The digestive system, especially the intestines, is the primary interface of our body for interaction and exchange of nutrients with the surrounding world. Evolutionarily, communities of microorganisms have settled at this interface, forming a spatially large and extremely complex ecosystem called the gut microbiome, which is in symbiosis with the host.
Thanks to insights accumulated over the past decade, we now know that the presence, metabolism, and activity of the gut microbiome have a regulatory function on human physiology and the functions of almost all organ systems. This includes the central nervous system. To understand the interaction between the gut microbiome and the central nervous system, the gut-brain axis model is used, which involves various pathways of communication through endocrine, nervous, and immune signals, as well as direct communication through bacterial metabolites that reach the central nervous system.
Metabolites in the intestines mainly arise from microbial fermentation. Microbial fermentation of fibers, carbohydrates, and proteins in various parts of the colon involves complex processes for obtaining energy. These processes mainly occur anaerobically and are necessary for the growth and development of a large number of intestinal microorganisms, including bacteria and fungi. However, many other roles of microbial metabolites have been established today.
Because of its pleiotropy, among the most important bacterial metabolites are short-chain fatty acids (SCFAs), small organic acids with a single carboxyl group with fewer than 6 carbon atoms, which are produced by anaerobic fermentation in the intestines.
Butyric acid is a short-chain fatty acid (SCFA) with four carbon atoms (IUPAC name: butanoic acid). Besides butyric acid, acetic and propionic acid are also classified as SCFAs.
Butyric acid is the most important mediator in communication between the gut microbiome and the host among SCFAs due to its versatile functionality. From the liver to the central nervous system, the production of this molecule by the gut microbiome regulates numerous physiological functions. The idea that we can modify its levels in the body through diet, supplementation, and lifestyle changes makes it even more fascinating.
Butyric acid, named after the Greek word for butter, is known for its intense smell of butter or spoiled milk. This scent occurs during the hydrolysis of triglycerides from fats catalyzed by lipase, either in butter, sweat (skin bacteria), or the stomach (salivary/stomach lipase), when butyric acid is released from them.
Butyric acid can enter the body in two ways. Either through diet by consuming foods rich in tributyrin triglyceride, such as butter (3g/100g), cheese (goat 1-1.8g/100g), and other dairy products, which lipases will degrade. Tributyrin in dairy products is of microbial origin, formed by the fermentation of cellulose in the rumen of cattle.
In the systemic circulation, under physiological conditions, butyrate usually comes from the endogenous gut microbiome. In the human gut microbiome, butyric acid is produced by anaerobic microbial fermentation of dietary fibers (polysaccharides) by so-called butyrate producers (Lachnospiraceae, Roseburia) from legumes, fruits, nuts, and cereals. The main metabolic pathways for production are through acetyl-coenzyme A from pyruvate or from lactate or through crotonyl-coenzyme A from glutarate, lysine, or succinate. This is why some butyrate is produced apart from the fermentation of exogenously ingested polysaccharides by interaction within the microbial ecosystem from other substrates. Cross-feeding of lactate and butyrate has been observed between butyrate-producing bacteria and lactate-producing bacteria, Bifidobacterium and Lactobacillus. In addition to this, butyrate can be produced by the degradation of mucins by, for example, Roseburia intestinalis and Eubacterium rectale. Butyrate-producing bacteria are not closely related (monophyletic group) which is why the possibility of butyrate production through the above-mentioned metabolic pathways is considered an evolutionary advantage used by different prokaryotes. However, most producers belong to the phylum Firmicutes, especially the class Clostridia, clusters Clostridium IV and XIVa.
It is interesting to note that butyric acid is also present in breast milk, and infants ingest about 30 mg of butyric acid per kilogram of body weight.
Butyric acid occurs in nature in two isoforms – n-butyric and iso-butyric acid. Since the concentrations of n-butyric acid in stool are five to eight times higher than in urine and the fact that only n-butyric acid has molecular and pharmacological activity, the focus is on n-butyric acid.
Since butyric acid is mainly present in the deprotonated form in the systemic circulation, it is often called butyrate. In the intestines, however, butyric acid contributes to lowering the pH value. The pH value as a determinant of the intraluminal milieu of the intestine is crucial for the efficiency of butyrate production: the concentration of butyrate is inversely proportional to the pH value. Butyrogenic bacteria that produce butyrate multiply at lower pH values compared to gram-negative species of the Bacteroides genus, which prefer a higher pH.
Butyrate, the anionic form of dissociated butyric acid, is involved in various physiological processes. From the regulation of energy homeostasis and related body weight to the regulation of the immune system, carcinogenesis, and the functioning of the central nervous system. Current scientific knowledge even suggests that the immunity and metabolism of the host depend on butyrate as a regulator of their processes.
Butyrate, like other SCFAs and chemically related ketone bodies such as β-hydroxybutyrate, has promising effects in the treatment of various diseases: from metabolic disorders such as obesity and insulin resistance to inflammatory bowel diseases, colorectal cancer to neurological diseases. Butyrate has been used experimentally in pharmacology and neuroscience in doses ranging from 100 to 1200 micrograms per kilogram of body weight.
Butyrate is considered a key factor in the interaction between the host and the gut microbiome, especially in the gut-brain relationship.
Butyrate exerts its activity by binding to receptors GPR43/FFAR2, GPR41/FFAR3, GPR109a/HCAR2, and by transport through MCT1/SLC16A1 and SMCT1/SLC5A8. These receptors are located on:
- epithelial cells of the intestine – colonocytes and enteroendocrine cells
- liver cells – hepatocytes
- nerve cells – in the vagus nerve, enteric nervous system, central nervous system
- immune cells – regulatory T cells, monocytes, neutrophils.
