Two 2021 reviews covered gut microbiota. The first was gut microbial origins of metabolites produced from our diets, and mutual effects:
“Gut microbiota has emerged as a virtual endocrine organ, producing multiple compounds that maintain homeostasis and influence function of the human body. Host diets regulate composition of gut microbiota and microbiota-derived metabolites, which causes a crosstalk between host and microbiome.
There are bacteria with different functions in the intestinal tract, and they perform their own duties. Some of them provide specialized support for other functional bacteria or intestinal cells.
Short-chain fatty acids (SCFAs) are metabolites of dietary fibers metabolized by intestinal microorganisms. Acetate, propionate, and butyrate are the most abundant (≥95%) SCFAs. They are present in an approximate molar ratio of 3 : 1 : 1 in the colon.
95% of produced SCFAs are rapidly absorbed by colonocytes. SCFAs are not distributed evenly; they are decreased from proximal to distal colon.
Changing the distribution of intestinal flora and thus distribution of metabolites may have a great effect in treatment of diseases because there is a concentration threshold for acetate’s different impacts on the host. Butyrate has a particularly important role as the preferred energy source for the colonic epithelium, and a proposed role in providing protection against colon cancer and colitis.
There is a connection between acetate and butyrate distinctly, which suggests significance of this metabolite transformation for microbiota survival. The significance may even play an important role in disease development.
https://www.hindawi.com/journals/cjidmm/2021/6658674/ “Gut Microbiota-Derived Metabolites in the Development of Diseases”
A second paper provided more details about SCFAs:
“SCFAs not only have an essential role in intestinal health, but also enter systemic circulation as signaling molecules affecting host metabolism. We summarize effects of SCFAs on glucose and energy homeostasis, and mechanisms through which SCFAs regulate function of metabolically active organs.
Butyrate is the primary energy source for colonocytes, and propionate is a gluconeogenic substrate. After being absorbed by colonocytes, SCFAs are used as substrates in mitochondrial β-oxidation and the citric acid cycle to generate energy. SCFAs that are not metabolized in colonocytes are transported to the liver.
- Uptake of propionate and butyrate in the liver is significant, whereas acetate uptake in the liver is negligible.
- Only 40%, 10%, and 5% of microbial acetate, propionate, and butyrate, respectively, reach systemic circulation.
- In the brain, acetate is used as an important energy source for astrocytes.
Butyrate-mediated inhibition of HDAC increases Nrf2 expression, which has been shown to lead to an increase of its downstream targets to protect against oxidative stress and inflammation. Deacetylase inhibition induced by butyrate also enhances mitochondrial activity.
SCFAs affect the gut-brain axis by regulating secretion of metabolic hormones, induction of intestinal gluconeogenesis (IGN), stimulation of vagal afferent neurons, and regulation of the central nervous system. The hunger-curbing effect of the portal glucose signal induced by IGN involves activation of afferents from the spinal cord and specific neurons in the parabrachial nucleus, rather than afferents from vagal nerves.
Clinical studies have indicated a causal role for SCFAs in metabolic health. A novel targeting method for colonic delivery of SCFAs should be developed to achieve more consistent and reliable dosing.
The gut-host signal axis may be more resistant to such intervention by microbial SCFAs, so this method should be tested for ≥3 months. In addition, due to inter-individual variability in microbiota and metabolism, factors that may directly affect host substrate and energy metabolism, such as diet and physical activity, should be standardized or at least assessed.”
https://www.hindawi.com/journals/cjidmm/2021/6632266/ “Modulation of Short-Chain Fatty Acids as Potential Therapy Method for Type 2 Diabetes Mellitus”
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