaginum – what is sometimes referred to as a ‘natural’ birth -  the baby will have a microbiome largely consisting of lactobacilli. On the other hand, if a baby is born by cesarean section, the initial bacteria that are acquired will be from the skin of the doctor, the nurse, or the birthing parent. It will therefore be a far more diverse microbiota than that of a baby born by cesarean section.

Now, at what we might think of as ‘the other end’ of life, the microbiota also alters as we age. It is now clear that, as we age, the loss of diversity in the microbiota precedes the onset of frailty. It is important to maintain a diverse microbiota as we age. The factors that can impact that microbiota as we get older would be things like stress, which can have quite a dramatic impact on the gut-microbiota; antibiotics – and medications in general - which can dramatically alter the gut-microbiota; and, of course, as regards a positive impact, diet and exercise. It is true that certain diets are associated with good microbiota, and there are certain diets associated with poor microbiota. Likewise, in terms of exercise, there is an abundance of evidence to indicate that people who engage in regular aerobic exercise tend to have a much healthier microbiota than subjects who do not.

How do gut microbes influence the brain?

Gut microbes influence the brain in a number of different ways. The vagus nerve is the long, meandering nerve that connects the brain and the gut, through which signals travel from the gut to the brain and from the brain to the gut. Certain microbes require an intact vagus nerve for brain-gut communication to take place.

A number of years ago, it was proven that certain lactobacilli could not transmit signals to the brain if the vagus nerve was severed. The vagus nerve is important, but there are other important roots of communication. Another of these is the production of 5-HT and the 5-HT metabolite, tryptophan. Tryptophan is the precursor of 5-HT.

We know that the human brain requires a constant supply of tryptophan. Tryptophan needs to cross the blood-brain barrier on a regular basis in order to maintain normal human mood, normal sleep patterns, and normal appetite. It is clear that certain bacteria, particularly bifidobacteria, which has been focused on previously in Atlantia’s studies, can increase the levels of tryptophan and the plasma. There is more tryptophan available to cross the blood-brain barrier.

Another important route of communication is the production of short-chain fatty acids. There are various short-chain fatty acids. The most extensively studied are butyrate, propionate, and acetate. They are produced by the metabolism of fiber in the intestine and can have a profound impact on brain function. Butyrate is an epigenetic modulator and, very specifically, an HDAC inhibitor. It can impact how genes in the brain function and also act on classic G-protein-coupled receptors. There are two receptors, FFAR2 and FFAR3, and butyrate connects through those receptors.

What are some of the other ways in which gut microbes can influence brain function?

There are other routes that seem to work in parallel. For normal brain function, we require the production of a variety of molecules by the gut-microbiota. Gut microbes produce molecules that we ourselves cannot otherwise produce and our brains require some of these molecules. It really is a synergistic interaction. We feed our gut microbes, and, in turn, they produce molecules that our brains require.

An essential point to note is that if we were to take all the genes from the gut-microbiota, whose products we require, and we were to put all those genes into ourselves, we would not be big enough to contain all that DNA. There is, therefore, an important synergistic interaction between humans and gut microbes.

Image Credit:Shutterstock/Christoph Burgstedt

What do studies into animals reveal about gut microbiota?

It is interesting to look at animals who have no gut-microbiota. It is possible to breed germ-free animals which have no microbes in their gut. These animals tend to be fairly abnormal in terms of their behavior. A number of papers have been published on the topic, which refers to a diagnosis of - an admittedly oft-used term these days - autistic patterns of behavior in these animals. 

For instance, if you allow the animal to socialize with another animal e.g., if you allow a mouse to socialize with another mouse and to spend time with an object (e.g., a pen), the animal with no gut microbiota is as likely to spend time with the object as it is with the other animal. Like ourselves, mice are sociable creatures, but there are many additional different anatomic abnormalities in animals that have no microbiota. Their myelination patterns and myelin is obviously a very important sheath in many neurons in the brain. Their myelination patterns are abnormal. Their neurotransmitter production, particularly serotonin production, is abnormal. Their synapse formation is also altered.

It has become clear in recent years that the mammalian brain, even in adulthood, is capable in some regions of producing new neurons. We know that these animals have abnormal neurogenesis. Their capacity to produce new neurons is actually altered. There are major changes, not just in behavior, but at a structural level in animals that are devoid of a gut-microbiota. Therefore, intriguingly, microbes are capable of producing most of the classic neurotransmitters.

