Link between gut and depression

Problems with mental health are one of the primary causes of the overall disease burden worldwide.1 According to the World Health Organization, over 300 million people are estimated to suffer from depression, which is equivalent to 4.4% of the world’s population.2

Serotonin reuptake inhibitors (SSRIs) have been the primary method of treatment for depression for many years; however they are not effective in all individuals, with success rates at about one in three.3 This has led researchers to investigate other potential factors which may be contributing to the growing body of mental dis-ease amongst the global population.

Enter: the gut. By now, it is unlikely to come as a surprise that there is a connection between the gut and the brain. However, what might be less familiar are the plethora of scientific studies which have been released over the past several years suggesting that inflammation (especially that which is generated within the gut) is a major driving factor of depression. Just like conditions such as obesity, heart disease and diabetes; the research is showing us that the origins of depression are inflammatory – with several neurological consequences.

And so, with this new understanding of the brain, scientists are looking at novel and alternative therapies for depression. One such therapy which has been yielding rather impressive results is the use of probiotics.

In this article, we will be looking at the new paradigm of depression, how the brain and gut communicate, as well as what the research is showing us regarding the use of probiotics as a novel therapy in cases of depression.

Skip to Key Takeaways

The old and the new: changing paradigms of depression

For a long time, it was believed that depression was caused by a deficiency of neurotransmitters such as serotonin, also known as the ‘happy’ chemical. This ‘serotonin model’ of depression led to the widespread use of selective serotonin reuptake inhibitors (SSRIs), which essentially increase levels of serotonin in the brain. However, how this neurotransmitter imbalance came to be was never fully understood and often assumed to be genetically driven. In recent years, a multitude of factors have come to be associated with the cascade of depression, including environmental, dietary, lifestyle and genetic; with chronic inflammation appearing as a central feature.

Several studies have implicated cytokines – chemical messengers released during the inflammatory process – in the pathology of depression. A variety of meta-analyses, cross-sectional and longitudinal studies have compared blood concentrations of pro-inflammatory and anti-inflammatory cytokines in patients with depression versus healthy controls. Across these studies, peripheral levels of IL-6, IL-10, IL-12, IL-13 and TNF-α were generally elevated, while IFN-γ levels tended to be lower in patients with depression.4–6 For instance, markers of inflammation such as CRP and IL-6 were found to be elevated by up to 50% in patients with clinical depression.7 In a study of over 1,000 women, increases in CRP triggered the onset of depression,8 while other studies show how symptoms of depression were resolved once these signs of inflammation returned to normal.9 Interestingly, even among healthy individuals, signs of depression would appear in tandem with increasing levels of inflammation.10

These cytokines appear to induce symptoms of depression through several mechanisms. When inflammation is high, the enzyme indoleamine 2,3-dioxygenase (IDO) is stimulated, which converts tryptophan into kynurenine. This does two things: (1) reduces the levels of tryptophan available to make serotonin and so levels of serotonin decline, (2) metabolites of the kynurenine pathway such as 3-hydroxy-kynurenine (3-OH-KYN) and quinolinic acid (QUIN) are released, which have toxic effects on brain function. 3-OH-KYN can produce oxidative stress by increasing the production of reactive oxygen species (ROS), while QUIN may overstimulate the hippocampal N-methyl-D-aspartate (NMDA) receptors, which leads to apoptosis and hippocampal atrophy – both of which have been associated with depression.11

Production of neopterin is also stimulated at the expense of tetrahydrobiopterin (BH4) under inflammatory conditions. This can impact the creation of neurotransmitters such as serotonin and dopamine, as BH4 plays a crucial role in their synthesis. From here a variety of symptoms of depression including a lack of enjoyment in the case of serotonin and a lack of motivation in the case of dopamine can be expressed.

As we can see, inflammation has very specific effects on brain behaviour, with multiple routes of creation including physical and psychological stress, pollution, infection, alcohol and poor diet.12–14 Gut dysfunction, too, appears to be a key factor – with systemic inflammation so radically able to alter the gut microbiome,12–14 it has led researchers to discover strong correlations between gut microbiome and brain health.15  In fact, differences in specific groups of gut bacteria present in those experiencing depression, anxiety and impaired brain function have been identified.16

Not-so-secret pals: how the gut and the brain communicate

The vagus nerve connects the brain with several parts of the gut, including the stomach, pancreas and intestines – making the gut a very influential path to the brain.17 This intricate, bidirectional network is a central means for communication between mind and body. It serves as a pathway for many things, including appetite regulation and ‘gut instincts’ – but also, when things go wrong, can be a channel for feelings of depression and anxiety.

