Does Your Gut Shape Your Mood?

The myriads of bacteria in the gut help the body digest food and stimulate the immune system, and they also impact your mood. Seriously, your intestines host 2000 different known species of bacteria.  Each person has a different proportion of bacterial species and strains in his or her gut.

The Gut-Brain Axis

The gut-brain axis is a bidirectional communication network between the intestines to the central nervous system. This network also significantly influences the endocrine and immune systems as well as metabolism. Additionally, the autonomic nervous system, hypothalamic-pituitary-adrenal axis, and nerves within the gastrointestinal (GI) tract, all link the gut and the brain.¹ Simply put, the brain “talks” to the gut, and conversely the gut communicates to brain, and consequently affects cognition and mood.

Gut Microbes and Brain Development

Research now shows that normal adult brain function depends on the presence of gut microbes during development.² Studies suggest the composition of the microbiota has the potential to either decrease or increase the activity of hippocampal brain-derived neurotrophic factor (BDNF), an important protein involved in neuronal plasticity and cognition. BDNF acts as a fertilizer for synapses in the brain.

Gut Bacteria Affects the Brain

Your gut bacteria help to determine how many new brain cells you produce; how permeable your brain is to toxic substances; and how well-nourished and protected your brain cells are.³

Diet, stress, and other lifestyle factors can also shape the distribution and balance of your gut microbiome. Some strains of intestinal bacteria manufacture compounds that have positive effects on the nervous system while other species of bacteria produce byproducts that exert detrimental effects on the brain and your mood. There have been a few trials where patients with depression have been given probiotic treatments and responded favorably. No doubt about it: gut bacteria seriously affect mood and influence behavior. ⁴

Anxiety, depression, and autism spectrum disorders now have well-established links to functional GI disruptions. Irritable bowel syndrome can have a psychological conpoment associated with gut dysbiosis in which harmful bacteria outnumber beneficial bacteria.⁵ 

Stressed? Could be Gut Bacteria!

Bienenstock and Forsythe used a “social defeat” scenario in which smaller mice were exposed to larger, more aggressive ones for a couple of minutes daily for 10 consecutive days. The smaller mice showed signs of heightened anxiety and stress–nervous shaking, diminished appetite, and less social interaction with other mice. The researchers then collected fecal samples from the stressed mice and compared them to those from calm mice. There was an imbalance in the gut microbiota of the stressed mice. The gut and bowels have a very complex ecology. Unfortunately, the less diversity there was, the greater the disruption to the body.

These scientists then fed live bacteria (probiotics) from the calm rodents to the stressed mice.  They were actually able to control the moods of the anxious mice by feeding them healthy microbes from fecal material collected from the calm mice. Using MRI technology, they also noted favorable changes in brain chemistry.  Interestingly enough, not only did the behavior of the mice substantially improve with the probiotic treatment, but it also continued to get better for several weeks afterward. The MRI technology enabled the scientists to see certain chemical biomarkers in the brain when the mice were stressed and when they were taking the probiotics. ⁶

Nerve Attack and Depression: The Gut Connection

The thinning of myelin (nerve insulation) is seen in such conditions as multiple sclerosis, depression, and social isolation. Myelin is the insulating sheath surrounding the axons of many nerve cells that allows for faster electrical impulse conduction. Without healthy myelin, nerve impulses from one nerve cell to another are significantly slowed. Rodent studies found that bacteria from the gut produce metabolites that can affect the myelin content in the brains of mice and induce depression-like symptoms.⁷

In one remarkable experiment, scientists relocated fecal bacteria from the gut of depressed mice to genetically distinct mice that did not manifest any signs of depression. What happened? The study showed that the transfer of microbiota was sufficient to induce social withdrawal behaviors and even change the activity of myelin genes and myelin content in the brains of the recipient mice. In other words, bacteria in your gut produce metabolites that have the capacity to affect the myelin content and your mental health irrespective of the genetics.⁸

Help for Depression

In a randomized, double-blind study, administration of a combination of L acidophilus, L casei, and Bifidobacterium bifidum was given to participants who were clinically depressed. After 8 weeks of intervention, patients who had major depression and who received probiotic supplements experienced significantly fewer signs and symptoms associated with depression. Once more, they had a lower inflammatory marker, known as hsC-reactive protein. This is important because depression fuels inflammation in the brain, and the converse is also true. ⁹ A systematic review of ten clinical trials found that most of the studies found positive results on measures of depressive symptoms.¹⁰ More studies are warranted to which gut microbes help depression

I am not saying that every case of depression is caused by an imbalance between the good and harmful gut bacteria, or can be treated successfully by probiotics. Other causes of depression such as harmful lifestyle practices, cognitive distortions, relationship problems, and spiritual issues need to be addressed. Gut bacteria, however, should not be overlooked in the treatment of this condition. Why? Emerging data reveals that there is communication between the gut and the brain in anxiety, depression, cognition, and autism spectrum disorder.¹¹

1. Appleton J., The Gut-Brain Axis: Influence of Microbiota on Mood and Mental Health. Integr Med (Encinitas). 2018 Aug; 17(4): 28–32.[↩]
2. California Institute of Technology. “Microbes help produce serotonin in gut.” ScienceDaily. ScienceDaily, 9 April 2015. www.sciencedaily.com/releases/2015/04/150409143045.htm[↩]
3. Sharon G. The Central Nervous System and the Gut Microbiome. The Central Nervous System and the Gut Microbiome. Cell. 2016 Nov 3;167(4):915-932.[↩]
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5. Appleton J., The Gut-Brain Axis: Influence of Microbiota on Mood and Mental Health.Integr Med (Encinitas). 2018 Aug; 17(4): 28–32. [↩]
6. S. Leclercq, P. Forsythe, J. Bienenstock. Posttraumatic Stress Disorder: Does the Gut Microbiome Hold the Key?The Canadian Journal of Psychiatry, 2016; 61 (4): 204 DOI: 10.1177/0706743716635535[↩]
7. Alimentary Pharmabiotic Centre – University College Cork. “Early gut bacteria regulate happiness.” ScienceDaily. ScienceDaily, 12 June 2012. www.sciencedaily.com/releases/2012/06/120612115812.htm[↩]
8. The Mount Sinai Hospital / Mount Sinai School of Medicine. “Transfer of gut bacteria affects brain function, nerve fiber insulation.” ScienceDaily. ScienceDaily, 20 April 2016. www.sciencedaily.com/releases/2016/04/160420104209.htm[↩]
9. Akkasheh G. Clinical and metabolic response to probiotic administration in patients with major depressive disorder: A randomized, double-blind, placebo-controlled trial. Nutrition. 2016 Mar;32(3):315-20.[↩]
10. Wallace CJK, Milev R. The effects of probiotics on depressive symptoms in humans: A systematic review. Ann Gen Psychiatry. 2017;16:14.[↩]
11. Sharon G. The Central Nervous System and the Gut Microbiome. The Central Nervous System and the Gut Microbiome. Cell. 2016 Nov 3; 167(4):915-932.[↩]