This exploratory case-comparison study found that a normal-BMI fast-finisher ultramarathoner showed smaller observed differences in gut microbial functional potential and selective SCFA changes after a 217 km race, while an obese slow-finisher showed larger differences in predicted functional potential and reductions across all analyzed SCFAs.
Key Findings
Results
The normal-BMI fast-finisher exhibited smaller observed differences in enzyme-coding functions and pathways between pre- and post-race samples compared to the obese slow-finisher.
Study used shotgun metagenomic sequencing and fecal metabolite analysis on two post hoc-selected runners from the same 217 km single-stage mountain ultramarathon
The normal-BMI fast-finisher showed a predominance of enzyme-coding functions associated with nucleic acid-related processes and protein biosynthesis in post-race samples
The obese slow-finisher showed larger observed differences in predicted functional potential between pre- and post-race samples
Findings are descriptive and hypothesis-generating, not causal, due to the n=2 case-comparison design
Results
Distinct short-chain fatty acid (SCFA) patterns were observed between the two runners, with the normal-BMI fast-finisher showing selective SCFA changes and the obese slow-finisher showing reductions across all analyzed SCFAs post-race.
Fecal metabolite analysis captured SCFA profiles before and after race completion
The normal-BMI fast-finisher showed selective (not uniform) changes in SCFAs after the race
The obese slow-finisher showed reductions across all analyzed SCFAs in the post-race sample
SCFAs are microbially-produced metabolites relevant to gut health and energy metabolism
Methods
The study design involved post hoc selection of two extreme and contrasting cases from a single ultramarathon event, introducing notable limitations including selection bias, confounding effects of adiposity, and unmeasured dietary intake.
Two runners were selected post hoc to represent extreme and contrasting outcomes: a normal-BMI fast-finisher and an obese slow-finisher
The race was a single-stage 217 km mountain ultramarathon
Authors explicitly acknowledge 'confounding effects of adiposity, unmeasured dietary intake, and post hoc selection bias'
Authors characterize findings as 'hypothesis-generating data rather than establishing causal relationships with performance'
Authors call for 'further investigation in larger cohorts'
Methods
Shotgun metagenomic sequencing was used to predict microbial functional potential, capturing enzyme-coding functions and metabolic pathways in pre- and post-race fecal samples.
Methodology involved shotgun metagenomic sequencing combined with fecal metabolite analysis
Samples were collected before and after race completion for both runners
Analysis focused on enzyme-coding functions and predicted metabolic pathways
The approach characterized functional potential rather than direct metabolic activity
What This Means
This research suggests that two ultramarathon runners with very different body compositions and finishing times showed distinct changes in their gut microbiome's functional capabilities after completing a grueling 217-kilometer mountain race. The runner with a normal body weight who finished faster showed relatively stable gut microbial functions after the race, with changes mainly in functions related to building proteins and processing genetic material. The heavier, slower-finishing runner showed much larger shifts in gut microbial functional potential after the race, along with reductions in all measured short-chain fatty acids — molecules produced by gut bacteria that play important roles in energy supply and gut health. The faster finisher, by contrast, showed only selective changes in these molecules.
This research matters because it hints that differences in body composition and athletic performance may be linked to differences in how the gut microbiome responds to extreme endurance exercise. Short-chain fatty acids in particular are thought to be important for energy metabolism and recovery, so differences in how they change during ultramarathon racing could be relevant to future research on athlete nutrition and gut health.
However, this study has very significant limitations: it only examined two people who were specifically chosen because they were different from each other, and the researchers could not account for differences in diet, training, or many other factors. The authors themselves emphasize that these are exploratory, hypothesis-generating observations and not evidence of cause and effect. Much larger studies will be needed to determine whether these patterns hold up across broader groups of athletes.
Check Your Own Numbers
Upload your bloodwork. We'll cross-reference your results against this study and 4,700 others.
Saragiotto G, de Oliveira L, Geciana Tomaz Dos Santos B, Nunes Sanches R, Merizzi de Oliveira M, Campos Freire F, et al.. (2026). Exploratory case comparison of gut microbiome functional potential in ultramarathoners differing in adiposity and finish time.. Journal of the International Society of Sports Nutrition. https://doi.org/10.1080/15502783.2026.2725874