The impact of actual and simulated space conditions on the gut microbiome and the modulatory role of nutrition: A systematic review and meta-analysis

This study aimed to evaluate the effects of spaceflight and simulated space environments on the human gut microbiome by examining also the role of nutrition as a modulator in these alterations. Relevant studies were identified through searches of PubMed, PubMed Central and Scopus from 2000 through July 10, 2025. Alterations to gut microbial diversity, the Firmicutes-to-Bacteroidetes (F/B) ratio, and other physiological values were reported. Statistically significant fluctuations in gut microbial composition were collected as mean differences and standard deviations and statistically analysed with 95 % confidence intervals (CI) using SPSS. Heterogeneity was assessed using the I 2 statistic. The link between these changes and other confounding factors was statistically evaluated. 15 studies were included, with 3 being actual spaceflights and 12 being space environment simulations. 6 out of the 11 studies that examined gut microbial diversity were included in the meta-analysis, and they presented a non-statistically significant trend toward increased diversity during the control phase (Hedges' g: -0.05, p = 0.84). The environment appeared to influence the observed alterations, with actual spaceflights having a greater impact (Hedges' g: 0.58, p = 0.19). Nutritional strategies, particularly those rich in fiber, probiotics, and vitamin D, were linked with stabilised or improved microbiome diversity, in contrast to ultra-processed foods (UPFs). Overall, the findings suggest a negative effect of space conditions on gut microbial diversity, along with an adaptive microbiome response, with targeted nutritional interventions being promising in preserving gut health. Further research is needed to develop effective countermeasures that can be applied in both space and on Earth. https://www.crd.york.ac.uk/PROSPERO/, identifier CRD420251068366 • Clear tendency of decrease of gut microbial diversity during space missions • Actual space flights have higher impact gut microbial diversity alternations • Nutrition has a promising modulatory role on space-induced gut microbial diversity alternations • There are negative effects of space conditions on gut microbial diversity