Of the 58 studies analyzed, nine of them were 9 human randomized controlled trials and the other 49 were animal studies. The human studies used probiotic strains from a variety of genera, including Lactobacillus, Lacticaseibacillus, Bifidobacterium, and Lactococcus.
“To the best of our knowledge, this work is the first systematic review to report the role of gut microbiota manipulation on the risk and outcomes of viral RTIs,” stated researchers from Capital Medical University (Beijing), the Beijing Digestive Disease Center, and the Chinese University of Hong Kong in the European Journal of Nutrition.
“We found that modulation of gut microbiota may prevent viral RTIs in humans. Animal studies showed that treatments with probiotics before viral challenge were effective in improving the outcomes of viral RTIs, in terms of reducing infection-induced mortality, mitigating symptoms, decreasing viral load and boosting host immunity against viral infection.
“Disturbance of gut microbiota deteriorated in viral RTIs, which could be reversed by microbiota restoration.”
Building the science
The new review is in-line with a recent paper from the International Probiotics Association (IPA) published in Frontiers in Cellular and Infection Microbiology, which concluded: “The interplay between the gastrointestinal microbiome, invasive viruses and host physiology is complex and yet to be fully characterized, but increasingly the evidence shows that the microbiome can have an impact on viral disease outcomes.
“While the current evidence base is informative, further well-designed human clinical trials will be needed to fully understand the array of immunological mechanisms underlying this intricate relationship.”
Interest in how the microbiome and probiotics can support immune function and response has never been higher, with scientific studies reporting a potential role of the gut microbiota in the severity and duration of COVID-19. Such links led prominent scientists like Professor Glenn Gibson and Dr Gemma Walton from the University of Reading and Dr Kirsty Hunter from Nottingham Trent University to ask the UK’s Health Secretary Matt Hancock to assess the role of the gut microbiome in coronavirus.
For the new paper in the European Journal of Nutrition, the China-based researchers explained that eight out of the nine studies identified investigated the viral infection rate reported, with six of these studies finding that probiotics were associated with a reduction in the infection risk.
The human studies used a variety of strains, including Lacticaseibacillus rhamnosus GG, Lactobacillus delbrueckii subsp. bulgaricus OLL1073R-1, Lacticaseibacillus casei Shirota, Bifidobacterium animalis subsp. lactis, and Lactococcus lactis JCM5805.
The study with the Bifidobacterium found that the probiotic was associated with a reduction in virus shedding in nasal secretion compared to placebo group. However, no significant effects were recorded in the studies for reducing the severity of virus-induced RTI symptoms.
Data from the animal studies indicated that probiotics consumption prior to an influenza viral challenge was associated with improved infection-induced survival, reduced symptoms, and reduced viral load.
More research urgently needed
“The gut-lung axis has been proposed in the pathogenesis of certain respiratory diseases,” wrote the reviewers. “Evidence has implied a gut-lung crosstalk in viral RTIs. Gut microbiota influences the susceptibility and severity of viral RTIs. Natural gut microbiota exhibiting more diverse microbiomes balanced systemic and local inflammatory responses upon lethal influenza virus challenge, resulting in higher survival rates and a milder disease course, compared with gut microbiota of laboratory mice from a restrictive environment.”
Despite the promising results, the reviewers note that research in this field is still in its infancy, and called for high-quality clinical trials, translational studies and mechanistic studies investigations to rapidly fill in the knowledge gaps.
“Unlike animal models, humans are highly heterogeneous in terms of diet, age, genetic background and gut microbiota configuration, and therefore may respond differently to the same intervention,” they wrote. “With the development of new technologies, individualized gut microbiota modification will become available to address specific consumer needs and issues. Next-generation probiotics specific to viral strains and individualized conditions of the hosts may become a promising therapy in the prevention and treatment of viral RTIs in the near future.”
Source: European Journal of Nutrition
Published online ahead of print, doi: 10.1007/s00394-021-02519-x
“Modulation of gut microbiota protects against viral respiratory tract infections: a systematic review of animal and clinical studies”
Authors: Hai Yun Shi, et al.