Not only are probiotics considered beneficial to digestive health, and immune health but increasing evidence suggests direct and indirect interactions between gut microbiota (GM) and the central nervous system (CNS).
A large body of research has supported the presence of a pathway of communication between the gut and the brain, modulated by the gut microbiota, giving rise to the term “microbiota-gut-brain” axis. It is now thought that, through this pathway, microbiota can affect behaviour and modulate brain plasticity and cognitive function. In particular, studies have illustrated an association between the gut microbiota composition and cognitive processes such as learning and memory. Research has shown that the intestinal microbiota additionally contribute to the early development of normal social and cognitive behaviours 1. While probiotic and prebiotic supplementation can have a positive effect on mood and psychological symptoms such as depression and anxiety, stress as well as mental health issues including depression, Alzheimers and Parkinson’s diseases. This new area of research, called “psychobiotics” is where the beneficial bacteria (probiotics) or prebiotics that influence bacteria–brain relationships can exert positive emotional, cognitive, systemic, and neural benefits 2. This process is thought to occur primarily through the central nervous system (CNS) as well as through metabolic, hormonal and immune pathways.
Recent evidence indicates a clear association between changes in the microbiota and cognitive behaviors and there is increasing evidence on the effects of supplementing with probiotics on improving cognitive disorders 3. One of the earliest studies found that the wrong type of bacteria added to the gut of germ free animals (containing no microbiota), can cause stress related negative behavious while administration of probiotics can improve cognitive behaviors including learning and memory 1.
In a clinical trial conducted among 60 Alzheimer's patients where the probiotic supplemented group took 200 ml/day probiotic milk for 12 weeks, the probiotic group showed a significant improvement in the MMSE (Mini-mental state examination) which is a measure of cognitive function. In addition, they reported lower levels of oxidation and inflammation, blood fats (tryglicerides) and improved insulin resistance and Beta cell (pancreas) function which controls and stores insulin in the probiotic group compared to the control group 4. A pretty good reason to supplement with probiotics.
Growing research also shows probiotic supplements may be used therapeutically to modify stress responses and symptoms of anxiety and depression 5,6,7,8. One study found that a short 3-week intervention with probiotics-containing milk drink improved mood scores compared to participants who received a placebo. Although, improvement in mood was only observed for participants who showed elevated symptoms of depression at the beginning 9. A second study found that a month of probiotic supplementation significantly improved depression and anger 10. In a triple-blind study of 20 healthy participants without a current mood disorder over 4 weeks, probiotics significantly reduced negative thoughts associated with a sad mood compared to placebo. The positive effect was mostly because of reduced rumination and aggressive thoughts 11. While a study of people with chronic fatigue syndrome found that supplementing with probiotics for 2 months significantly reduced their anxiety scores 12.
Similar results have also been shown in animal studies. In a study of healthy mice researchers observed a reduction in anxious and depressive behavior after feeding healthy mice with probiotics 13. A similar result was shown in adult rats with a reduction in depressive-like behaviors after feeding them with Bifidobacterium species. The effect was comparable to the effects of administering the antidepressant citalopram 14.
Stress and anxiety disorders are two of the most common psychiatric illnesses worldwide, affecting both children and adults. Increasing numbers of studies have suggested that the gut microbiota is involved in the pathophysiology of stress-related disorders. For example, a number of studies have now shown that certain strains of bacteria increase anxious behaviour while others reduce it. One study found that participants who were given a mixture of probiotics containing Lactobacillus and Bifidobacterium species showed significantly less psychological distress than matched controls 15. A recent (November 2016) meta-analysis of seven studies with around 300 participants showed that supplementation with probiotics resulted in a significant improvement in psychological symptoms of depression, anxiety, and perceived stress in healthy human volunteers 16.
However, probiotics and the effects on our moods appears to be a two way street. While the gut microbiome can alter moods, our moods can also alter our gut microbiome. Chronic stress can cause behavioral, cognitive, biochemical, and gut microbiota aberrations. In a study of 1002 cases and controls followed up for 12 years, those with gut disorders had elevated levels of anxiety and depression at baseline, but also those with higher levels of anxiety and depression were more likely to have gut disorders at follow-up 17.
Animal studies have also shown certain strains of Lactobacillus exert a positive effect on anxiety-related behavior and responses to stress 13,18. In a study on rats subjected to 21 days of stress the results showed that administration of probiotics improved the stress-induced behavioral (anxiety and depression) and cognitive dysfunction, showing an effect similar to and better than that of an antidepressant. It also resulted in lower stress hormones and improved blood parameters suggesting it was dealing with some of the underlying mechanisms 18.
Other studies have noted that microbiota have an important influence on the development of cognitive processes in young mice 1. Depletion of a normal gut microbiome in early life, especially during the post-weaning period, may affect cognitive and social behaviours in the brain through the alteration of neuropeptides (chemical messengers) such as vasopressin and oxytocin 19,20. In fact, the research suggests a strong role of the gut microbiota in autism spectrum disorder. Interestingly, treatment of mice with autism with probiotics has shown to ameliorate autism related traits 21. In another twist on this, a study of 75 pregnant women given probiotics 4 weeks before their due dates and then continued giving the probiotics to the infants, or to the mothers if they were breastfeeding for 6 months did not develop any ADHD or Asperger’s after being followed for 13 years. 17.1% of the children in the placebo group developed ADHD or Asperger’s. Not one child in the probiotic group did 22.
Importantly, studies have shown that multispecies probiotics (i.e., combining different strains of specific genera) can have increased effectiveness through an additive effect of specific strain properties such as colonization of different niches, enhanced adhesion and induction of an optimal pH range, as compared to mono-species supplements 23,24. So take a mixed species supplement for the best results.
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