Unlike the brain, the gut is designed to come into contact with intruders from the outside world. It is through the intestinal wall that the majority of nutrients, water and lipids are absorbed into the bloodstream. On the other hand, microbes, large food particles and other antigens cannot cross this barrier. Therefore, maintaining intestinal integrity is vital to ensuring that only what is deemed safe gets through to internal body systems. For this reason, the gut houses the bulk of our immune cells (the GALT – gut associated lymphoid tissue) which allow it to handle these interactions with foreign matter.18

Another central feature of the gut is the microbiome. The gut microbiome is made up of trillions of bacteria, viruses, fungi and other microscopic living things which are collectively referred to as ‘microbes.’ The balance of these microbes is central to our body’s ecosystem and when things get knocked out of balance, it can have a systemic effect on the global function of the body; impacting digestion, immunity and even brain health (to name a few). For example, when levels of inflammation are increased in the gut, inflammatory mediators such as cytokines can travel to the brain,19 particularly along the vagus nerve.

These alterations or ‘dysbiosis’ can damage the integrity of the intestinal wall and lead to increased intestinal permeability or ‘leaky gut’. When this happens, levels of LPS (lipopolysaccharides – endotoxins found in the outer membrane of Gram-negative bacteria like E. coli) can enter the bloodstream, cross the blood-brain barrier and bring about even more inflammation in the brain.

This is where probiotics come in…

With dysregulation of the microbiome and subsequent inflammation as a clear driving factor to depressive symptoms,20 researchers have been investigating the potential therapeutic role of probiotics in managing these symptoms.

In an eight week randomised controlled trial, patients with major depression were administered a combination of Lactobacillus acidophilus, Lactobacillus casei, and Bifidobacterium bifidum or placebo. At the end of the trial, not only were significant differences noted in mood (assessed using the Beck Depression Inventory), but significant decreases in serum insulin, insulin resistance and serum C-reactive protein were recorded.21 Furthermore, a rise in plasma total glutathione (GSH) levels was noted in the intervention group when compared with placebo.21

Another study also found that oral probiotics, specifically of the Lactobacilli genera attenuated depressive and anxious behavioural symptoms in a chronic mild stress mouse model.22 In addition, a recent meta-analysis and systematic review of human controlled clinical trials on the use of probiotics showed significant effect on symptoms of depression, with an increased effect when results were restricted to psychiatric samples only.23

While further research is required to clarify dose and strain specific impacts on our brain chemistry, one thing that is very clear in the research so far is the low risk of using probiotics therapeutically due to their multimodal effects. Many researchers comment that probiotics appear to be very safe to use even while investigations continue in an effort to fully understand the precise mechanisms of impact.

Key Takeaways

  • According to the World Health Organization, over 300 million people are estimated to suffer from depression, which is equivalent to 4.4% of the world’s population.
  • Just like conditions such as obesity, heart disease and diabetes; the origins of depression are inflammatory rather than neurological. Increased inflammation can impact the body’s ability to produce important neurotransmitters such as serotonin and dopamine.
  • The vagus nerve connects the brain and the gut; allowing for bidirectional communication and influence.
  • Unlike the brain, the gut is designed to come into contact with intruders from the outside world. However, when levels of inflammation are increased in the gut this can result in increased inflammation in the brain.
  • This kind of inflammatory cascade can be triggered by a variety of things. For example, consuming processed, nutrient-poor foods; experiencing real or perceived stress, infection and pollution, can all radically alter the gut microbiome and thus trigger an inflammatory response.
  • Several studies demonstrate that a healthy gut microbiome is essential for maintaining brain health, and markers of inflammation are generally elevated in patients with depression.
  • Studies have shown that Lactobacillus acidophilus, Lactobacillus casei, and Bifidobacterium bifidum resulted in significant improvements in mood, insulin resistance and inflammation in patients with major depression.

