This 2021 human review subject was interactions of gut health and disease with brain health and disease:
“Actions of microbial metabolites are key for appropriate gut-brain communication in humans. Among these metabolites, short-chain fatty acids (SCFAs), tryptophan, and bile acid metabolites / pathways show strong preclinical evidence for involvement in various aspects of brain function and behaviour.
Dietary fibres, proteins, and fats ingested by the host contain components which are metabolized by microbiota. SCFAs are produced from fermentation of fibres, and tryptophan-kynurenine (TRP-KYN) metabolites from dietary proteins. Primary bile acids derived from liver metabolism aid in lipid digestion, but can be deconjugated and bio-transformed into secondary bile acids.
One of the greatest challenges with human microbiota studies is making inferences about composition of colonic microbiota from faeces. There are known differences between faecal and caecal microbiota composition in humans along with spatial variation across the gastrointestinal tract.
It is difficult to interpret microbiome-host associations without identifying the driving influence in such an interaction. Large cohort studies may require thousands of participants on order to reach 20 % explanatory power for a certain host-trait with specific microbiota-associated metrics (Shannon diversity, relative microbial abundance). Collection of metadata is important to allow for a better comparison between studies, and to identify differentially abundant microbes arising from confounding variables.”
https://www.sciencedirect.com/science/article/pii/S0149763421001032 “Mining Microbes for Mental Health: Determining the Role of Microbial Metabolic Pathways in Human Brain Health and Disease”
Don’t understand why these researchers handcuffed themselves by only using PubMed searches. For example, two papers were cited for:
“Conjugated and unconjugated bile acids, as well as taurine or glycine alone, are potential neuroactive ligands in humans.”
Compare scientific coverage of PubMed with Scopus:
- 2017 paper: PubMed citations 39; Scopus citations 69.
- 2019 paper: PubMed citations 69; Scopus citations 102.
Large numbers of papers intentionally missing from PubMed probably influenced this review’s findings, such as:
- “There are too few fibromyalgia and migraine microbiome-related studies to make definitive conclusions. However, one fibromyalgia study found altered microbial species associated with SCFA and tryptophan metabolism, as well as changes in serum levels of SCFAs. Similarly, the sole migraine-microbiota study reported an increased abundance of the kynurenine synthesis GBM (gut-brain module).
- Due to heterogeneity of stroke and vascular disease conditions, it is difficult to make substantial comparisons between studies. There is convincing evidence for involvement of specific microbial genera / species and a neurovascular condition in humans. However, taxa were linked to LPS biosynthesis rather than SCFA production.
- Several studies suggest lasting microbial changes in response to prenatal or postnatal stress, though these do not provide evidence for involvement of SCFA, tryptophan, or bile-acid modifying bacteria. Similar to stress, there are very few studies assessing impact of post-traumatic stress disorder on microbiota.”
These researchers took on a difficult task. Their study design could have been better.