Background: There is a bidirectional communication system between the gut-microbiota and the brain. Probiotics are live microorganisms that should have health benefits on the host. Modulations of the gut-microbiota using probiotics to target inflammation, stress and depression have shown promising results in rodent studies and are now to be proven in human subjects. 

Method: 20 male participants aged 25-35, were aliquoted into three groups. All groups consumed placebo tablets for two weeks. For additionally six weeks, two of the groups were administered different commercially available probiotic tablets containing Lactobacillus reuteri and the last group continued the placebo tablets. Venous blood samples and a questionnaire about general wellbeing were collected at baseline, after two weeks, and at the end of the eight weeks study. Cortisol was analysed by electrochemiluminescence immunoassay (ECLIA). An enzymatic colorimetric test was used to determine glucose concentrations (GOD-PAP). Soluble urokinase plasminogen activator receptor (suPAR) concentrations determined with ELISA. Oxidative stress was carried out using Free Oxygen Radicals Testing (FORT).

Results: For all biomarkers no significant differences were found (p>0.05). For the questionnaires, a significant difference was found in the placebo group. Here, a general wellbeing improved throughout the study. 

Conclusion: The present study was affected by the COVID-19 pandemic, which led to a decrease in number of participants, change of environment and sampling at different times of the day which could have had an impact on diurnal variation. Although none of the biomarkers showed statistically significant results, further investigations of Lactobacillus reuteri in human subjects are necessary. In addition, this study highlights the importance of developing a robust model for translating findings in rodent studies to healthy human subjects.