There is an emerging field of nutritional mental health that looks at holistic factors when treating cognitive disorders.
Mental health is not merely a question of what happens inside your head. It’s difficult to fully separate what occurs in your brain, and what occurs in the rest of your body.
In particular, new research is shedding light on how your gut health may influence cognition.
Your intestines are filled with helpful bacteria that help digest food and regulate metabolism. Some researchers estimate that about 100 trillion bacteria from up to 1000 distinct bacterial species co-inhabit the human GI tract, in different proportions among individuals.1Aziz Q, et al. <a href=”https://www.ncbi.nlm.nih.gov/pubmed/23279728″
Keeping this biome of microorganisms happy and healthy could play a huge role in lowering inflammation, oxidative stress, and improving mitochondrial function, all of which can have a drastic impact on systems well outside the GI tract, including cognition.2Bonnie J. Kaplan, et al. <a href=”https://www.researchgate.net/publication/271908350_The_Emerging_Field_of_Nutritional_Mental_Health_Inflammation_the_Microbiome_Oxidative_Stress_and_Mitochondrial_Function”
How Does Gut Bacteria Affect The Brain
Researchers are just now exploring all the different ways that the gut biome can influence mental health.
Advances in sequencing technology have enabled the exploration of the role of the gut microbiota in a broad range of neurological and psychiatric disorders and diseases.
This enables large-scale analysis of self-reported conditions such as the Earth Microbiome Project, which includes sample contributions from over 10,000 citizen-scientists together with an open research network to compare human microbiome specimens.3McDonald D, et al. <a href=”https://www.ncbi.nlm.nih.gov/pubmed/29795809″
What researchers are finding is that gut microbiota plays an active role in processes linked to brain development and physiology, psychology, and behavior.
Neural, endocrine, and immune communication lines tightly link the human gut microbiota with the host central nervous system. In other words, your brain and stomach are inextricably connected.4Cryan JF, Dinan TG. <a href=”https://www.ncbi.nlm.nih.gov/pubmed/22968153″
Microbial metabolism can influence the brain through the production of hormones, neurotransmitters, immune system modulators, and short-chain fatty acids like Butyrate.
Tryptophan metabolism in the gut produces important neurotransmitters like serotonin and melatonin, which are critical for mental and emotional regulation.5Gao J, et al. <a href=”https://www.ncbi.nlm.nih.gov/pubmed/29468141/”
Diseases Affected By Gut Bacteria
The dysregulation of gut bacteria can at least partially cause multiple diseases.
Multiple sclerosis (MS) is a chronic neuroinflammatory disease of the central nervous system. Several studies show that patients with MS exhibit poor metabolism in their gut biome.
Although the mechanism by which this might influence MS is unknown, it is clear that some connection deserves further study.6Freedman SN, et al. <a href=”https://www.ncbi.nlm.nih.gov/pubmed/29204955/”
Emerging data reveal an interaction between psychiatric disorders and Irritable Bowel Syndrome (IBS). This includes generalized anxiety disorder, panic disorder, major depressive disorder, and bipolar disorder.
IBS is partially characterized by disturbed gut flora and inflammation.7Fadgyas-Stanculete M, et al. <a href=”https://www.ncbi.nlm.nih.gov/pubmed/25408914/”
Gut flora may be one of several environmental factors that contribute to Autism.8Karimi P, et al. <a href=”https://www.ncbi.nlm.nih.gov/pubmed/28413424/”
Gut Microbiome and Depression
Major Depressive Disorder (MDD) is a debilitating psychiatric illness that affects an estimated 300 million people worldwide.
Despite significant advances in our understanding of MDD, existing knowledge is incomplete, treatments are often inadequate, and new insights are urgently needed.
One new area of investigation related to depression is the gut microbiome.
A growing body of literature supports and characterizes a gut-brain axis, and investigates a possible role of gut microbiome dysfunction in major depression.