If you have questions regarding the topics that have been raised, or any other health matters, contact our nutritionist team by phone or email at any time:, 01684 310099


  1. Vos, T., et al (2015) ‘Global, regional, and national incidence, prevalence, and years lived with disability for 301 acute and chronic diseases and injuries in 188 countries, 1990–2013: a systematic analysis for the Global Burden of Disease Study 2013.’ Lancet, 386(9995), pp. 743-800.
  2. (2017) ‘Depression and Other Common Mental Disorders Global Health Estimates.’ Retrieved from:
  3. John M (2005) ‘Eisenberg Center for Clinical Decisions and Communications Science JMEC for CD and C. Treatment Options When Your SSRI Antidepressant Is Not Working Well.’ Agency for Healthcare Research and Quality (US).
  4. Köhler, C. A., et al (2017) ‘Peripheral cytokine and chemokine alterations in depression: a meta-analysis of 82 studies.’ Acta Psychiatr Scand, 135(5), pp. 373-387.
  5. Himmerich, H., et al (2019) ‘Cytokine Research in Depression: Principles, Challenges, and Open Questions.’ Front Psychiatry, 10, pp. 30.
  6. Kim J-M, et al (2018)’ Changes in pro-inflammatory cytokine levels and late-life depression: A two year population based longitudinal study.’ Psychoneuroendocrinology, 90, pp. 85-91.
  7. Kiecolt-Glaser, J, K., et al (2015) ‘Inflammation: Depression Fans the Flames and Feasts on the Heat. Am J Psychiatry, 172(11), pp. 1075-1091.
  8. Pasco, JA., et al (2010) ‘Association of high-sensitivity C-reactive protein with de novo major depression.’ Br J Psychiatry, 197(5), pp. 372-377.
  9. Dahl J, et al (2014) ‘The plasma levels of various cytokines are increased during ongoing depression and are reduced to normal levels after recovery.’ Psychoneuroendocrinology, 45, pp. 77-86.
  10. Haroon E, et al (2012) ‘Psychoneuroimmunology Meets Neuropsychopharmacology: Translational Implications of the Impact of Inflammation on Behavior’ Neuropsychopharmacology, 37(1), pp. 137-162.
  11. Mangoni A (1974) ‘kynurenine and depression.’ Adv Biochem Psychopharmacol, 11, pp. 293-298.
  12. Zinöcker MK and Lindseth IA (2018) ‘The Western Diet-Microbiome-Host Interaction and Its Role in Metabolic Disease.’ Nutrients, 10(3).
  13. Maier L, et al (2018) ‘Extensive impact of non-antibiotic drugs on human gut bacteria.’ Nature, 555(7698), pp. 623-628.
  14. Karl JP, et al (2018) ‘Effects of Psychological, Environmental and Physical Stressors on the Gut Microbiota.’ Front Microbiol, 9, pp. 2013.
  15. Dinan TG and Cryan JF (2017) ‘Microbes, Immunity, and Behavior: Psychoneuroimmunology Meets the Microbiome.’ Neuropsychopharmacology, 42(1), pp. 178-192.
  16. Bercik P, et al (2011) ‘The Intestinal Microbiota Affect Central Levels of Brain-Derived Neurotropic Factor and Behavior in Mice.’ Gastroenterology, 141(2), pp. 599-609.
  17. Dinan TG and Cryan JF (2017) ‘The Microbiome-Gut-Brain Axis in Health and Disease.’ Gastroenterol Clin North Am, 46(1), pp. 77-89.
  18. Smolinska S, et al (2016) Biology of the Microbiome 1.’ Gastroenterol Clin North Am, 46(1), pp. 19-35.
  19. Jacka FN (2017) ‘Nutritional Psychiatry: Where to Next?’ EBioMedicine, 17, pp. 24-29.
  20. Dinan TG and Cryan JF (2015) ‘The impact of gut microbiota on brain and behaviour.’ Curr Opin Clin Nutr Metab Care, 18(6), 552-558.
  21. Akkasheh G, et al (2016) ‘Clinical and metabolic response to probiotic administration in patients with major depressive disorder: A randomized, double-blind, placebo-controlled trial’ Nutrition, 32(3), pp. 315-320.
  22. Li N, et al (2018) ‘Oral Probiotics Ameliorate the Behavioral Deficits Induced by Chronic Mild Stress in Mice via the Gut Microbiota-Inflammation Axis.’ Front Behav Neurosci, 12, pp. 266.
  23. Liu RT, et al (2019) ‘Prebiotics and probiotics for depression and anxiety: A systematic review and meta-analysis of controlled clinical trials.’ Neurosci Biobehav Rev, 102, pp. 13-23.

Last updated on 10th August 2023 by cytoffice


14 thoughts on “Link between gut and depression

  1. It would be great if you had one product that had all the things you need or there was some explanation about the differences in the products you do sell.

    1. Dear Sue

      Thanks for your comment on our blog. You ask about an all-round product. We do have an all-round gut product if this is what you mean – CytoProtect GI Tract. This includes the probiotic Bacillus coagulans, along with marshmallow, slippery elm, pectin (these have prebiotic properties), plus we include vitamins A, D, and zinc. Vitamin A contributes to the maintenance of mucous membranes as well as normal function of the immune system. Vitamin D contributes to normal function of the immune system. Zinc is needed for protein synthesis and immune function. All three nutrients are often used as part of gut support programmes. In addition we have included lactoferrin which has a number of properties relevant for gut health; and the antioxidant quercetin.

