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A gut feeling about probiotics

Probiotics are food supplements that contain live microorganisms which when administered in adequate amounts can confer benefits to the health and functioning of the digestive system, as well as modulation of immune function.

In the general population, studies have shown that probiotic intake can improve rates of recovery from diarrhoea, increase resistance to gut and respiratory infections, promote anti-tumour activity and alleviate some allergic and respiratory disorders (1). Several studies in athletes also indicate that some strains of probiotic – lactobacillus and bifidobacterium species in particular – can be effective in reducing the incidence of the common cold (5). The evidence for this was explained in a previous blog. However, the benefits of probiotics may extend beyond immunomodulation improvements of immune function and reduction of illness risk according to new research that indicates the gut microbiota can communicate in a bidirectional manner with the brain and thus influence mood, stress responses and sleep quality.

The benefits of probiotics may extend beyond immunomodulation improvements of immune function, with new research suggesting an influence on mood, stress response and sleep quality.

The microbiota-gut-brain axis

The basis of this is the recognition of the existence of a microbiota-gut-brain connections between the microbiota, the gut and the brain (referred to as the microbiota-gut-brain axis) axis that can influence behaviour and cognition via the production of neurochemicals chemicals (neurochemicals) by commensal certain bacteria and probiotics (3). This system has probably evolved over the millennia during which animals have hosted bacteria in their gut for mutual benefit. The microbe-derived neurochemicals are the same as those produced by the host and there is some evidence from animal studies that these neurochemicals activate nerve endings in the gut and information is then transmitted to the central nervous system (6). Microbes and their metabolism can also be influenced in turn by neurochemicals including catecholamines (e.g. adrenaline and noradrenaline) produced by the human body in response to stress. These interactions likely explain what we like to call our “gut feelings”!

Reduction of stress responses by probiotics

The body’s stress response system maintains homeostasis. When we are exposed to stresses (heat, exercise, mental challenges etc) against various external stimuli our body adapts. However, an excessive response to stress can trigger both mental and physical health problems. Studies in animals have shown that probiotics and gut microbiota can reduce stress reactivity by modulating the neuroendocrine system and can have positive effects on behaviour and cognitive function (e.g. reduced anxiety, depression and defeatism) under stressful conditions. In humans, recent investigations of the effects of probiotics on both stress-related physical symptoms and stress biomarkers have shown beneficial effects.

New studies

A series of double-blind, placebo controlled trials in medical students studying towards a nationwide academic examination were conducted by researchers in Japan (2,6). For 8 weeks before the exam, two groups of 70 students, consumed either Lactobacillus casei Shirota (Yakult) in a fermented milk drink or a placebo milk drink daily. The self-reported scores for abdominal dysfunction and cold-like symptoms and the number of genes with more than a 2-fold change in gene expression in white blood cells were significantly suppressed in the probiotic group compared with the placebo group during the study period. Levels of the salivary stress hormone, cortisol, were also lower in the probiotic group close to the time of the exam. Thus, administration of the probiotic was shown to reduce stress-related physical symptoms and a hormonal biomarker of stress. At a recent conference that I attended in Tokyo, one of the researchers who led these human studies, Dr Kensei Nishida reported the results of his latest study which indicated that regular daily ingestion of Lactobacillus casei Shirota also improved sleep quality (earlier onset of sleep and longer sleep duration with probiotic compared with placebo) in medical students preparing for their academic exams (4).

Administration of a milk drink containing probiotics (Yakult) has been been found to reduce stress-related physical symptoms and reduce salivary cortisol levels.

Possible benefits for athletes

These findings raise the possibility that probiotics could be of benefit for athletes recovering from intensive training and competition. Recovery involves both physical and psychological issues and a supplement that could reduce physical symptoms of stress and improve both mood and sleep quality would almost certainly be good for the athlete or games player. The importance of mental as well as physical recovery after intense competition is often underestimated, but a quote from someone who has a lot of experience of these issues, Dr Sam Erith, head of sport science at Manchester City FC, is illuminating: “the longer I do this job the more you see that mood and mind state are such powerful drivers for recovery”. Although further research is required to confirm these effects in the sporting population, it seems likely that probiotic supplements may provide multiple benefits for athletes and games players.


  1. Hao Q, Dong BR and Wu T (2015) Probiotics for preventing acute upper respiratory tract infections. Cochrane Database Systemic Review 2:CD006895.

  2. Kato-Kataoka A, Nishida K, Takada M et al. (2016) Fermented milk containing Lactobacillus casei strain Shirota prevents the onset of physical symptoms in medical students under academic examination stress. Beneficial Microbes 7(2):153-156.

  3. Lyte M (2014) Microbial endocrinology: Host-microbiota neuroendocrine interactions influencing brain and behaviour. Gut Microbes 5(3):381–389.

  4. Nishida N (2016). Stress relief effect of probiotics through gut-brain axis. Proceedings of the 8th Yakult Shirota Conference, Tokyo, 2nd November, 2016.

  5. Pyne DB, West NP, Cox AJ and Cripps AW (2015) Probiotic supplementation in athletes: clinical and physiological effects. European Journal of Sports Science 15:62-72.

  6. Takada M, Nishida K, Kato-Kataoka A et al. (2016) Probiotic Lactobacillus casei strain Shirota relieves stress associated symptoms by modulating the gut-brain interaction in human and animal models. Neurogastroenterology and Motility 28(7):1027-1036.


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