Inflammation and gut barrier health often show up in studies between the gut microbiome and depression. Depression and anxiety symptoms increase with functional gut disorders.9Maes M, Kubera M, Leunis JC. <a href=”https://www.ncbi.nlm.nih.gov/pubmed/18283240/”
Animal studies support the idea that a disruption of the microbiome may play a role in depression-like behaviors. Antibiotic administration in mice leads to dysbiosis, depression-like behavior, and altered neuronal hippocampal firing.
However, this can be reversed by repopulating their intestines with probiotics.11Guida F, et al. <a href=”https://www.ncbi.nlm.nih.gov/pubmed/28890155/”
In cell culture studies, researchers found that gut microbiota makes precursors to neurotransmitters, such as tryptamine.
This can lead to an increase in important transmitters such as serotonin, dopamine, and norepinephrine in the brain, which are primary factors in mood and emotional stability.12Williams BB, et al <a href=”https://www.ncbi.nlm.nih.gov/pubmed/25263219/”
One animal study found that probiotics containing Lactobacillus plantarum led to both antidepressant-like effects in mice as well as increases in levels of serotonin and dopamine in the brain.
Hopefully, more trials on human subjects will be forthcoming soon.14Liu WH, et al. <a href=”https://www.ncbi.nlm.nih.gov/pubmed/26522841/”
Can Gut Bacteria Cause Anxiety and Depression?
Abnormalities in this microbiota-gut-brain axis have emerged as a key component in depression, leading to more research attempting to understand the neuroactive potential of the products of gut microbial metabolism.15Caspani G, et al. <a href=”https://www.ncbi.nlm.nih.gov/pubmed/31646148″
The future of gut-brain axis research involves moving towards the mechanisms underlying the relationship between gut bacteria and depressive behavior.
Gut microbes might affect depressive behavior through mechanisms including direct stimulation of central receptors, peripheral stimulation of neural, endocrine, and immune mediators, and epigenetic regulation of histone acetylation and DNA methylation
In experiments, Brain Derived Nerve Factor (BDNF) expression was reduced in the brains of “germ-free” mice, and this reduction in BDNF correlates to increased anxiety behavior and progressive cognitive dysfunction.16Carlino D, et al. <a href=”https://www.ncbi.nlm.nih.gov/pubmed/23242909″
Gut Bacteria and Alzheimer’s Disease
Alzheimer’s disease (AD) is a progressive neurodegenerative disorder and the leading cause of cognitive impairment in industrialized societies. The cause of AD is unknown, although the major risk factor for AD is age.17James M. Hill, et al. <a href=”https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4058571/”
There is now evidence that AD may be related to gut biome health.
Most of the changes seen in AD, such as inflammation, brain cell atrophy, immune dysfunction, and cognitive deficits, can also be a consequence of microbial infection.18Heintz C, Mair W. <a href=”https://www.ncbi.nlm.nih.gov/pubmed/24485451/”
There is currently no cure for AD, and it remains unclear how AD originates and propagates throughout the brain and central nervous system.
However, results from recent studies indicate that a significant portion of AD gene signals may be related to environmental and epigenetic factors, including microbes.
The potential contribution of pathogenic microbes to aging and AD is increasingly recognized.19James M. Hill, et al. <a href=”https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4058571/”
Cyanobacteria in the gut can produce BMAA, a neurotoxic amino acid not normally incorporated into protein. This is especially true during periods of stress and inflammation.
BMAA may be a primary contributor to amyloid plaques, which are a hallmark of Alzheimer’s Disease.20Brenner SR. <a href=”https://www.ncbi.nlm.nih.gov/pubmed/23146671″
Lactobacillus in the gut is capable of metabolizing glutamate to produce gamma-amino butyric acid (GABA), the major inhibitory neurotransmitter in the CNS. Dysfunctional GABA-signaling may play a role in cognitive impairment, including AD.21Aziz Q, et al. <a href=”https://www.ncbi.nlm.nih.gov/pubmed/23279728″
Gut Bacteria and Neurotoxicity
The microbiome can influence neuroprotection and overall brain health.
