Trillions of bacteria reside in the human gut and play a crucial role in gut-brain communication through our gut microbiome. Our gut consists predominantly of bacteria but also includes viruses, protozoa, fungi, and archaea. Given the enormous genetic potential of the microbiota, it is unsurprising that it appears to play a role in almost all physiological processes in the human body. When our microbiome produces substances, our bodies may abort these, eventually finding their way to our brain. The link of how our microbiome affects mental health stems from scientific findings. Some of these studies featured in this article have been conducted on animals and summarize advances in the field based on human research.

Understanding our gut microbiomes!

To understand the complexity of our microbiomes, it first makes sense to know what a microbiome is? In this article, I will be using microbiome(s) and gut microbiome interchangeably. Microbiome means the plethora of bacteria that resided in our gut. The gut microbiome vastly exceeds the amount of human DNA present in the body, such that, for every human gene, we have over 100 bacterial genes. Bacterias can be classified as "good" vs "bad." Due to the number of strains, I will not dive into specific bacterias unless mentioned in a study, which the effects have been noted and observed. It makes sense from a high level that what we eat is ultimately digested by enzymes and bacterias alike. We are what we eat nonetheless.

How does food intake alter our gut microbiome?

Experiments show that dietary alterations can induce considerable, temporary microbial shifts within 24 hours, with a complete reversal to baseline within 48 hours. Our bodies are amazing machines that can adapt and reset quickly. Even high-stress situations can alter our microbiome, which tells me we are sensitive to our environment and interactions—studies conducted about different food intakes, such as fats, proteins, carbohydrates. Researchers also looked at diet types; the Mediterranean diet, vegans and vegetarians, etc.

The conclusion seems to be favourable towards the Mediterranean diet'. Distinguished by a beneficial fatty acid profile rich in monounsaturated and polyunsaturated fatty acids, high polyphenols, and other antioxidants, including high fibre intake and other low glycemic carbohydrates, relatively more significant vegetable than animal protein intake. The literature suggests that diet can modify the intestinal microbiome, which profoundly impacts overall health and well-being.

What does a healthy microbiome entail?

Due to the relatively new field of research, I don't think we know enough to distinguish what bacteria is ultimately healthier versus not. There are billions of strains of bacteria that flourish in our gut. To determine which bacteria we need more of compared to others is a demanding task with various outcomes. People are unique, and perhaps one set of bacteria is required more for your body than mine. More studies are needed to begin to point out specific strains. A healthy diet with regular exercise and good sleep is a great baseline we can all achieve. Adding in supplementation at that point will help our bodies with any imbalances.

What is the relationship between the gut microbiome and mental health?

The mechanisms gut bacteria communicate with and influence the central nervous system are gradually being uncovered and span neural, endocrine, and immune systems. As this area of study is relatively new, there seems to be a correlation and findings that our gut microbiome does affect our mental health in numerous ways. Science indicates that our gut plays a significant role in our overall health, more so than initially thought. As such, there have been studies that show potential treatments to mitigate mental health-related issues.

Treatments and Findings from Research Studies

Treatments included various prebiotics, probiotics and synbiotics. Prebiotics has been studied for potential antidepressant properties, but a recent meta-analysis has found no benefit over placebo concerning mood improvement.  

Fecal transplants, effectively transplanting the "healthy" individual's fecal microbiome to the affected patient, positively affected those who suffer from mental health issues.

A new and exciting method of altering the microbiome is through the use of phage therapy. Phages, short for bacteriophages, are viruses that infect specific bacteria. This particular therapy has been around for some time but has recently regained interest from the science community as another treatment method.

Postbiotics and Butyrate appear to have neuroprotective properties and have demonstrated antidepressant potential in animal models, although human studies are lacking. In Probiotic trials, ten clinical trials with a total of 1349 patients found a significant mood improvement in individuals with pre-existing depressive mood symptoms after an eight-week trial of probiotics compared to placebo.

In a study out of Belgium with 1,045 people, they found Coprococcus and Dialister were depleted in patients with depression irrespective of antidepressant treatment, and butyrate-producing Faecalibacterium and Coprococcus bacteria were consistently associated with a higher quality of life measures.

