Our lungs and respiratory system are vital for many bodily functions. Lung diseases remain as leading causes of death worldwide . 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.
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.
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.
Recent technological advances have discovered the presence of microbes in our lungs, which were traditionally thought to be sterile . 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 .
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 .
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. Just as a healthy set of gut microbiota supports our cardiovascular health  and immune system , 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)  and asthma . 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 .
Other evidence that the gut and lungs communicate with each other :
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 .
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 . Hence, dysbiosis at the gut level may affect immune responses in the lungs.
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.