Our lungs and respiratory system are vital for many bodily functions. Lung diseases remain as leading causes of death worldwide [1]. More people are now taking steps to maintain a healthy respiratory system and prevent lung diseases. In recent years, researchers have begun to uncover the association between the gut microbiota and respiratory health. Read on to find out more about the gut-lung axis.

Gut Health and Gut Microbiota

Our gut consists of trillions of such microorganisms, including bacteria, viruses, fungi and protozoa species. This set of gut microbiota is unique to everyone and varies due to several factors including environmental, lifestyle, dietary habits, and consumption of medications such as antibiotics[2].

This innate set of gut microbiota (the set that we are born with) is thought to be the most optimum for oneself. This optimum composition deteriorates as we age. Along with poorer modern-day diets lacking in prebiotics (food for the good gut bacteria) as well as increased use of medications, the composition and amount of good gut bacteria decrease even faster.

An optimum gut microbiota consists of a healthy balance of both good and bad gut bacteria species, which leads to good gut health.

Introduction to the Lung Microbiota

Recent technological advances have discovered the presence of microbes in our lungs, which were traditionally thought to be sterile [3]. In healthy lungs, it is estimated that there are about 10³to 10⁵ bacteria per gram of tissue. This number is much lower compared to the lower gastrointestinal tract, where there are about 10¹¹ bacteria cells per gram of tissue [3].

The colonization of bacterial species in the lungs depends on several factors such as air inhalation, migration of microbes from the oral cavity, and local conditions in the lungs such as pH, temperature and oxygen levels [3,4]. The bacterial species that reside in the lungs are similar to those in the gut, which consists of mainly Firmicutes and Bacteroidetes [5].

Gut-Lung Axis: Evidence

Although anatomically distinct, the gut and lungs communicate bi-directionally via their respective sets of microbiotas. This communication is referred to as the gut-lung axis[4]. Just as a healthy set of gut microbiota supports our cardiovascular health [6] and immune system [7], the health of the gut also affects our lungs.

Dysbiosis refers to an alteration of the gut microbiota and is associated with many inflammatory diseases both within and outside the gastrointestinal (GI) tract. Inflammatory diseases include inflammatory bowel disease (IBD) [8] and asthma [9]. Evidence of the gut-lung axis is further demonstrated in patients with chronic GI-related diseases such as IBD: these patients also tend to have a higher prevalence of respiratory-related diseases [10].

Other evidence that the gut and lungs communicate with each other [4]:

• Gut to lung microbiota: changes to an infant’s diet have been found to alter the lung microbiota
• Lung to gut microbiota: influenza infection was found to induce changes in the gut microbiota, including a decrease in Lactobacillus species, a gut microbe species known to be beneficial to humans [11]

Gut-Lung Axis: How They Interact

Good gut bacteria feed on prebiotics in a process called fermentation, which produces short-chain fatty acids (SCFAs). SCFAs such as butyrate and propionate have been found to have immunomodulatory and anti-inflammatory effects in the gastrointestinal tract [12].

These SCFAs also travel from the gut to the lungs, leading to local effects on the lungs such as modulating lung immune response and anti-inflammatory effects, which can potentially play a role in respiratory inflammatory diseases such as asthma [4]. Hence, dysbiosis at the gut level may affect immune responses in the lungs.

Conclusion

Ever since the COVID-19 pandemic started, more emphasis has been placed on the importance of lung health and ways to proactively support our lungs. The existence of the gut-lung axis shows that improving our gut microbiota through dietary changes and supplements such as prebiotics provides an opportunity to improve our lung health and protect against respiratory diseases such as asthma and COPD.

Related articles

Gut-Brain-Immune Axis: An Introduction

Prebiotic: Gut Microbiota, Gut Health, and Beyond

Keeping Your Gut Microbiota Healthy to Combat Long COVID and Severe COVID-19 Infection

References

1. GBD Chronic Respiratory Disease Collaborators. Prevalence and attributable health burden of chronic respiratory diseases, 1990-2017: a systematic analysis for the Global Burden of Disease Study 2017. Lancet Respir Med. 2020 Jun;8(6):585-596.
2. Rutsch A, Kantsjö JB., Ronchi F. The Gut-Brain Axis: How Microbiota and Host Inflammasome Influence Brain Physiology and Pathology. Immunol. 2020;11.
3. Yagi K, Huffnagle GB, Lukacs NW, Asai N. The Lung Microbiome during Health and Disease. Int J Mol Sci. 2021 Oct 8;22(19):10872.
4. Enaud R, Prevel R, Ciarlo E, et al. The Gut-Lung Axis in Health and Respiratory Diseases: A Place for Inter-Organ and Inter-Kingdom Crosstalks. Front Cell Infect Microbiol. 2020 Feb 19;10:9.
5. Faner R, Sibila O, Agustí A, et al. The microbiome in respiratory medicine: current challenges and future perspectives. Eur Respir J. 2017 Apr 12;49(4):1602086.
6. Witkowski M, Weeks TL, Hazen SL. Gut Microbiota and Cardiovascular Disease. Circ Res. 2020 Jul 31;127(4):553-570.
7. Yoo JY, Groer M, Dutra SVO, et al. Gut Microbiota and Immune System Interactions [published correction appears in Microorganisms. 2020 Dec 21;8(12):]. Microorganisms. 2020;8(10):1587.
8. Nishida A, Inoue R, Inatomi O, et al. Gut microbiota in the pathogenesis of inflammatory bowel disease. Clin J Gastroenterol. 2018 Feb;11(1):1-10.
9. Hufnagl K, Pali-Schöll I, Roth-Walter F, Jensen-Jarolim E. Dysbiosis of the gut and lung microbiome has a role in asthma. Semin Immunopathol. 2020 Feb;42(1):75-93.
10. Dang AT, Marsland BJ. Microbes, metabolites, and the gut-lung axis. Mucosal Immunol. 2019 Jul;12(4):843-850.
11. Ahrne S, Hagslatt ML. Effect of lactobacilli on paracellular permeability in the gut. Nutrients. 2011 Jan;3(1):104-17.
12. Vinolo MA, Rodrigues HG, Nachbar RT, Curi R. Regulation of inflammation by short chain fatty acids. Nutrients. 2011 Oct;3(10):858-76.