By Jordan Haworth, Clinical GI Physiologist
Food and Mood
Prebiotic and probiotic therapy may be useful for patients with depression and/or anxiety disorders, according to a recent review published in the BMJ Nutrition, Prevention & Health.
Probiotics contain live microorganisms, usually bacteria or yeast, that exert some health benefits. While prebiotics are what feed and nourish the microbes already in the gut. You can read more about the differences between pre/probiotics here. Their use has been studied mostly in digestive disorders. Still, there has been a growing interest in using pre/probiotics for conditions associated with the brain, including cognition, such as Alzheimer’s and Parkinson’s, as well as mood, such as depression and anxiety.
Gut Health and The Brain
The gut-brain axis is a two-way communication between the central nervous system, which supplies the brain, and the enteric nervous system, which supplies the gut. This links the emotional and cognitive centres of our brain with the intestinal tract which may be part of the reason why people with digestive issues, such as irritable bowel syndrome (IBS), also report high rates of depression.
Recent scientific research has highlighted the importance of our intestinal microbiota in influencing the interactions between the gut and the brain, so it may be more appropriate to call it the gut-microbiome-brain axis. Specific gut bacteria can make neurotransmitters that are used by the brain for signalling – it is estimated that gut bacteria produce 95% of the body’s serotonin.
Serotonin is a neurotransmitter with many roles in the body, including appetite, sleep, sexual desire, learning and mood. Low levels of serotonin have been associated with depression. Gut bacteria metabolise the amino acid tryptophan into serotonin (5-hydroxytryptophan). However, tryptophan metabolism may shift from serotonin to kynurenine in the presence of gut inflammation. Serotonin produced in the gut cannot cross the blood-brain-barrier, but kynurenine can, and this is where it plays a critical role in neuroinflammation and psychiatric disorders. Interestingly, mice receiving gut microbiota from depressed patients exhibit increased anxiety-like behaviours, in line with higher kynurenine levels.
Studies on The Effect of Pre/Probiotics On Mood
Studies have shown that some probiotic species belonging to Lactobacillus and Bifidobacterium can suppress the kynurenine pathway, which makes them a potential treatment for brain disorders. Another possible treatment is prebiotics that support beneficial gut bacteria. When certain bacteria break down prebiotics, they produce short-chain fatty acids (SCFAs), notably butyrate. Butyrate reduces intestinal inflammation and downregulates the kynurenine pathway.
A group from Brighton and Sussex Medical School in the UK conducted a systematic review of the current literature to assess pre/probiotics as treatments for depression and anxiety. They included data from seven studies, which looked at 12 probiotic strains – eleven of these appeared to have some beneficial effect, mostly in depression outcomes.
The one probiotic strain that did not show significant improvements in depression and anxiety was Lactobacillus plantarum 299V. Despite this, it showed significant improvements in cognition and a reduction in kynurenine. This suggests L. plantarum may be more beneficial to memory than mood.
The review highlighted two studies with the highest quality. The probiotics tested in each study showed improvements in depression scores. The first looked at the benefits of a synbiotic in conjunction with antidepressant medication. A synbiotic contains both prebiotics and probiotics. In this study, it contained a multi-strain probiotic (Familact H®) with species of Lactobacillus, Bifidobacterium and Streptococcus, as well as a prebiotic called fructooligosaccharide. The synbiotic, or a placebo, was given in conjunction with antidepressant medication for patients with moderate severity depression. Patients saw significant improvements in depression with the synbiotic compared to placebo, which suggests it may be complementary to take alongside antidepressant medication.
The second study looked at a probiotic and a separate prebiotic in patients with major depressive disorder (MDD).  The patients received either the probiotic (Lactobacillus helveticus R0052 and Bifidobacterium longum R0175), the prebiotic (galactooligosaccharide), or placebo. Only those taking the probiotic had a significant improvement in MDD. In addition, the probiotic group showed a reduction in kynurenine levels. Interestingly, this is not the only study to assess the combination of L. helveticus and B. longum. Their use for the treatment of mood disorders has been approved by national health bodies in Canada and Brazil.
While disruption to the gut-brain axis is common in mood disorders, our understanding of it is still in its infancy. Much of the current research has been carried out in animal models, but gut bacteria appear to play an essential role in how we think and feel. While more research in humans is needed, probiotics and prebiotics may be an adjunctive therapy for improving emotional health. In the meantime, check out our A-Z of gut health lifestyle to keep your gut and your brain in good shape.
1. Noonan, S., Zaveri, M., Macaninch, E., Martyn, K.: Food &amp; mood: a review of supplementary prebiotic and probiotic interventions in the treatment of anxiety and depression in adults. BMJ Nutrition, Prevention &amp; Health, bmjnph-2019-000053 (2020). doi:10.1136/bmjnph-2019-000053
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