Scientists from the California Institute of Technology (Caltech) and the University of California Los Angeles (UCLA) in November 20161 have demonstrated that the health of the gastrointestinal tract plays a key role in regulating healthy brain function. In a paper in the very prestigious international peer reviewed scientific journal Cell, the researchers reviewed 194 published papers that provide evidence for gut bacteria being “integral contributors to development and function of the nervous system and to the balance between mental health and disease”.
They presented evidence that showed the gut microbiome (the bacteria that reside in the gastrointestinal tract of humans) and/or probiotics could play a key role in a range of neuropsychiatric conditions, including depression and anxiety, autism spectrum disorder, schizophrenia, Parkinson’s disease (PD) and Alzheimer’s disease (AD) . This was further supported by researchers from the Chinese Academy of Sciences who provided evidence for the role of the microbiome, and, by implication, probiotics, in dementia2.
The role of the gut microbiome and probiotics in human health.
The authors acknowledge that the gut microbiome plays a significant role in the wellbeing of its host. Consumption of probiotics supplements the gut microbial population with a specific microbe, either transiently or permanently, and can change the microbiome profile and function, as well as interact with the host to produce a range of beneficial health effects. The effect of probiotics on immune health was clearly stated by the researchers: “Research in animal models and humans has inextricably linked gut bacteria to the development and function of the immune system. The presence of entire immune cell types requires the microbiome, and specific microbes have been discovered that either promote or ameliorate immunologic disorders such as type 1 diabetes, asthma, and inflammatory bowel disease.”
"Research in animal models and humans has inextricably linked gut bacteria to the development and function of the immune system"
In both papers the authors predominantly focused on evidence for the role of the gut microbiome in nervous system disorders.
Evidence for the ability of probiotics and the gut microbiome to influence mood:
- A study in healthy volunteers who consumed a fermented milk product containing probiotic bacteria showed alterations in brain activity during an attention task, in brain regions that control processing of sensation and emotion, in comparison to a control group who consumed a milk drink without probiotic bacteria.
- Another study in humans associated the gut microbiome in IBD with stress disorders.
- Gastrointestinal symptoms are associated with depression, with around 20% of patients reporting such symptoms.
- The composition of the gut microbiome in major depressive disorder (MDD) patients is significantly different from that of healthy controls3. When fecal samples from MDD
"Mouse and human studies provide tantalizing suggestions that the microbiome may play an active role in driving depressive-like behaviors suggesting potential new avenues for therapeutic development"
Sharon G, Sampson TR, Geschwind DH, Mazmanian SK. The Central Nervous System and the gut microbiome. Cell 2016; 167: 915-32.
Alkasir R, Li J, Li X, Jin M, Zhu B. Human gut microbiota: the links with dementia development. Protein Cell 2016; DOI 10.1007/s13238-016-0338-6
Significantly more Actinobacteria and less Bacteroidetes in MDD-associated microbial populations
patients and controls were transplanted to mice, the recipients of MDD samples exhibited much more depressive-like behaviour compared to controls
Evidence for the role of probiotics and the gut microbiome in autism spectrum disorder (ASD):
- The gut microbiome of ASD patients has an increased abundance and diversity of “unhealthy” bacteria4.
- The gut microbiome of ASD patients often lacks probiotic bacteria.
- Gastrointestinal symptoms such as diarrhea are significantly more prevalent in children with ASD.
Evidence for probiotics and the gut microbiome in dementia
Dementia is a syndrome that affects memory and other cognitive functions to the extent that it interferes with daily function. There are many conditions that can cause dementia, including neurodegenerative disorders [e.g. Alzheimer’s disease (AD) and Parkinson’s disease (PD)], cerebrovascular disease, brain injury, and certain infections. There are several studies cited by the authors that imply a role of the gut microbiome in AD and PD:
- Individuals afflicted with PD display significantly different fecal and mucosal microbial populations:
- There are increased numbers of tissue-associated E. coli in the gut of PD patients compared to healthy controls,
- Furthermore, short chain fatty acid (SCFA) concentrations in feces from PD patients are decreased as compared to controls. SCFA are produced by bacterial fermentation in the gut, and are needed for normal nervous system development.
- Recent evidence suggests that molecules from bacteria present in the microbiota can trigger autoimmune responses that promote neurodegeneration during AD and PD.
- There are marked differences in the composition of the gut microbiome between AD mice and healthy mice and strongly indicated that a distinct microbial population in AD mice may play a role in the development of the disease.
From their extensive review of the literature, the American group conclude that there is a growing understanding of how different gastrointestinal tract microbial populations, beneficial or pathogenic, regulate the nervous system in health and disease. They contend that, “this holds promise in the diagnosis, treatment, and prevention of some specific neuropathologies. Determining how a microbiome, changing with Westernisation and other environmental factors, impacts a human population with growing rates of neurodevelopmental disorders and increasing life expectancy represents an urgent challenge to biomedical research and to society.”
The Chinese group reports a similar result. They propose that modulation of the gut microbiota (by probiotics or other dietary intervention) is a growing area of interest for the pharmaceutical and functional food industries, with the goal of decreasing the incidence of diseases associated with Westernisation and longevity, including Alzheimer’s disease.