by Sandra Saville RD, IPA Education and Communications Committee, reviewed by Artur Ouwehand PhD and IPA Scientific Committee
With health presently top of mind, many questions surround what we can do to keep healthy and support the immune system. Maintaining a healthy immune system allows the body’s defense system to fight pathogens such as viruses. At the same time, immune responses are negatively affected by ageing (Pawelec, 2018), various health conditions, obesity (Smith et al., 2009) and stress (Locke et al., 1984) (Nishihira et al., 2018).
The immune system is similar within the digestive tract, lungs and mouth/throat. Substances produced by the immune system help us defend against or respond to bacterial pathogens and viruses.
Microbiota and Upper Respiratory Tract Infection
Viruses such as influenza can cause severe upper respiratory tract infection (URTI) (Nishihira et al., 2018). However, studies indicate that beneficial changes to the gut microbiota can reduce complications and intensity of URTI (Gao et al., 2020), and symptom severity and duration (Vouloumanou et al., 2009).
Probiotics and the Immune System
There is a general consensus that consuming probiotics supports immune function by adjusting the microbial balance or by interacting with the host immune system (Corthésy et al., 2007; Ouwehand, 2007). The effects can include an increase in the count and activity of immune cells such as leukocytes, neutrophils and natural killer cells (Guillemard et al., 2010a). Azad et al. (2018) noted that probiotics and synbiotics have the potential to enhance immune responses. Similarly, Nishihira et al. (2018) observed that “Among various potential candidates, the use of probiotics is one possible way to prevent influenza virus infection.” Furthermore, short chain fatty acids and other compounds produced by probiotics and stimulated by prebiotics also have a positive effect on the immune system (Frei et al., 2015). Overall a consistent body of evidence suggests that consumption of probiotics may decrease the incidence or severity of common infections, including acute respiratory infections (King et al., 2014).
Probiotics: Evidence for Impact in Infections
Studies indicate that certain lactic acid bacteria, strains of Lactobacillus and Bifidobacterium may help the immune system by producing or triggering beneficial compounds. However, this is limited to certain probiotic strains that may support the immune system (Rask et al., 2013; Van Puyenbroeck et al., 2012).
Several large clinical trials have been conducted with selected strains of probiotics and synbiotics (probiotic and prebiotic combination) to assess their ability to reduce URTI, influenza, and rhinovirus infection (common cold).
- A fermented dairy product containing a strain of L. casei has been shown to modulate the immune system, improving the antibody response to influenza vaccination and increasing the rate of seroprotection and seroconversion (Boge et al., 2009). In another study, daily consumption of the same fermented dairy product containing a strain of L. casei could reduce the risk of common infections, including respiratory and gastrointestinal common infectious diseases (Guillemard et al., 2010b).
- A study with L. plantarum and L. paracasei concluded reduced frequency of colds, fewer days with cold symptoms, and less severe symptoms (Berggren et al., 2011).
- A study with a different strain of L. plantarum administered for 3 months found a reduced incidence of URTI after 1 and 2 months, and reduced nasal symptoms after 3 months (Chong et al., 2019).
- A study of a group consuming L. gasseri in yogurt showed better immune response and antibodies in response to the influenza vaccine (Nishihira et al., 2016, 2018).
- A study of people consuming a combination of L. gasseri, B. longum and B. bifidum concluded that the severity and duration of URTI could be reduced, but there was no change in the likelihood of infection (de Vrese et al., 2005).
- A multi-year trial of several groups taking a combination of probiotics (L. plantarum, L. rhamnosus, B. lactis) with either fructo-oligosaccharide (FOS) or galacto-oligosaccharide (GOS) prebiotics also indicated decreased frequency, severity and duration of URTI (Pregliasco et al., 2008).
- A study in children showed that L. rhamnosus in yogurt had lower incidence of URTI, pharyngitis, and tonsillitis (Villena et al., 2012), while in vivo models support the notion of the probiotic priming the immune system to fend off infections (Salva et al., 2010).
- A study with a strain of L. casei also modulated the systemic and upper airways immune response in marathon runners (Vaisberg et al., 2019), reduced the incidence and duration of URTI in healthy middle-aged office workers (Shida et al., 2017), elderly inhabitants of day care centers (Fujita et al., 2013) and athletes (Gleesson et al., 2012).
- Finally, a study found that children consuming B. lactis and L. acidophilus had less severe and shorter duration of cold and flu-like symptoms, including cough, fever, and runny nose (Leyer et al., 2009).
While it is difficult to extrapolate results from studies with different strains and different types of respiratory infections, collectively, these results point to support of the immune system using select strains of probiotics, certain prebiotics and specific combinations of synbiotics. In certain cases, these may potentially reduce the incidence, severity, and duration of some viral URTI. These results cannot be extrapolated to other populations such as the immune compromised. Finally, while it may be tempting to look at these results in the light of the current COVID-19 epidemic, it should be noted that currently no studies have been performed with probiotics in this field.
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