      I hope this helps. If there are any other areas where you feel we need a specific type of product please do email me –

      Best wishes, Clare

  2. You talk about the blood-concentration of cytokines. Then you say that they travel up the vagus nerve. How does that happen?

    1. Dear Shabra

      Thank you for your question. Inflammatory cytokines can activate the vagus nerve and send signals to the brain. The vagus nerve is part of a neural reflex mechanism called the inflammatory reflex that controls innate immune responses and inflammation during pathogen invasion and tissue injury. If you are interested in getting more detail you may be interested in this paper:

      Best wishes, Clare

      1. Is there any research on specific probiotics for specific conditions. Is anxiety also covered best with the lactobacillus which you mention for depression.

        1. Hi Duka,

          Thank you for your question. You may be interested in the book The Psychobiotic Revolution by Scott Anderson which includes some discussion of specific strains which have been researched in relation to mental health conditions (anxiety and depression). The specific strains mentioned in the book include Bifidobacterium longum, Bifidobacterium infantis, Bifidobacterium breve, Bifidobacterium bifidum, Lactobacillus acidophilus, Lactobacillus bulgaricus, Lactobacillus rhamnosus, Lactobacillus reuterii, Lactobacillus plantarum, Lactobacillus casei, Lactobacillus paracasei, Streptococcus thermophilus and Saccharomyces boulardii.

          Our product Acidophilus Plus contains 9 species, of which 8 are mentioned above.

          I hope this helps.


  3. A well written and credible piece enlarging on the gut/brain connection and one which I will certainly act upon. Thank you for this valuable information

  4. In your article on Depression you say that 4.4% of the World’s population suffer from depression.
    I would say that it’s more correct to say that it’s 4.4% or less of the World’s population who don’t.
    I have a booklet called Why people feel Depressed and how to Cure it.
    It’s by Fr.Colmcille Murphy OFM Conventual.
    If you wish I’ll send it to Cytoplan and include a sae for the return. (I have a booklet and a photocopy of the booklet).
    I’ll pay postage because I strongly believe in this excellent booklet.
    If people put this booklet into practice,(with medication as well where necessary),the number of people with depression would greatly reduce.
    Please send me your address though I’ll look for it on your website.
    If you prefer me not to send it, please let me know.
    I won’t do anything until I’ve heard from you.
    I just want people to be free of depression because I’ve gone through Hell with it and still do everyday though not as so severe.
    I look forward to hearing from you.
    Simon Slater.
    God bless all at Cytoplan.

    1. Thanks for your comment, Simon. Yes the 4.4% figure does sound low. This is data from World Health Organization and is global data so perhaps not representative of Western world where rates may be higher. Thank you for the offer of sending your booklet but I wouldn’t want to risk it getting lost in the post. I did have a look if it is available online but haven’t found it.

      Sorry to hear that you have experienced long-term depression. We do have a free health questionnaire service that you may be interested in. If you complete and return a health questionnaire we will send you some written diet and supplement recommendations.

      Hope this helps.


  5. My husband and I recently had mild Covid and ended up with anosmia (and loss of taste) for 10 days at the end. During that time we both felt very “low” which is unusual for us, not wanting to socialise in any way (even talking on the phone) and worrying about plans we had for a couple of weeks later to visit friends and family as we felt so down. Thankfully, through various means (vitamin A, “smell training”, homeopathic remedies) those remaining symptoms disappeared (almost overnight, the way they came originally!!) and our moods lightened again.

    Can you explain the mechanism as to why that should have happened, what the connection with anosmia and mood might be?

    1. Hi – Yes the anosmia and mood relation is because the virus travels up the olfactory nerve, damaging the olfactory epithelium in the process leading to loss of smell. It is thought it passes across the crib form plate into the brain and affects glymphatic drainage in the brain increasing pressure and inflammation in the brain, which can contribute to low mood.

      I have emailed you the COVID recovery protocol; main focus on zinc, A and B12, as well as cell active curcumin.


  6. Your blog articles are superb! Detailed, yes, sometimes highly complex and thorough but that’s the way to push the boundaries of complete understanding. Thank you

    1. Hi Kelly, thank you for your kind comments. We are thrilled you find them informative. Our nutritional team and guest editors work hard to create them so it’s always great to hear that they are enjoyed.

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