The process of fermentation and short-chain fatty acid production
You have probably heard about the benefits for overall health of a diet high in fiber, but do you know where those benefits come from? Dietary fibers, which are mostly found in fruit, vegetables, cereals and legumes, are not absorbed by the host and instead reach the colon where they are fermented (eaten) by the gut bacteria. Bacteria thrive on these substrates and release beneficial compounds known as short-chain fatty acids (SCFAs), along with gases that can induce bloating.
First and foremost, fiber content helps with stool consistency and transit time. Then, by releasing CO2 and other gases, the process of fermentation itself makes the gut environment more anaerobic (low oxygen) and inhibits the growth of pathogens that thrive in an aerobic (high oxygen) environment.
SCFAs and their benefits for gut health
Around 90%–95% of SCFAs are absorbed by the body and the remaining 5%-10% feed other bacteria before being eliminated in feces. The three main SCFAs are acetate, propionate and butyrate, which occur in a ratio of 60:20:20 in the large intestine, with butyrate being the most closely studied.
SCFAs, as their name implies, make the colon environment more acidic. That low pH favors the growth of beneficial microorganisms while preventing the colonization of pathogens and enhances the absorption of minerals such as iron. In addition, an acidic environment resulting from SCFA production decreases the amount of secondary bile acids, which are known to produce deleterious effects on gut health.
SCFAs provide energy to colonic cells while also assisting them with regeneration and maintaining optimum gut barrier impermeability. Indeed, butyrate stimulates the formation of tight junctions in between cells, causing them to pack tightly together, thus making the gut more impermeable.
Butyrate also aids in the formation of mucus, which is a gel-like protective barrier that lies over intestinal cells and protects them from pathogens. This mucus also constitutes a habitat and food source for beneficial bacteria.
Butyrate also has other highly beneficial properties. For instance, it regulates the body’s inflammatory response and serves as an anti-inflammatory molecule, which is key considering that intestinal inflammation plays an important role in the development of metabolic diseases such as obesity and diabetes. Furthermore, butyrate has been found to restrict the proliferation and induce the death of cancerous cells, indicating that it may possesses anti-cancer effects. Additionally, butyrate may improve the efficiency of cancer-related treatments such as chemotherapy, radiotherapy and immunotherapy.
Microbial metabolites beyond short chain fatty acids
In addition to SCFAs, the gut microbiota secretes a variety of compounds. Vitamins such as vitamin K and B12 are secreted by some microorganisms, while other bacteria that feed on leftover proteins produce neurotransmitters, which are small molecules that interact with the nervous system. Tryptophan, which is found in protein-rich foods such as lean chicken, fish, eggs, tofu, oatmeal and nuts and seeds, has been shown to have butyrate-like properties, regulating inflammation and the gut barrier. And through that protein-rich food, the gut microbiota can produce branched-chain fatty acids, although it remains to be seen whether they are beneficial or detrimental to gut health.
In conclusion, the gut microbiota produces a variety of helpful metabolites that promote not only gut health but overall health. We cannot stress this enough: eating a healthy diet rich in fruit and vegetables is your best ally for staying healthy.
Jardon KM, Canfora EE, Goossens GH, et al. (2022). “Dietary macronutrients and the gut microbiome: a precision nutrition approach to improve cardiometabolic health.” Gut 71:1214-1226.
Armet, A. M., et al. (2022). “Rethinking healthy eating in light of the gut microbiome.” Cell Host Microbe 30(6): 764-785.
Lavelle, A. and H. Sokol (2020). “Gut microbiota-derived metabolites as key actors in inflammatory bowel disease.” Nature Reviews Gastroenterology & Hepatology 17(4): 223-237.
Roager, H. M. and T. R. Licht (2018). “Microbial tryptophan catabolites in health and disease.” Nature Communications 9(1): 3294.
Kaźmierczak-Siedlecka, K., et al. (2022). “Sodium butyrate in both prevention and supportive treatment of colorectal cancer.” Frontiers in Cellular and Infection Microbiology 12.
Rios-Covian, D., et al. (2020). “An Overview on Fecal Branched Short-Chain Fatty Acids Along Human Life and as Related With Body Mass Index: Associated Dietary and Anthropometric Factors.” Frontiers in Microbiology 11.