Brain-gut axis

Pioneering probiotics application in the brain-gut axis

We have all experienced the incredible link between our brain and our gut. Just when we need to be at our best (i.e. an important event, an exam, the first day in a new job, a big meeting, etc.), our gut seems to have a mind of its own. Who is to blame? More and more scientific evidence points at the Brain-Gut axis, also known as our “second brain”, referenced amongst scientists as a bi-directional dialogue between our nervous and digestive systems.

Probiotics have spurred many hopes and promises as a result of their interactions with the Brain-Gut axis. As a pioneer in the probiotic industry since 1934, Lallemand Health Solutions has committed to contribute strongly in this new field of research since 2006; documenting the specific benefits of three of its flagship probiotics (Probio’Stick®, Lacidofil®, Bifidobacterium bifidum Rosell®-71) with the publication of 17 mechanistic studies and 4 clinical studies.

A historical perspective on the science in the BGA with LHS probiotics

Learning from rodents

The idea that a dietary intake of live beneficial bacteria could support mood (“melancholia”) has been published as early as 1910 by Porter. However, it took another sixty years to get the first indications that the microbiota was involved in the brain-gut dialogue [Bähr, 1970].  In the years that followed, in vivo studies using microorganisms-challenged models demonstrated an impact of microbiota on behavior, through the activation of immune-neural mechanisms.

It is only in 2006 that the first in vivo study testing a probiotic in a model of chronic psychological stress was conducted. Using the water avoidance stress, Zareie et al. showed that Lacidofil® was effective in maintaining intestinal structure during stress. The following year, the same team [Gareau 2007] confirmed these results in a different model. Moreover, they also showed an improvement of gut permeability with the same probiotic and the fact that the probiotic was able to normalize the level of stress hormone in the body.

In 2008, Pr Desor, an expert in cognitive science from Pointcaré University in France, demonstrated that in a specific stressful condition, rats given another probiotic, Probio’Stick exhibited relative behavior to valium-treated rats.

In 2010, Gareau et al. conducted a new in vivo study with Lacidofil® confirming the involvement of the HPA axis to mediate probiotics benefits in stress models. The team went then one step further by looking at the effect of the microbiota on cognitive functions. They demonstrated that the microbiota plays a role in memory formation and that the probiotic helps with this function.

Our probiotics have been studied in available stress models in rats: myocardial infarction, water avoidance and maternal separation.

In parallel, the team of Prof. Rousseau, from Montreal University in Canada, conducted several studies using the model of myocardial infarction. This model of depression is accompanied by a production of immune stimulation cytokines, the naturally occuring structural remodelling (apoptosis) of brain cells, in the limbic system, the parts of the brain implicated in emotions. It is also associated with an increased intestinal permeability and depression-like behavior. In a first study, Girard et al, 2009 showed that the prophylactic administration of Probio’Stick® prior to MI was able to support rats limbic system by reducing apoptosis/structural changes. This innovative study showed a direct link between probiotic administration and the brain. They also showed that the probiotic treatment was able to significantly reduce abnormal behavior signs in rats.

Later, an INRA team in France lead by Dr. Ait-Belgnaoui showed that pre-treatment with Probio’Stick formula had direct effects at the brain level in a rodent stress model and that these effects were strain specific.

In 2013, Ohland demonstrated in wild-type mice (“normal” mice) and IL-10 deficient mice (lowered immune system) that anxiety-like behavior and memory were negatively affected by Western-style diet depending on the inflammatory status. L. helveticus Rosell®-52 supplementation can help support a wild-type state..

In 2016, Cowan team (University of New South Wales, Sydney, Australia) demonstrated that Lacidofil® could restore normal developmental trajectories of emotion-related behaviors in infant rats exposed to early life stress. Then, they went one step further with experiments in male rats over two generations to assess whether generational effects of stress on learning is evident in infant offspring and whether probiotics can contribute to reverse the effects of paternal stress on offspring. The team demonstrated, for the first time, longer-lasting aversive associations and greater relapse in the offspring (F1 generation) of rats exposed to maternal separation (F0 generation) and that those effect of paternal stress on the development of memory and extinction in offspring were reversed by the probiotic.

The first clinical evidence

So many convincing results in animal models could only give way to a growing wave of clinical studies. Between 2005 and 2018, we have identified a total of 27 published clinical studies using probiotics in the brain-gut axis. 15 of them concerned severe or pathological condition, the other 12 were conducted on healthy adults with a non-pathologic level of stress; we shall focus on the latter category.

In 2008, for the first time in human, a specific probiotic formula, Probio’Stick®, was demonstrated to improve gastrointestinal symptoms related to stress in healthy people.