Brain Imaging trials investigated the effect of four-week probiotic treatment on brain activity in healthy volunteers. Probiotics were found to improve impulsivity and decision-making compared to placebo. Still, no pain, quality of life, anxiety or depressive symptoms in a double-blind, placebo-controlled, randomized study in patients diagnosed with fibromyalgia. Patients with depression have significant compositional differences compared with healthy controls.

Participants receiving probiotics reported higher positive affect (mood state). They showed differences in the BOLD (blood-oxygen-level-dependent) signal pattern in response to emotional decision-making and emotional recognition memory tasks compared to participants receiving placebo.

A trial evaluating the effect of multiple probiotics on anxiety in healthy college students found improvement in panic, anxiety, neurophysiological anxiety, negative affect, worry, and improved negative mood regulation in the probiotic group compared to the placebo group.

Specific Mental Health Disorders

  • Bipolar Disorder (BD) some studies have found BD to be associated with a decreased bacterial diversity compared to healthy controls.
  • Anxiety and Stress undertook a 4-week trial of long-term use of Lactobacillus gasseri CP2305 (CP2305) found improvement in mental state, sleep quality, and gut microbiota under stressful conditions by attenuating the stress-induced decline of Bifidobacterium.
  • Post Traumatic Stress Disorder (PTSD) lower fecal microbial diversity was linked to the type of gut microbiota.
  • Pregnant women who had an adverse childhood had more significant "bad" bacteria than those who didn't. The uterus is a sterile environment, and that bacterial colonization begins during birth varies according to the mode of delivery of vaginally delivered infants resembling the maternal vaginal microbiome and that of those provided by cesarean section. Microbiome differences based on delivery mode are no longer evident by the sixth week of life.
  • Human Touch, Interestingly, while environmental proximity to another person does not increase the similarity of microbiome composition between individuals, the quality of human relationships does seem to have an impact.

Wrapping up

Our gut microbiomes play an integral part in overall health, and not surprisingly, our mental health. This study area is still relatively new; however, optimistic signals and encouraging study results have emerged. Ultimately more studies are needed to fully understand the impacts of probiotics, prebiotics and other means of intervention concerning mental health.

Although much remains to be discovered about how the gut microbiome influences the brain and mental functioning, nutrition and gut health are beginning to represent an essential component in holistic psychiatric care. The sentiment that one might consider most appropriate regarding the field of the gut microbiome and nutritional psychiatry is cautious but justifiable optimism.

Reviewed studies report improved anxiety and stress symptoms using probiotics in healthy subjects and altered gut microbiome in patients with generalized anxiety disorder compared to healthy controls. Studies support the relevance of the bidirectional gut microbiota–brain communication in mood disorders in humans, such as the effect of probiotics on brain connectivity and mental health outcomes and pregnancy-related Stress on gut microbiota in the newborn child. However, additional studies need to be conducted as current sample sizes and demographics have been limited.


Our gut microbiome and bacterias makeup play a significant role in our health. Studies have linked microbiome and absorption of certain substances created by our gut, being absorbed and ultimately making their way to our brains. We have over 100 bacterial genes for every human gene, which is essential in living a healthy life. Various diets and food intake affects the balance of bacteria in our gut. Our bodies can adapt and change bacteria culture within 4 hours of digestion. Treatment options include probiotics, prebiotics, fecal transplants and phage therapy. Most treatments resulted in a 'good' bacteria growth, leading to positive results in various mental health conditions. Gut microbiomes play an integral part in overall health in unison with mental health.

CITE & References

Butler MI, Mörkl S, Sandhu KV, Cryan JF, Dinan TG. The Gut Microbiome and Mental Health: What Should We Tell Our Patients?: Le microbiote Intestinal et la Santé Mentale : que Devrions-Nous dire à nos Patients? Can J Psychiatry. 2019 Nov;64(11):747-760. doi: 10.1177/0706743719874168. Epub 2019 Sep 17. PMID: 31530002; PMCID: PMC6882070.