These neurotransmitters are confined to the gut or the enteric nervous system surrounding the gut, but it is still remarkable that they produce most of the neurotransmitters like GABA, serotonin, dopamine, and noradrenaline that we have in our brain as active neurotransmitters.

Can human beings change genes in ourselves or our microbiota?

At present, human beings cannot change the genes within themselves, but there are a  variety of ways in which we can change the genes in our microbiota. Diet is a key component, as is the use of drugs like antibiotics as well.

Some of the common ways in which the gut microbiota can be altered include probiotics, which are now referred to as Live Biotherapeutics by US regulatory authorities, and prebiotics, which are fibers that promote the growth of good bacteria.

In addition to the more generative dietary approach, there is a more radical way of altering the gut microbiota through fecal microbiota transplantation. This approach is used for treating conditions like C. difficile infection in the elderly, but it must be noted that over 75% of the commonly prescribed drugs in clinical practice, and all commonly prescribed drugs, impact the gut-microbiota - some in very dramatic ways.

Proton pump inhibitors are used very widely for the treatment of hyperacidity or peptic ulceration, and while they do have a dramatic impact on the gut microbiota, so do many different drugs that are routinely prescribed in clinical practice.

Stress is another important driving factor for many psychiatric disorders like anxiety disorders and depression, along with physical ailments such as irritable bowel syndrome. This is why it has been described as “the disease of the 21st Century”. Although perhaps that is slight hyperbole, stress certainly is an important driving force to be analyzed.

Can stress responses be altered by altering the gut microbiota?

The short answer is yes. Data from a study published several ago in biological psychiatry was the first study to show that when animals were stressed - whether by specific stress or early life stress – has less diverse gut microbiota than animals who had not been subjected to this particular stress. The study revealed that they had an overactive hypothalamic pituitary adrenal axis by comparison; they released more corticosterone and had a less diverse microbiota.

In terms of translating this to human understanding: I was interested in exploring the situation in relation to patients who were attending my clinic at Cork University Hospital, where I primarily tended to see patients with depressive illnesses. We took a group of patients who were attending with a clinical diagnosis of depression. We compared their gut microbiotas with that of healthy subjects.

What we found as a result was that the gut-microbiota of the depressed subjects lacked the diversity and richness that is seen in healthy subjects. We then went on to do a transplant from the gut microbiotas of depressed patients. We transplanted them into animals - rats in this case – and these animals received a transplant from a healthy object of a depressed patient.

To our surprise, we found that when animals had a transplant from a depressed patient - in marked contrast to what happens when they had a transplant from a healthy subject - their behavior changed: the rats seemed to develop a depressive behavior pattern. The rats additionally had more inflammatory markers post-transplant, and their metabolism of tryptophan, which is the key building block for serotonin, was altered. This was certainly unexpected and potentially very significant.

The gut microbiota is altered in stress-related conditions, but there are a number of other scenarios in which the gut microbiota has been implicated: within disorders like schizophrenia and autism. It has also been implicated in a variety of degenerative conditions. The most widely-studied degenerative condition in relation to the microbiota is Parkinson’s disease, but there have recently been suggestions that the gut microbiota may also be involved in addiction, particularly in relation to alcoholism.

What are psychobiotics, and how are they relevant to abnormalities of gut-microbiota?

Today, a wide variety of conditions are associated with abnormalities of the gut microbiota. The concept of psychobiotics was introduced into the literature a few years ago. We defined it as bacteria, which when ingested in adequate amounts, had a positive mental health benefit.

When it was introduced, we were conceptualizing psychobiotics as something that might be beneficial in treating milder forms of depression or anxiety. The concept has since caught on, and to briefly recap present data in support of that view - there are bacteria out there that do seem to have psychobiotic activity. This was a Bif. longum study, in which the Bif. longum was given to mice. It was demonstrated that these mice were less anxious when they were taking Bif. longum. The study is somewhat complicated: escitalopram, the anti-depressant, was utilized as well, but the take-home message is that the animals were less anxious when they were taking Bif. longum. They also seemed to have enhanced cognitive function.