A second pioneering study on this probiotic was published in 2010. It was clinically proven for the first time that a probiotic could help moderate general feelings of anxiety and promote a healthy mood balance in those experiencing mild to moderate stress resulting from life events, in other words: healthy active people [Messaoudi 2010]. In this randomized double-blind placebo-controlled study, psychological signs of occasional stress were evaluated using a range of validated psychological assessment scales. One month daily intake of Probio’Stick significantly improved general signs and the subject’s ability to cope with the stress of the events of everyday life. Moreover, those results were correlated by a decrease in urinary free cortisol levels, a biomarker for stress. It was concluded that the improvement in symptoms among subjects with low to mild stress levels suggests the value of prophylactic intake of Probio’Stick in terms of digestive comfort and general well-being.

Scientists have validated university examination as a model of psychological stress in healthy adults. It is a relatively easy way to access a large population of healthy individuals submitted to a comparable psychological stress at the same time. A randomized clinical trial published in 2015 was performed in the US at the University of Florida on healthy students for six weeks around semester exams. Primary outcomes of the study were the effect of three different probiotic strains on the proportion of days with cold/flu-like symptoms in this model of occasional stress, in order to evaluate the probiotic effect on natural defenses (Langkamp- Henken et al., 2015). However, levels of self-reported stress, salivary cortisol, and gastrointestinal symptoms were also evaluated in the trial. Further statistical analysis of the study enabled to evaluate the effect of the probiotics on stress and stress-related symptoms. The results of this secondary analysis (Culpepper et al., 2016), indicated that with probiotic strain Bifidobacterium bifidum Rosell®-71:

  • Stressed-induced occasional diarrhea symptoms were reduced
  • The effect of lack of sleep on intermittant stress was diminished: at fewer hours of sleep, participants reported lower stress levels when on Bifidobacterium bifidum Rosell-71 than on placebo.

Microbiota-brain-gut axis, what we know on how it works

The stress response in mammals is mainly regulated and controlled by the Hypothalamus-Pituitary-Adrenal axis (HPA axis). This dominant component of the neuroendocrine system controls stress response and regulates bodily processes such as digestion and immunity. One of the main biochemical hormonal mediators of this pathway is the glucocorticoid, cortisol.

External factors (i.e. a traffic jam, fear, hostility, work, exams, etc.) can be the culprit of the stress felt in everyday life. In the efferent pathway (from the brain to the gut), this stress causes a stream of hormones along our HPA axis which releases cortisol. Cortisol reacts systemically and leads to the opening of tight junctions (the space between epithelial cells in the intestine), and thus induces a loss of intestinal permeability.

Opportunistic microorganisms or antigens can then enter the body, causing a reduction of immune response, and stimulate a release of additional cortisol. Our brain and our gut communicate by means of an intestinal network comprising 100 to 200 million nerves. This is why the gut is also known as the second brain. The enteric nervous system allows a bi-directional communication through blood vessels (after passing through intestinal barriers) and nervous pathways through the vagus nerve. It is essential that our central nervous system and our enteric nervous system communicate from the brain for digestion (motility) but also satiety and abdominal discomfort.

It has been recently hypothesized that signals from the brain can impact the microbial composition to meet the body’s needs. The intestine can also send a message to the brain through neuroactive compounds such as serotonin which goes systemically through blood circulation up to the brain. The gut microbiota is a major factor in this communication as it metabolizes ingested food into tryptophan, gamma-aminobutyric acid (GABA), opioid-like compounds and other potentially neuroactive biochemicals. Specialized intestinal endocrine cells, enterochromaffin, located in the gut produce large quantities of the neurotransmitter serotonin from tryptophan and histamine which may be responsible for some of the major effects on signaling to the brain. Some bacterial species are able to produce these neuroactive indolamines from tryptophan. Bacteria can also modulate the release of cytokines from our enteric immune cells which in turn through systemic actions can modulate our limbic system (i.e. the portion of the brain involved in emotion, learning, and memory) via the vagus nerve. The interactive relationship between the immune cells and the microbiota has been extensively studied and is reasonably well-defined. Additionally, some of the microbiota can digest carbohydrates into Short Chain Fatty Acids (SCFAs, such as acetate and butyrate) which are then taken up by the intestinal epithelial cells. These metabolites may also have a role in enteric neuro-signaling.

Everything that can cause a dysbiosis can have an impact on the Brain-Gut axis. This could occur as a result of changes in diet, ageing, lifestyle changes, or after taking antibiotics/other medications. The dysbiosis means a change in the microbiota and potentially a loss of bacterial diversity which could disrupt the positive mechanisms and result in the production of different neuroactive signals, less production of metabolites, and production of more cytokines. All of these can send negative messages to the brain which can result with loss of neuroplasticity in the limbic regions of the brain associated with mood, behavior, learning, and memory (i.e. the hypothalamus, hippocampus and the amygdala). Certain probiotics, when administered orally, have been shown to reduce the cytokine production promotint the healthy brain structure, and decrease feelings of mild anxiety in healthy adults