Deep Dive: Can synbiotics help prevent respiratory tract infections?

This meta-analysis found a modest, but reliable, effect in adults, but no apparent effect in children. However, what doses and strains work best is far from clear. There’s also no strong reason to suspect that these results carry over to more serious RTIs, like COVID-19.

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Study under review: Preventing Respiratory Tract Infections by Synbiotic Interventions: A Systematic Review and Meta-Analysis of Randomized Controlled Trials

Introduction

The human gastrointestinal tract is colonized by trillions^[1] of microbes from more than 1,000^[2] bacterial species that are involved in several critical physiological functions. These include the modulation^[3] of intestinal barrier function, nutrient absorption and energy metabolism^[4], regulation^[5] of the immune system, and protection^[6] against exogenous pathogenic bacteria. However, there is potential for these functions to be disrupted as a result of gut dysbiosis^[7]. Gut dysbiosis refers to the disturbance of the gut microbiota homeostasis due to (i) qualitative and quantitative changes in the gut microbiota itself, (ii) changes in the metabolic activities of the microbiota, or (iii) changes in the local distribution of the microbiota.

Since the gut microbiome is involved in physiological functions that relate to immunity and protection from pathogens, and considering that there is “crosstalk” between the gut microbiota and the lungs (termed the gut-lung axis^[8]), it’s no surprise that gut dysbiosis has been associated^[9] with respiratory tract infections^[10] (RTIs)—infectious diseases of the upper or lower respiratory tract, which include the common cold, laryngitis, pharyngitis/tonsillitis, bronchitis, and pneumonia.

One potential strategy to help prevent RTIs is the use of synbiotics^[11], which are products that combine probiotics (live microorganisms that supposedly provide health benefits when used in adequate amounts) and prebiotics (compounds that provide nutrition for the growth of beneficial microorganisms). Since pro- and prebiotics may have independent beneficial effects on the immune system, it has been suggested^[12] that the appropriate combination of both may have synergistic effects. However, trials examining the use of synbiotics for RTI prevention have yielded conflicting results. The study under review is the first meta-analysis to have pooled the available trials together to examine the overall impact of synbiotic use on RTI prevention.

The human gastrointestinal tract is colonized by microorganisms that are involved in critical physiological functions, some of which relate to host immunity. Since probiotics (live microorganisms) and prebiotics (food for live microorganisms) may have independent beneficial effects for preventing respiratory tract infections (RTIs), combining both into synbiotics may have synergistic effects. However, trials examining the use of synbiotics for RTI prevention have provided mixed results. The meta-analysis under review pooled the available trials together to examine the overall impact of synbiotic use on RTI prevention.

What was studied?

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