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The Gut-Brain Axis: Can Probiotics Affect Our Mood?

If you've ever had an upset stomach and anxiety before giving a big presentation, you're well aware of the intimate connection between the gut and the brain. This is not surprising, because a major part of our nervous system resides in our gastrointestinal tract and is known as the enteric nervous system. Irritable bowel syndrome (IBS) is a classic example that demonstrates the close relationship between the digestive tract and the brain. Physical symptoms are often triggered or worsened by stress, and the majority of people with IBS also suffer from anxiety or depression.

Much research is being done on this bi-directional gut-brain axis. One of the more recent and exciting aspects of this research involve the bacteria that live in our intestines. Known collectively as the intestinal microbiota, these microorganisms weigh 1 to 2 kilograms, outnumber the number of cells in the human body by more than 10 times, and contain 150 times as many genes as our genome. The composition of gut bacteria are at least partially determined by our genetics, and can be affected by diet, stress, infection, disease, and antibiotics.


The intestinal flora affects gut motility and permeability, as well as nutrient production and absorption. However, they influence more than just our digestion. These bacteria can impact our weight and the distribution of fat in the body, memory, pain perception, immunity, and the physiological effects of stress.

Intestinal microorganisms can even impact our brains and nervous systems, partly by influencing neurotransmitter production and metabolism. For example, Lactobacilli spp. increase the activity of enzymes that breakdown tryptophan, which is a precursor of serotonin. In germ-free mice (mice born with sterile digestive tracts), norepinephrine and the serotonin precursor 5-HT were significantly lower than in control mice. Rats that were given B. infantis for 14 days had markedly increased levels of tryptophan compared to controls. Other studies have suggested that gastrointestinal microbes may play a role in the development of multiple sclerosis and autism.


Furthermore, certain probiotic strains can modulate mood. B. longum had an anti-anxiety effect in mice with mild colitis, while B. infantis had anti-depressive effects and normalized tryptophan levels in rats. L. rhamnosusgiven to healthy mice reduced increases in stress-induced corticosterone and anxiety- and depression-related behavior. Changes in gamma-aminobutyric acid (GABA) receptors are associated with anxiety- and depression-like behavior; L. rhamnosus changed GABA receptor expression in certain areas of the brain. Behavior is also influenced by intestinal flora. Germ-free mice that were colonized with gut bacteria from their own species had similar exploratory behavior as their counterparts. However, if they were colonized with bacteria from another species, their behavior was similar to that of the donors.

Gut Flora In Human

Probiotics can reduce anxiety, stress response, and improve mood in individuals with IBS or chronic fatigue syndrome (CFS). Patients with CFS often have anxiety and also have altered intestinal flora. 39 patients with CFS were randomly given 24 billion CFUs of L. casei strain Shirota (LcS) or placebo for 2 months. Stool samples revealed a significant increase in Lactobacilli and Bifidobacteria, and a significant decrease in anxiety in the probiotics group versus the control group. A specific combination of L. helveticus and B. longum reduced anxiety-like behavior in animals and had beneficial psychological effects and reduced serum cortisol levels in humans.


So how can probiotics affect mood? First, they can make neurotransmitters. For example, Lactobacillus spp. and Bifidobacterium spp. make GABA; Escherichia spp., Bacillus spp., and Saccharomyces spp. produce norepinephrine; Candida spp., Streptococcus spp., Escherichia spp., and Enterococcus spp. produce serotonin; Bacillus spp. make dopamine; and Lactobacillus spp. make acetylcholine. B. infantis and Lactobacilli both influence tryptophan metabolism. Gut microbes can also make metabolites that have neuroactive properties. For example, when they digest fiber, they produce short-chain fatty acids, which affect the nervous system.

Second, probiotics can increase docosahexaenoic acid (DHA) and arachidonic acid concentrations in the brain. These fatty acids affect neurodevelopment and neurogenesis, and their concentrations in the brain influence anxiety, depression, learning, and memory.

Third, probiotics can affect brain activity. A combination of B. lactis, L. lactis, L. bulgaricus, and Streptococcus thermophilus altered brain activity in the region involved in anxiety disorders during an emotional reactivity test in human subjects.


Future directions in research involve determining which strains affect which psychological and neurological disorders, with the goal being the development of specific probiotic combinations to treat such conditions. Another interesting issue being studied is how early events in our lives shape the intestinal microorganism populations we have as adults. We are born with a sterile gut, but vaginal delivery introduces certain bacterial species to the neonatal digestive tract; babies born through Cesarean section miss out on this exposure. Furthermore, breast-fed babies get lots of Lactobacilli growth in their intestines from the lactose in breast milk, which is something bottle-fed babies don't get. Antibiotics and their effects on intestinal flora is another concern. A 1-week course of antibiotics can cause changes in the gut microbiota for up to two years. Although antibiotics are beginning to be prescribed with more caution, giving them to children for chronic ear infections is still a common practice. Research has shown that intestinal microbes influence neurodevelopment in young animals, which makes us wonder what effect these changes to a child's gut bacteria have on their developing brains and adulthood behavior and disorders. So, the next time you eat some yogurt, remember that you're not only doing your digestive tract a favor, but also your brain!


When selecting yogurt, make sure that they are labeled as having live active cultures. We don't know enough yet about which strains affect human neurophysiology. I have my patients eat a variety of probiotic-rich foods, so that they get a wide range of good bugs. Examples of other probiotic-rich foods include kefir, kombucha, fermented foods such as sauerkraut, kim chi, miso soup, and tempeh. Again, they all contain different strains of bacteria, so it's good to eat them all to get a variety of good bugs.

Eating fiber or prebiotics is a good habit, as the probiotics feed on these. Also, trying to avoid antibiotics as much as possible, and taking probiotics during or after an antibiotic regimen in order to re-populate the gut.


Bercik P, Collins SM, Verdu EF.Microbes and gut-brain axis. Neurogastroenterol Motil. 2012 May;24(5):405-13.

Bienenstock J, Collins S. 99th Dahlem conference on infection, inflammation and chronic inflammatory disorders: psycho-neuroimmunology and the intestinal microbiota: clinical observations and basic mechanisms.ClinExpImmunol. 2010 Apr;160(1):85-91.

Cryan JF, Dinan TG. Mind-altering microorganisms: the impact of the gut microbiota on brain and behaviour.Nat Rev Neurosci. 2012 Oct;13(10):701-12.

Cryan JF, O'Mahony SM. The microbiome-gut-brain axis: from bowel to behavior. Neurogastroenterol Motil. 2011 Mar;23(3):187-92.

Forsythe P, Sudo N, Dinan T, Taylor VH, Bienenstock J. Mood and gut feelings.Brain Behav Immun. 2010 Jan;24(1):9-16.

James WP, Garza C. Summary of the 24(th) Marabou Symposium: Nutrition and the human microbiome. Nutr Rev. 2012 Aug;70Suppl 1:S87-94.

Maiko Ochi, N.D., LAc. practices at SageMED in Bellevue WA

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