Background The prevalence of allergic rhinitis has been estimated to range from 10 to 30% in adults and up to 40% in children. Probiotics have been tested as for this condition, but no census on which probiotic types are most effective. Objective Our aim of this study was to provide probiotic strain-specific evaluation for the treatment of allergic rhinitis. Methods Databases (PubMed, Google Scholar, and ScienceDirect) were searched (from inception to October 2020) to identify randomized controlled trials (RCTs) of probiotic treatments in allergic rhinitis patients. A systematic review was done comparing types of reported outcomes, clinical and immunological parameters and safety data. A meta-analysis was done for probiotics with at least two RCTs per probiotic strain(s) and sharing a common outcome. Results A total of 13 randomized, double-blind, placebo-controlled trials (N = 1591 participants) were included in the systematic review. Overall, 8 of 9 probiotic types alleviated at least one clinical symptom of allergic rhinitis. The meta-analysis results showed that, neither L. paracasei Lp33 (SMD = −1.61, 95% CI −4.67, 1.45) nor L. rhamnosus GG (SMD = −1.00, 95% CI −3.01, 1.00) had significant efficacy for reducing nasal symptom scores. Conclusions Our analysis showed probiotics produced a slight improvement in some clinical and immunological measurements on allergic rhinitis. Due to the diversity of outcome measurements and lack of sufficient trials for each probiotic strain, future trials are needed with similar study design and uniform outcomes to better compare the effect of probiotics on allergic rhinitis.

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Research on the probiotic effect in preventing or treating diseases has attracted scientists’ attention for many decades. Findings of probiotics effects on human health indicate that they are not only no detrimental but also may have a beneficial effect on the host. Indeed, the effectiveness of probiotics depends on the type of utilized strain, duration, dose administration, and single or combined strains used that can be different in a specific disease. Therefore, probiotics can play a significant role in the treatment and prevention of different diseases through several mechanisms; for instance, stimulating respiratory immunity in the airway and enhancing resistance to respiratory tract infections, can prevent or reduce the duration of respiratory system diseases. By ameliorating glucose metabolism, reducing inflammation and oxidative stress in pancreatic cells, and preventing the destruction of β-pancreatic cells, may prevent the onset of diabetes and the pathogenesis of diabetic retinopathy as well. Moreover, using their metabolites, especially short-chain fatty acids production, probiotics may have an important effect on weight modifications. Finally, from the regulation of important neurotransmitters and regulation of inflammatory markers, it may be effective in mental disorders improvement.

The study of the probiotic effect in the prevention or treatment of diseases has long attracted the attention of many researchers. Here, we collected close to 300 meta-analysis articles for 20 years, investigating the effect of probiotics in the prevention and treatment of diseases. The goal of this study is to provide an overview of all meta-analysis articles of the effects of probiotics on various human diseases. For this purpose, different online databases, Pubmed, ScienceDirect, and Google Scholar, were searched with the keywords "probiotics" + "disease" + "meta-analysis" in the title, abstract, and keywords. Papers studied and categorized and investigated in order to present valuable insights for researchers in the field. Some of main categories are based on publication year, publishing journals, gender, age, effect type, disease type, contradicting reports and etc. According to the results, most meta-analyses indicated probiotics were 79% effective in preventing or treating the diseases. Some articles have also reported no positive effects, but there is not any paper in our study confirming the detrimental influence of probiotic effect on human health. For the future works, Cochrane reviews, meta-analysis including dozens of articles (as e.g. for NEC and AAD) may be investigated.