The study in humans was a placebo-controlled trial. The results showed that when human subjects were taking this Bif. longum, they reported themselves as less stressed, and their awaking cortisol - typically a very good measure of stress - was decreased. They reported that they felt less stressed, and their cortisol level in the morning was reduced. In addition, I believe this was the first study to demonstrate changes in the electrophysiological activity of the brain following intervention with a probiotic. This has been replicated recently by several other groups, but what was shown was that frontal mobility, which is a marker of anxiety in humans, was altered upon ingestion of this particular Bif. longum.

A psychobiotic therefore exists that does seem to have an anxiety action in both rodents and in humans. This was another study undertaken with this particular probiotic, in which we took healthy students over two semesters. In one semester, they were treated for four weeks before their exams with a probiotic. In the other semester, they were treated for four weeks with a placebo, rendering the study placebo-controlled.

The most important finding was that there was an improvement in sleep duration when the subjects were taking the probiotic, as opposed to when they were taking the placebo. It does seem to indicate that not only does this probiotic reduce anxiety levels, but it improves sleep quality. Not all probiotics translate from animals to humans.

It must be noted, however, that studies on humans failed miserably. We found no evidence whatsoever to indicate that this probiotic had any impact on any aspect of psychological functioning or any aspect of immune functioning; and yet, we had previously published a paper on a placebo-controlled trial in rodents in PNAS to show that it was potent as an anxiolytic agent in rodents. Though we considered it fairly robust, the findings did not translate.

Do the bacteria need to be alive to produce a psychobiotic effect?

Some do, but this may not always be the case. In a study that Colin Hill and I published some time ago where we used heat-killed lactobacilli, quite marked changes could be seen -  not only in the gut microbiota but in behavior as well. One other notable probiotic study was performed, which was a study of pregnant women who were given a probiotic or placebo for six. This was a large study with a large sample size of 423 women. This study demonstrated that the women who were taking the probiotic had reduced postpartum depression in comparison to the women who were not taking the study. Postpartum depression is a major clinical issue in clinical practice. Though there are some limitations to the study overall, and it needs to be repeated, I believe that it obtained strong and important results.

What are prebiotics?

Prebiotics are fibers, which when ingested, promote the growth of good bacteria and are found in a variety of food substances. Inulin is the most well-studied prebiotic. You find high levels of inulin in things like Jerusalem artichoke, but you find it in potatoes and in onions as well. Taking in high levels of prebiotics is certainly regarded as positive in terms of overall health, particularly one’s mental health.

Phil Burnet from Oxford conducted a placebo-controlled study with two prebiotics. One was B-GOS and the other was FOS. He essentially showed that B-GOS in healthy subjects reduces the awaking cortisol response.

What are polyphenols, and how do they impact brain-gut access?

Polyphenols are important chemicals that we find in various fruits and in various vegetables. There are well over 1,000 polyphenols described. Resveratrol in red wine is probably the most widely studied. We have previously worked on two different polyphenols. We were looking at two well-defined polyphenols; xanthohumol and quercetin.

It is known that corticosterone, the stress steroid, if it is at very high levels, actually kills neurons. It also decreases brain-derived neurotrophic factors. Now, what we find is that both of these polyphenols, xanthohumol and quercetin, actually protected the neurons from the deleterious effects of corticosterone. They also prevented the decrease in BDNF.

In relation to brain-gut access, in a subsequent study in rodents, we showed that both of these polyphenols have a significant antianxiety effect, and in so doing, they dramatically impact the gut-microbiota. Their impact seems to be both at the level of the gut-microbiota and at the level of the brain.

Image Credit;Shutterstock/ FOTOGRIN

How is olanzapine analysis useful in this field?

Olanzapine is one of the most widely-used antipsychotic drugs in the world. It can be very effective in treating schizophrenia or bipolar illness, but one of its side effects is that, in some people, it can cause substantial weight gain. The mechanism of this effect is not well established. It is postulated that this may be due to the blockade of histamine receptors or the blockade of 5-HT2C receptors in the brain, but the precise mechanism has not been elucidated.

Several years ago, it was decided that the impact of olanzapine should be studied in rats, and it was revealed that it does cause weight gain in rats just as it does in humans. However, what was found is that when the animals were given olanzapine, there was a shift in the balance between firmicutes and bacteroidetes. The firmicutes went up, and the bacteroidetes phylum decreased.

