Gut Microbiota Dynamics During Extreme Altitude Climbing: A Longitudinal Case Report.

Álvarez-Herms, Jesús, Kilian Jornet, Martin Burtscher, Adriana González, and Adrián Odriozola. Gut microbiota dynamics during extreme altitude climbing: a longitudinal case report. High Alt Med Biol. 27:123-127, 2026.-The present case study would explore the acute changes of gut microbiota (GM) and some physiological responses of a world-class mountaineer during a 4-week stay at altitudes between 3,600 and 4,200 m, with four rapid ascents to different extreme altitudes, that is, 6,700, 7,000, 8,000, and 8,400 m. Severe hypoxemia, hyperglycemia, ketoacidosis, and sympathetic dominance were found to be proportionally higher with increasing altitude. Thus, the GM changed during the acclimatization period, decreasing bacterial diversity (Shannon index from 5.88 to 4.56). Remarkably, during the third week, a transient increase of the pathogenic bacteria Ruminococcus gnavus (+2,800%) was found. Thus, this significant elevation coincided with the impaired systemic health and gastrointestinal and respiratory complications. In conclusion, the present exploratory study found that a favorable and healthy GM profile, characterized by elevated mucolytic and short-chain fatty acids-producing bacteria, may improve hypoxic tolerance, reduce the risk of developing gut leakage syndrome and systemic endotoxemia, and protect against acute mountain sickness.