A significant portion of our immune system is in our gut.

Gut mucosal lining is one of the largest surfaces in the human body that is continuously under attack by invaders. It comes into constant contact with potential pathogens and food-related antigens. While the main function of the gut is to absorb nutrients from the food we eat, it must also protect the body from these invading microorganisms and food-related toxins. In this context, it is not surprising that the gut harbors more than 70% of our body’s immune cells and that the intestinal wall is the largest reservoir of tissue macrophages (a type of immune cells which destroy pathogens by engulfing them) [1].
Gut is also home to more than 40 trillion microbes and the gut immune system is specialized not to attack these commensal microbes while fending off potentially pathogenic microorganisms [2].

The gut houses a collection of specialized structures which are collectively known as gut-associated lymphoid tissue (GALT). These structures are important features of the innate and adaptive immune systems and they contribute to the immune function in different ways. For example, goblet cells, which are a type of highly specialized epithelial cells, secrete mucin, a gel-like substance. This mucin acts as a glue which traps pathogens, and thus prevent them from binding to the epithelial cells. Another type of cells that help prevent the invasion by microorganisms is Paneth cells that secrete defensins—a microbicidal substance—thus destroying the potentially pathogenic microorganisms. They also produce antibacterial substances such as lysozyme and cathelicidins, thus further preventing the entry of microorganisms to the mucosal layer of the intestines[3].

In addition to the gut-associated lymphoid tissue, the vast number of commensal bacteria living inside the gut also help the immune system by competing with invaders for attachment sites and nutrients, and producing antimicrobial compounds that destroys pathogens. They also help educate the immune system from birth, thus teaching it to distinguish between commensals and intruders.

Imbalances in the gut commensal microflora composition (known as dysbiosis) can result in gut infections such as Clostridium difficile (C. diff.)-associated diarrhea as well as inflammatory conditions such as the inflammatory bowel disease (IBD). Several studies have also linked gut dysbiosis with pathological conditions outside the gut such as rheumatoid arthritis, and diabetes mellitus.
Given this background, it is evident that the gut plays a major part in keeping us healthy and that the gut microbiota plays a major part in maintaining a healthy gut.

What is a healthy gut?

A healthy gut is defined as one that promotes effective digestion, is free of gastrointestinal diseases and inflammations, harbors a healthy gut microbiota, and sustains a healthy immune system[4].

Gut health and gut microbiota

Impairment of the gut microbial composition can directly influence the healthy status of the gut while on the other hand gut-associated pathological conditions can influence the composition of the gut microbiota.

Usage of antibiotics is one major factor that contributes to the gut dysbiosis. While antibiotics are essential to treat infections, it is important not to misuse or over-use them. Studies have linked the antibiotic usage to gut dysbiosis and gut inflammation. In one such study carried out by Lin Sun and colleagues, they treated mice with antibiotics and measured the effect of antibiotics on the gut microbiome and inflammatory response. The study revealed that the antibiotics altered the gut microbial composition and these changes induced inflammation of the gut tissues[5].

Another factor that largely influence gut health is our diet. Mounting scientific evidence suggest that diets rich in carbohydrates and fat and low in indigestible fiber changes the composition of the gut microbiome considerably and these changes can result in altered metabolic and inflammatory conditions. A study conducted by Peter J. Turnbaugh and the group using mice, revealed that, switching the diet to a “Western-style” diet which is high in fat and sugar from a low-fat, plant fiber-rich diet, resulted in drastic changes in the gut microbial population of mice within one day.

Studies suggest that exercise can also modulate gut microbial composition. Megumi Matsumoto and colleagues observed that the gut microbiota composition of rats who have undergone wheel-running exercise regimen was significantly different from the gut microbiome of sedentary rats. They suggested that this change of the microbial composition may be one of the underlying factors of the positive effects of exercise on gut disorders.

Researchers also claim that gut microbiota are important in the development of the gut immune system. Several studies done using germ-free mice point out that without gut microbiota, the immune system doesn’t develop properly[6].

How to keep our gut healthy?

Evidently, gut health is directly linked to the food we eat and the lifestyles we maintain. Our gut microbiota also plays a crucial part in maintaining a healthy gut and a healthy immune system.

According to science, taking a diet rich in fruits and vegetables, exercising regularly and staying away from antibiotics unless they are absolutely necessary will help maintain a healthy gut microbiome. Research also say that taking prebiotics and probiotics can help rebuild an imbalanced gut microbiome[7]. A healthy and well-balanced gut microbiome will keep the gut healthy and in turn, keep the immune system functioning efficiently.

References:

[1]        J. B. FURNESS, W. A. A. KUNZE, and NADINE CLERC, “Nutrient Tasting and Signaling Mechanisms in the Gut,” Am J Physiol Gastrointest Liver Physiol, vol. 277, no. 11, pp. 922–928, 1999.

[2]        U. M. Mörbe et al., “Human gut-associated lymphoid tissues ( GALT ); diversity , structure , and function,” Mucosal Immunol., no. March, 2021, doi: 10.1038/s41385-021-00389-4.

[3]        M. Schenk and C. Mueller, “The mucosal immune system at the gastrointestinal barrier,” Best Pract. Res. Clin. Gastroenterol., vol. 22, no. 3, pp. 391–409, 2008, doi: 10.1016/j.bpg.2007.11.002.

[4]        S. C. Bischoff, “‘Gut health’: A new objective in medicine?,” BMC Med., vol. 9, 2011, doi: 10.1186/1741-7015-9-24.

[5]        L. Sun et al., “Antibiotic-induced disruption of gut microbiota alters local metabolomes and immune responses,” Front. Cell. Infect. Microbiol., vol. 9, no. APR, 2019, doi: 10.3389/fcimb.2019.00099.

[6]        J. L. Round and S. K. Mazmanian, “The gut microbiota shapes intestinal immune responses during health and disease,” Nature Reviews Immunology, vol. 9, no. 5. Nature Publishing Group, pp. 313–323, May 2009, doi: 10.1038/nri2515.

[7]        A. Gagliardi et al., “Rebuilding the gut microbiota ecosystem,” International Journal of Environmental Research and Public Health, vol. 15, no. 8. MDPI AG, Aug. 07, 2018, doi: 10.3390/ijerph15081679.