Probiotics may reduce respiratory tract infections (RTI) and lung diseases
International studies are in agreement that probiotics may have a positive effect in preventing or reducing the risk of viral RTI, as well as lung diseases such as asthma and chronic obstructive pulmonary disease (please refer to references).
The physiology and pathology of the respiratory and gastrointestinal tracts are closely related. Research found that probiotics probably act as immune modulating agents and activators of host defence mechanisms. This is an indication that probiotics can also exert an effect on the prevalence and intensity of diseases at a position distant from the gut [1, 2, 3].
The general mechanisms by which probiotics are beneficial to the human body are the following  (Note the four i’s):
1. Suppression of invasion, epithelial binding and growth of pathogenic bacteria;
2. Improvement of intestinal barrier function;
3. Modulation of the immune system (these include inducement of protective cytokines and suppression of pro-inflammatory cytokines);
4. Modulation of intestinal pain perception.
The intestinal epithelial cell is the presumed first target of probiotic action. Probiotic bacteria stimulate several intestinal epithelial cell protective responses, including enhancement of epithelial barrier function, mucin synthesis and secretion . Mucin is a glycoprotein found in saliva, gastric juice etc. that form viscous solutions and act as lubricants or protectants on external and internal surfaces of the body. Gastric mucin plays a major role in protecting the gastrointestinal tract from acid, proteases, pathogenic microorganisms and mechanical trauma .
In addition to oral delivery of probiotics, efficacy has also been demonstrated after nasal administration . Intranasal delivery of Lactobacillus rhamnosus strain GG protected mice from H1N1 influenza virus infection by regulating respiratory immune responses  A lower incidence of ventilator associated pneumonia was reported after probiotic use , while reduced respiratory infections in healthy and hospitalized children was observed [9, 10]. Reduced duration of infection with the common cold was reported by .
It has been proposed that the protective effects of probiotics in the airways (both intranasal and oral applications) are associated with the activation of pro-inflammatory NK cells and/or macrophages within the airway mucosa [7, 12]. As it is unlikely that lactic acid bacteria (LAB) will enter into the circulation and then into the lungs, this effect can more likely be attributed to the fact that the LAB exert an immunomodulatory action affecting the function of immune cells that migrate into the lungs. “Probiotics therefore improve the barrier function of the intestinal mucosa, reducing leakage of antigens through the mucosa and thereby the amount of allergen that the lung may be exposed to via the circulatory route” . Direct modulation of the immune system may occur through the induction of anti-inflammatory cytokines or through the increased production of secretory IgA or via activation of Treg cells or alterations in macrophage function resulting in reduced allergic responses .
A meta-analysis conducted in 2016 by an international team of researchers, showed that regular probiotic use to prevent respiratory tract infections in Canada, could save as much as $100 million per year in terms of health-system savings, as well as productivity losses due to illness . Benefitting the most would be children under the age of 10, people living in a community setting and working in open spaces, as well as those not vaccinated against influenza . A study published in the Cochrane Database for Systematic Reviews  reported that “probiotics were better than placebo in reducing the number of participants experiencing episodes of acute upper respiratory tract infections (URTI), the mean duration of an episode of URTI, antibiotic use and cold-related school absence”. This study was a meta-analysis of 12 trials involving 3720 participants from eight countries. For several reasons the quality of the data in this study was however found to be low .
Although it cannot be claimed that probiotics will definitely prevent RTI, there is enough evidence to conclude that probiotics can/may play a role in activating the immune system, while at the same time providing a physical barrier to infection. This may reduce the risk of contracting respiratory tract infections, as well as the duration and severity of such an episode.
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