There is also a mechanism that is associated with weight gain in a variety of other settings. It does seem as though the shift in the gut microbiota plays a role in the weight gain that is induced by antipsychotics. A few months after this initial paper was published, users were able to block the effect of the olanzapine on weight gain using an antibiotic, but a few months later, a study was published from research undertaken in Houston, Texas, which clearly showed the same shift in phyla in humans who were given risperidone, another related antipsychotic drug, namely firmicutes increasing and, bacteroidetes decreasing. The gut-microbiota seems to play a role here in relation to weight gain.

Do any other drugs impact the activity of microbiota in the gut?

Data from a recent publication reveals that many of today’s commonly-used anti-depressants have antibiotic-like activity when they encounter certain microbes in the gut. Many people might consider this strange. If we were to look at the history of anti-depressants, around 1951-1952, iproniazid was the first anti-depressant described - an anti-tuberculous drug, or to be more precise, an antibiotic. Of course, anti-depressants have developed a great deal since that time, but the mechanisms of the action have remained relatively the same, so it is not altogether surprising that current anti-depressants have antibiotic-type activity.

Another discovery was the continuing prevalent use of lithium. Lithium is a very effective drug for stabilizing the mood of patients with bipolar illness. There is some evidence to indicate that people who are taking lithium may live longer and have less cardiovascular disease and possibly even a reduction in cancer. Surprisingly, it was found that lithium increases the diversity of the gut microbiota. Several other psychotropic drugs were found to have similar effects, but lithium notably increased the diversity of the gut microbiota.

What is the most appropriate diet for healthy gut microbiota?

Brain-gut microbiota access plays a very important role in regulating our stress responses. Undoubtedly, as we have noted, diet can impact the gut microbiota. The Mediterranean diet - and probably the Japanese diet - have been the most extensively studied and are known to be associated with optimal health metrics and outcomes. The Mediterranean diet has been scientifically linked to both a good physical health outcome and a good mental health outcome. That seems to be the most appropriate diet.

Psychobiotics, as a form of live biotherapeutic, have the capacity to treat a number of psychiatric and physical conditions. This is most notable in anxiety-related disorders, depression and irritable bowel syndrome, but increasingly we are learning how commonly used drugs impact the gut microbiota. This is, of course, not always in a positive manner conducive to good health - but can also encompass the opposite effect.

For example, we know that one of the products of eating red meat is metabolized by the gut microbiota, which can result in poor cardiovascular effects. This is one of the ways in which gut microbiota can negatively impact health. We also know that patients are sometimes digitalized with digoxin, which is a very old drug. Digoxin is actually metabolized by the gut microbiota and can be associated with significant side effects when used.

Gut microbiota is an appropriate target for intervention in relation to mental health. At Atlantia, we do clinical trials for the food, as well as those for the farming industry, from both our Cork and our Chicago offices. Atlantia purchases design trials for companies, and we also perform actual clinical trials on their behalf.

About Atlantia Clinical Trials

Atlantia Clinical Trials Ltd is a CRO that specializes in conducting studies on foods, beverages, and supplements for companies worldwide that want to scientifically validate their functional ingredients to support an: EFSA (European Food Safety Authority) Health Claim; FDA (Food & Drug Administration) Structure Function Claim; or General Product Marketing Claim.

Atlantia works with world-leading scientists (among the top cited 1% internationally, in the areas of digestive health and functional foods) at the: APC Microbiome Institute in University College Cork, Ireland; Teagasc, Moorepark, Ireland, and recognized centers of excellence globally.

Atlantia runs and operates its own clinic sites and conducts all studies to ICH-GCP standard (International Council for Harmonization of Technical Requirements for Pharmaceuticals for Human Use - Good Clinical Practice). Its team includes physician experts in digestive health, mental health (psychological stress and cognition), cardiovascular health, sports performance, metabolic disease, bone health, immune health, and healthy ageing. The clinical team also includes project managers, research nurses, nutritionists, certified sports trainers and lab researchers.

Atlantia manages all elements from protocol design, placebo manufacture, recruitment, and study execution, to sample and data analysis, statistics, and report/dossier preparation to provide a service that is technically, scientifically, and clinically superior.

The clinical studies cover a broad spectrum of functional food and beverage categories, such as dairy, cereal, probiotics, different protein forms, infant-specific foods, vitamins/minerals, plant or marine extracts, and medical foods.


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