Review published in Advances in Nutrition suggests probiotics “modestly reduce the incidence, duration, and severity of respiratory tract infection”

Respiratory tract infections, illnesses such as COVID-19 that lead to infections of the sinuses, throat, airways and lungs, are common.  They exist on a spectrum from nuisances such as sore throat and coughs to potentially life-threatening ailments such as pneumonia.  Worldwide, respiratory tract infections are responsible for more than 2 million deaths annually.

Despite the high prevalence and disease burden of respiratory tract infections, current therapies are limited and often palliative rather than preventative, especially for the most common infections.  Although vaccines are available to prevent certain types of respiratory tract infections, developing new vaccines is a lengthy and costly process that frequently results in products with limited or variable effectiveness.  As a result, identifying new and cost-effective strategies for managing a broad spectrum of respiratory tract infections is needed.

In response, authors Julie L. Coleman et al. explored the effects of probiotics, prebiotics, and synbiotics on respiratory tract infection incidence, duration, and severity among adults aged 18 to 65.  The results of their scientific review, “Orally Ingested Probiotics, Prebiotics, and Synbiotics as Countermeasures for Respiratory Tract Infections in Non-elderly Adults: A Systematic Review and Meta-analysis,” were published in Advances in Nutrition, the international review journal of the American Society for Nutrition.

To conduct their research, the authors searched for randomized-controlled trials that examined the effects of orally ingested probiotics, prebiotics or synbiotics versus a placebo.  Their research led them to 42 studies that met their criteria, with the majority focusing on probiotics.  The studies under review were conducted in 16 countries across Europe, North America, Asia, Australia/Oceania, and South America.  Study populations included healthy adults, healthy physically active adults, and adults with chronic illness.

According to the authors’ findings, “orally ingested probiotics, relative to placebo, modestly reduce the incidence, duration, and severity of respiratory tract infection in non-elderly adults.”  Further research indicated that those benefits did not significantly differ by the genus, dose, number of strains of probiotic administered, or the duration of treatment.

Due to a lack of studies, the authors were not able to make a determination about the effects of prebiotic and synbiotic interventions.

Interestingly, the authors found that “orally ingested probiotics may not be effective, or may be less effective, for reducing respiratory tract infection incidence, duration and severity in physically active populations.”  The authors hypothesized that “physically active adults generally also have other lifestyle behaviors that may reduce respiratory tract infection risk, such as high-quality diets.  Possibly, probiotics do not provide any additional immune benefit in this context.”  On the other hand, high levels of exercise without adequate rest and recovery may compromise immune function and increase the risk of respiratory tract infection among physically active adults.  In either case, more research is needed to determine why physically active populations did not respond as well to probiotics compared to physically inactive populations.

The authors also found that consuming probiotics via fermented dairy products may result in a greater reduction in the number of days of illness due to respiratory tract infection.  One reason may be linked to the fermentation process itself.  The authors noted that “lactic acid bacteria used in dairy fermentations and as probiotics, may increase the bioavailability of immuno-modulatory nutrients in milk.”

In conclusion, the authors believe their systematic review and meta-analysis strengthens the findings of previous studies that relied primarily on pediatric populations by “demonstrating that orally ingested probiotics also reduce the incidence, duration, and severity of respiratory tract infection in non-elderly adult populations.”  The authors believe that “replication of positive findings for individual probiotic strains and strain combinations in high quality randomized controlled trials is necessary in order to conclusively identify the most effective strains and dosing strategies.”