Tens of trillions of microorganism populate the human intestine. We often talk about how they are relevant for obesity, diabetes, and in autoimmune disorders. But there is growing evidence that neuropsychiatric disorders, including autism, anxiety and major depressive disorders are affected as well.
Recent studies point towards two directions. On the one hand, there are indications that probiotics might help to improve mood and to cope with the some effects of pyschological stress. On the other hand, other studies are unable find benefits over placebo on mood, anxiety, or sleep quality in healthy volunteers.1
Why is it so hard to be conclusive?
Part of the difficulty in making a definitive judgement is that the field is still very young. The notion that there may be a link between gut and mental health first developed just over a decade ago with research into obesity and its effect on gut microbiota. This led to research in the early and mid-2010s which found poor microbiota diversity in the intestine was linked to behavioural impairment. Researchers then wondered in what other ways gut microbiota may affect mental and neurological health.
The first (and still most) studies have been on mice. An early study revealed that mice without intestinal microbiota acted strangely and were unusually stressed. When given probiotics, their behaviour normalised.2 In mice, it turned out that microbiota were essential for developing the neuronal circuits for motor control, anxiety behaviour, and social responses.3 In other animal models, probiotics proved to have positive effects for mood, anxiety, and cognition.4
- “A Randomized Controlled Trial to Test the Effect of Multispecies Probiotics on Cognitive Reactivity to Sad Mood – ScienceDirect.” Accessed June 5, 2018. (link)
- “Fermented Milk Containing Lactobacillus Casei Strain Shirota Preserves the Diversity of the Gut Microbiota and Relieves Abdominal Dysfunction in Healthy Medical Students Exposed to Academic Stress.” Accessed June 5, 2018. (link)
- “Harnessing Gut Microbes for Mental Health: Getting From Here to There – Biological Psychiatry.” Accessed June 5, 2018. (link)
- “Lost in Translation? The Potential Psychobiotic Lactobacillus Rhamnosus (JB-1) Fails to Modulate Stress or Cognitive Performance in Healthy Male Subjects – ScienceDirect.” Accessed June 5, 2018. (link)
- “Postnatal Microbial Colonization Programs the Hypothalamic–pituitary–adrenal System for Stress Response in Mice. 2004. The Journal of Physiology Wiley Online Library.” Accessed June 5, 2018. (link)
- “Effects of the Probiotic Bifidobacterium Infantis in the Maternal Separation Model of Depression – ScienceDirect.” Accessed June 5, 2018. (link)
- “Postnatal Microbial Colonization Programs the Hypothalamic–pituitary–adrenal System for Stress Response in Mice – Sudo – 2004 – The Journal of Physiology – Wiley Online Library.” Accessed June 5, 2018. (link)
- “Probiotics Normalize the Gut-Brain-Microbiota Axis in Immunodeficient Mice | American Journal of Physiology-Gastrointestinal and Liver Physiology.” Accessed June 5, 2018. (link)