Astronauts' Blood Tests Expose How Microgravity Rewires Gut Bacteria Within Weeks

Constipation has long been an uncomfortable reality for astronauts, but the biological reason has remained poorly understood. A new study from the University of Copenhagen, conducted in collaboration with NASA, now offers a compelling mechanistic explanation rooted in how microgravity disrupts gut bacteria and digestive transit time.

Researchers analyzed blood metabolites — small molecules circulating in the bloodstream that reflect internal biological processes — from 52 astronauts during months-long stays aboard the International Space Station. They detected clear signs that gut bacteria shifted toward protein fermentation rather than their usual preference for dietary fiber. This shift appeared within weeks of entering space and persisted until the astronauts returned to Earth.

The working hypothesis is that microgravity slows the movement of food through the intestines. When transit time increases, bacteria exhaust available fiber and turn to protein as a fuel source instead. Protein fermentation produces metabolic byproducts associated with negative health outcomes, including kidney damage, mood disruption, and reduced cognitive focus — concerns that become especially serious on multi-year missions to Mars.

Beyond space medicine, these findings have broader relevance. Gut motility naturally slows with aging, and older adults often experience constipation, microbiome shifts, and increased protein fermentation for similar reasons. Understanding the microgravity model could illuminate mechanisms behind age-related gut dysfunction and the downstream effects on brain and organ health.

The researchers suggest that targeted dietary interventions — such as increased fiber intake or probiotic supplementation — may help counteract these shifts during prolonged spaceflight. Similar strategies could also benefit aging populations on Earth who experience slowed gut transit. The study reinforces the gut-brain-body axis as a critical system to monitor and support across all environments where normal physiological cues are disrupted.