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Billions of microbial cells sculpt the gut ecosystem, playing essential roles in human physiology. Since endurance athletes’ performance is often physiology-limited, understanding the composition and interactions within these athletes’ gut microbiomes could lead to improved performance. Previous studies describe differences in the relative abundance of bacterial taxa when comparing athletes versus controls or athletes before and after an endurance event, suggesting the existence of an “endurance microbiome”. However, there are inconsistencies among studies in which taxa correlate with extended physical exertion. Although these studies employed similar barcoding methods, variation in downstream bioinformatic analyses makes it difficult to determine whether inconsistencies are due to methodological differences or biological factors. Herein, we created a metagenomic bioinformatics workflow reanalyzing four 16S rDNA sequence datasets reflecting endurance athletes’ gut microbiomes, looking at alpha diversity, changes in relative abundance of gut microbiome genera, changes in pairwise correlations between bacterial genera and compared bacterial association networks. There were no significant differences in alpha diversity between any of the four treatment group comparisons. For relative abundance, there were no consistent differences in all four datasets, and only two genera were significantly different in 50% of the datasets. Although many genera showed changes in pairwise correlations in endurance microbiome samples from individual datasets, none were consistent across datasets. Collectively, these results suggest that either there is no universal endurance microbiome, or that it remains elusive even after controlling for the bioinformatic workflow and statistical analyses. Using this data, a power analysis indicates that sample sizes 150- to 800-fold larger than these published studies would be necessary to detect a 10% difference in relative abundance. Furthermore, 10- to 20-fold more samples will be needed to control for the multitude of covariates (genetic, metabolic, dietary, environmental, and pharmacological factors) that mold the gut microbiome of athletes and non-athletes alike.I’m going to work so that it’s a pure guts race at the end, and if it is, I am the only one who can win it. - Steve Prefontaine