In addition to acting on the above receptors in the mentioned cells, the mechanisms of action of butyrate also involve its use by cells as an energy source through mitochondrial β-oxidation processes and the inhibition of histone deacetylase (HDAC) and thus the promotion of genetic expression (epigenetic modulation). Interestingly, butyrate is the first endogenous substance discovered to have inhibitory activity on HDAC and is the most potent natural inhibitor of it. However, not all mechanisms of butyrate action have been fully elucidated yet.
Considering that almost all butyrate in the body is actually produced in the intestines, its main action is in the digestive system and on organ systems closely related to it, such as the immune and enteric nervous systems. The largest number of receptors for butyrate is found on colonocytes, and their number is proportional to the concentrations of butyrate within the intestines. Butyrate is the main source of energy for colonocytes through mitochondrial β-oxidation. In the digestive system, butyrate also plays an important role in the integrity of the intestinal barrier. Butyrate and other SCFAs affect the expression of tight junction proteins that connect intestinal membrane cells. Through the activation of 5-HT3 receptors on enteroendocrine cells, butyrate stimulates serotonin production, which affects intestinal motility. Additionally, butyrate in enteroendocrine cells stimulates the production of GLP-1, peptide YY, and other peptide hormones that regulate appetite and metabolism. Only micromolar amounts of butyrate reach the systemic circulation. The highest concentration outside the digestive system is in the portal vein. In the liver, a large portion of butyrate is used for energy needs through β-mitochondrial oxidation, where it metabolizes into ketone bodies and acetyl-coenzyme A. In peripheral circulation, the concentration of butyrate is only 20% of that in the portal vein. Excess butyrate is excreted in the intestines, and concentrations in stool range from 3.5 to 32.6 grams per kilogram of stool.
Butyrate is an important immunomodulator. It is believed that due to the coevolution of the gut microbiome and the host immune system, the immune system has become very sensitive to the action of butyrate. Human immunity matures and then is regulated by the microorganisms of the human body, their mere presence, and their metabolites. Butyrate, like other metabolites of the gut microbiome, is signaling molecules that inform the immune system about the composition and activity of the gut microbiome. Depending on the levels of butyrate with which the immune system in the intestines and circulation comes into contact, it will increase or decrease its activity. A rapid decrease in butyrate indicates an increase in pathogenic bacteria, which requires increased activity of the immune system. Butyrate itself has a proven anti-inflammatory potential. Through the inhibition of HDAC and activation of FFAR2 receptors, butyrate promotes the maturation of regulatory T lymphocytes that regulate the proinflammatory immune response. On the other hand, through the inhibition of HDAC, butyrate reduces the levels of CD4+ and CD8+ T lymphocytes, which are crucial for the pro-inflammatory response.
Physiological levels of butyrate in the body indirectly affect the function of the central nervous system through the regulation of the immune response and stimulation of the vagus nerve. Low concentrations of butyrate in circulation and the high selectivity of receptors in the central nervous system are reasons why butyrate probably does not directly affect the central nervous system. However, changes caused by butyrate in the immune system are reflected in the immune milieu of the brain, and direct stimulation of the peripheral nervous system, vagus nerve, and enteric nervous system indirectly affects brain function. Besides the integrity of the intestinal barrier, butyrate and other SCFAs also have a positive, very likely indirect effect on the integrity of the blood-brain barrier. The application of exogenous butyrate as a drug has proven effective in experimental conditions, which is why it is considered a potential neuropharmacological agent, primarily due to its ability to modulate epigenetics (inhibition of HDAC). Many neuropsychiatric diseases are associated with increased proinflammatory response in the body, and by reducing it, butyrate may be effective in therapy. Through the inhibition of HDAC, butyrate affects the activity of microglia and thus achieves its neuroprotective effects. However, the inhibition of HDAC by butyrate is also associated with increased neuroplasticity due to increased expression of proteins crucial for memory formation in the context of neurodegenerative diseases, addiction, and learning. However, in the context of treating brain diseases, many aspects of the central action of butyrate are not yet fully understood.
The volatile butyrate, originating from the gut microbiota, may also be involved in the host’s behaviour, i.e., social communication. Butyrate has one of the most intense smells. Two hypotheses try to explain this phenomenon: one claims that evolution has led to the association of this smell with biological decay, fermentation, and toxins, and the other claims that the presence of butyrate in the smell indicated a good composition and activity of the gut microbiome, and therefore good health. The proportion of volatile SCFAs, which is associated with the composition of the gut microbiome, is an important component of the individual’s “olfactory fingerprint,” as confirmed in animal studies. The smell is extremely important for social interaction, which is why it is believed that due to its influence on the olfactory fingerprint, there is also a potential role of butyrate in social interaction. This claim is supported by research on the scent glands of hyenas: the microbiome within the glands, dominated by bacteria from the Firmicutes phylum that produce volatile fatty acids, depending on its composition, affected the scent characteristics of hyenas related to gender and species.
We will likely learn much more about butyrate in the coming years. The biggest questions concern its mechanisms of action, especially regarding the central nervous system. Butyrate has great potential in treating many diseases: from functional and inflammatory bowel diseases to various diseases of the central nervous system. It is not surprising when we know that butyrate is a regulator of gut health, metabolism, and the immune system, and in addition to all that, it has the ability of epigenetic modulation. While butyrate can be introduced from the outside, it seems that perhaps the most important thing is to ensure adequate endogenous production of butyrate in our gut microbiome.
Adapted from: Stilling R.M. et al., The neuropharmacology of butyrate: The bread and butter of the microbiota-gut-brain axis? Neurochemistry International, 2016.
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