Gut bacteria seem to interact with the N-methyl-D-aspartate (NMDA) glutamate receptor, a prominent component in the nervous system that regulates synaptic plasticity and cognition.22Shaheen E. Lakhan, et al. <a href=”https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3677126/”
For example, the NMDA-targeting and oxidative-stress-inducing neurotoxin BMAA found elevated in the brains of patients with cognitive damage.
Researchers believe that BMAA is caused by gut microbe dysregulation. So, it stands to reason that lowering BMAA through regulating gut bacteria may actually protect the brain.23Schneider C. <a href=”https://www.ncbi.nlm.nih.gov/pubmed/2314667″
Gut-Brain Connection and Diet
Ensuring a healthy gut biome (and, therefore, brain) starts with proper nutrition. Eating certain foods can help you maintain the right balance of microbes in your gut. These include:
- Fermented foods (sauerkraut, kimchee, cheese, kefir, yogurt)
- Starchy roots
- Meat and fish.24Robert Hutchins. <a href=”https://scholar.google.com/scholar_lookup?title=Microbiology+and+Technology+of+Fermented+Foods&author=R.W.+Hutkins&publication_year=2018&”
target=”_blank”>Microbiology and Technology of Fermented Foods,</a> 2006
How To Improve Good Gut Bacteria
Fermented foods contain probiotics, which are beneficial live bacteria that can populate your gut after consumption. It is especially important to consume these foods so that you can form and maintain colonies of helpful bacteria.
Many of the other foods, especially legumes and vegetables, contain prebiotics, which are starches that help nourish the good bacteria in your stomach and help them flourish.
Consuming a balanced diet full of both probiotics and prebiotics can go a long way towards keeping your stomach bacteria happy.
Do Probiotics Affect Your Mood?
Besides the potential application of probiotics in the treatment of various health conditions like allergies, GI and urogenital tract infections, inflammatory disease, cystic fibrosis, and certain cancers, there is an increasing interest of positive microbiome effects toward the central nervous system.25Duncan SH, Flint HJ. Probiotics and prebiotics and health in ageing populations, Maturitas. 2013
For example, there is preliminary research on the influence of probiotics and nutritional factors on cognition and mental health.26Camfield DA, et al. <a href=”https://www.ncbi.nlm.nih.gov/pubmed/21338538″
One double-blind, placebo-controlled study saw a significant improvement in mood after six weeks of consuming a probiotic blend. Researchers observed a reduction in depressive mood state, anger, and fatigue, and an improvement in sleep quality.
These findings suggest that probiotics may improve psychological well-being by improving aspects of mood and sleep quality.27Marotta A, et al. <a href=”https://www.ncbi.nlm.nih.gov/pubmed/30971965″
A meta-analysis of 10 clinical trials found significant improvements in the moods of individuals with mild to moderate depressive symptoms and non-significant effects in healthy individuals. 28Ng QX, et al. <a href=”https://www.ncbi.nlm.nih.gov/pubmed/29197739″
However, this metanalysis did note some discrepancies between studies that made comparison difficult. Hopefully, there will be further high-quality studies on this subject in the future.
Which Probiotics Are The Best?
What probiotic is best? The short answer is it depends on the person. Each strain should be used for your specific situation.29Francisco Guarner, et al. <a href=”https://www.worldgastroenterology.org/guidelines/global-guidelines/probiotics-and-prebiotics/probiotics-and-prebiotics-english”
For example, Lactobacillus plantarum strain 299v has been shown to effectively reduce IBS symptoms, whereas Lactobacillus plantarum strain MF1298 was found to actually worsen IBS symptoms.30Ducrotté P, Sawant P, Jayanthi V. <a href=”https://www.ncbi.nlm.nih.gov/pubmed/22912552″
For general physical and mental health, spore-based probiotics ( a mixture of spores of five Bacillus species) have shown to be the most effective at showing statistically significant improvement in measurements of quality of life.31Catinean A, et al. <a href=”https://www.ncbi.nlm.nih.gov/pubmed/31438618″
For mental health, B. longum, B. infanti, L. plantarum, L. helveticus, and L. Rhamnosis all have noted positive effects on mood and anxiety.
So, it is advisable to do your research on which particular strain suits your needs.