The Impact of Ultra-Marathon Running on the Gut Microbiota as Determined by Faecal Bacterial Profiling, and Its Relationship with Exercise-Associated Gastrointestinal Symptoms: An Exploratory Investigation

<b>Background/Objectives:</b> This exploratory study aimed to evaluate the impact of an 80 km ultra-marathon trail running event on changes in faecal bacterial composition, and to investigate whether any correlations exist between exercise-associated gastrointestinal symptoms (Ex-GIS) with faecal bacterial profiles. Such events represent a unique physiological stressor and may impact the composition of the gut microbiota. Studying this impact may provide insights into acute (i.e., <24 h) gut microbiota changes under extreme conditions. <b>Methods:</b> Thirteen endurance athletes (<i>n</i> = 7 males, <i>n</i> = 6 females) aged 41 ± 8 years completed the 80 km Margaret River (Australia) ultra-marathon race in 2022. Faecal samples were collected pre- and post-race. Faecal bacterial profile, as per relative abundance (RA) of operational taxonomic units and the determination of α-diversity (Shannon Equitability Index (SEI)), was achieved by 16S rRNA amplicon gene sequencing. Changes in RA% and SEI pre- to post-race were assessed by the Wilcoxon signed-rank test. Correlations between Ex-GIS with bacterial profile and changes pre-, during, and post-ultra-marathon race were determined by Spearman's rank correlation coefficients. <b>Results:</b> Bacterial calculations of phyla (<i>n</i> = 5), family (<i>n</i> = 23), and genus (<i>n</i> = 41) were detected for RA (≥0.5%). A significant decrease pre- to post-race of <i>Actinobacteriota</i> (<i>p</i> = 0.035) phyla, <i>Bifidobacteriaceae</i> (<i>p</i> = 0.007), and <i>Clostridiaceae</i> (<i>p</i> = 0.010) family, and <i>Blautia</i> (<i>p</i> = 0.039) and <i>Subdoligranulum</i> (<i>p</i> = 0.023) genus was determined; meanwhile, <i>Oscillospiraceae</i> (<i>p</i> = 0.016) and <i>Monoglobaceae</i> (<i>p</i> = 0.039) family significantly increased pre- to post-race. No other bacterial group changes were observed. No correlations were observed between pre- to post-ultra-marathon RA change and Ex-GIS. <b>Conclusions:</b> The completion of an 80 km ultra-marathon did not invoke substantial changes in the gut microbiota as determined by faecal bacterial profiling. Very strong and strong correlations were observed between certain bacterial groups and Ex-GIS; however, no significant correlations were observed between pre- to post-ultra-marathon changes in RA ≥ 0.5% and Ex-GIS.