Probiotics and mental health – The Food Medic

This article was written by specialist dietitian, Kaitlin Colucci

The importance of gut health has rapidly grown in popularity over recent years, and most would have heard of probiotics – live microorganisms that can benefit the body and the gut microbiome – and their potentially positive effects on digestive health. However, the health benefits of probiotics may extend beyond the physical. Evidence shows a link between an imbalance of the organisms that make up the gut microflora (gut dysbiosis), and several mental illnesses, including anxiety and depression. 

Mental health issues are now thought to affect one in six people in the UK (1). However, since the COVID-19 pandemic, this has placed even further strain on our mental health and wellness, and the estimated cost this is placing on the economy is staggering (several billions), with anti-depressant medication often first line medical treatment. 

With mental health problems being at an all-time high, there are emerging drug-free approaches to potentially treating these conditions. Some of these methods include exercise, heart-rate regulation and ‘psychobiotics’. But what does this newly conceived term actually mean? And does it represent a significant scientific breakthrough, or is it just a new name for exciting science? 

What are psychobiotics?

The term ‘psychobiotics’ is used by experts when probiotics are being studied or used to improve mental health. According to the ‘Society of Biological Psychiatry’ the term psychobiotics is defined as a ‘live organism that, when ingested in adequate amounts, produces a health benefit in patients suffering from psychiatric illnesses’. The way that these probiotic bacteria exert beneficial effects on mental health and mood is thought to happen in several different ways. 

According to Harvard Medical School (2), psychobiotics are involved in: 

  • Producing neurochemicals, such as serotonin (the happy hormone) and gamma-aminobutyric acid (GABA) (the relaxation hormone), which modulate our mood
  • Activating neural pathways between the gut and brain
  • Limiting the production of inflammatory cytokines (cell signalling molecules), reducing inflammation in the body and brain
  • Improving general nutritional status
  • Reducing pathogenic (bad) bacteria in the intestines, which can cause lots of different health problems in the rest of the body

Probiotic foods consumed to improve mental health are also considered psychobiotics. This includes fermented foods such as fermented yogurts (kefir), kombucha, sauerkraut and kimchi. However, because these foods are full of other beneficial nutrients, it’s difficult to isolate specific bacterial strains and identify their effects on the body. For example, antioxidants and fibre have both been linked to benefiting mental health and are often found in probiotic foods.

Psychobiotic research 

There are now emerging studies looking at specific probiotic strains and their effect on mental health. In a double-blind, placebo-controlled clinical trial run by Dr Philip Burnet at Oxford University (3), they investigated the very question of whether probiotics can help with mood. The study recruited 70 people who suffered with either low mood, tiredness, lack of pleasure and/or sadness. They were divided into two equal groups and one group was given a 14-strain probiotic for 4 weeks, whilst the other group received a placebo. Participants were asked to track their mood and symptoms throughout the study period. 

At the end of the 4 weeks, 50% of those in the probiotic group reported an improvement in mood and concentration, compared to only 20% in the placebo group. In addition, the study also analysed salivary cortisol levels from each participant at the start of the study, and then again after the 4 weeks. Cortisol is an adrenal hormone that is produced in response to stress. At the end of the study, the results showed a clear reduction in cortisol levels in the probiotic group. No reduction was seen in the placebo group. 

How do probiotics benefit mood?

The gut microbiome can influence the brain directly via the vagus nerve, which travels from the brain stem to the gut. This is known as the gut-brain axis. When we’re stressed and anxious, our brain sends signals to the gut to tell us that we are stressed. This switches on the sympathetic (‘fight or flight) nervous system and can increase the sensitivity of the gut, a term called visceral hypersensitivity. Similarly, however, if there is an imbalance of good and bad bacteria in the gut (gut dysbiosis) then the gut sends signals to the brain and can impact our mood, making us more susceptible to low moods and depression. 

However, not all probiotics are the same, and therefore have the same effect. Certain strains of probiotic bacteria have been clinically researched and shown to exert a positive influence on our mood, but not all strains have this ability. Therefore, it is important to select strains that have good quality evidence supporting their use for specific health issues. For example, strains such as Lactobacillus acidophilus Rosell-52 and Bifidobacterium longum Rosell-175 have been clinically trialled and shown to be helpful against stress and low mood (4).

There are numerous limitations to such studies and probiotics shouldn’t be thought of as the quick fix to improving mental health. For example, many studies examine several psychophysiological variables, and there are issues with placebo effect of probiotics. That being said, it appears psychobiotics offer an exciting new avenue for research into mental health. Watch this space!


  1. Mental Health Taskforce NE. The Five Year Forward View for Mental Health. 2016 [cited 2022 Aug 10]; Available from:
  2. Fermented foods, microbiota, and mental health: ancient practice meets nutritional psychiatry’ Eva M. Selhub. Journal of Physiological Anthropology, 2014.
  3. Baião, R., Capitão, L., Higgins, C., Browning, M., Harmer, C., & Burnet, P. (2022). Multispecies probiotic administration reduces emotional salience and improves mood in subjects with moderate depression: A randomised, double-blind, placebo-controlled study. Psychological Medicine, 1-11. doi:10.1017/S003329172100550X
  4. Messaoudi M. et al., (2011), ‘Assessment of psychotropic-like properties of a probiotic formulation (Lactobacillus helveticus R0052 and Bifidobacterium longum R0175) in rats and human subjects’. British Journal of Nutrition, 105(5):755.

Probiotics and mental health was last modified: August 17th, 2022 by Kaitlin Colucci

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