Why do astronauts get constipated in space? Blood tests offer new clues
Blood Markers Reveal How Microgravity Slows the Astronaut Gut
Poinews.com – For decades, crew members on long-duration space missions have reported sluggish bowel function, yet the physiological mechanism behind the slowdown has eluded explanation. A team at the University of Copenhagen, collaborating with NASA, now points to a biochemical signature in astronauts' blood that illuminates what happens to the digestive tract once the body is removed from Earth's gravitational pull.
A Consistent Biochemical Signal Across Missions
The investigators pooled blood samples drawn from 52 astronauts who had each spent months aboard the International Space Station. By combining data from three separate studies spanning multiple missions over several years, the team achieved a sample size uncommon in spaceflight research. Using a non-targeted metabolomics strategy—scanning a wide spectrum of circulating metabolites rather than hunting for one predetermined compound—they identified a uniform elevation in markers of intestinal protein fermentation.
The shift appeared within weeks of departure from Earth, persisted for the duration of each mission, and faded once the crew members returned to normal gravity. The researchers interpret this pattern as evidence that the gut environment itself is altered by microgravity.
From Fibre to Protein: What Happens When Food Lingers
Under ordinary conditions, the intestines propel a meal forward through rhythmic muscular contractions called peristalsis. In the near-absence of gravity, the team hypothesises that this transit becomes markedly slower. When chyme dwells in the intestinal lumen for extended periods, resident microbes exhaust the available dietary fibre and pivot to fermenting protein instead. That fermentation yields small-molecule metabolites capable of crossing into the bloodstream and distributing throughout the body.
The rise in protein-fermentation products aligns with the hypothesis that microgravity retards intestinal transit, thereby contributing to the constipation astronauts experience.
The Gut–Brain Axis and Cognitive Consequences
Not every consequence of altered fermentation stays localised to the abdomen. Certain metabolites generated during protein breakdown have been linked in prior work to shifts in mood and diminished capacity for sustained attention, operating through what scientists term the gut–brain axis—a bidirectional signalling channel between the digestive tract and the central nervous system. The Copenhagen–NASA team flags these connections as a priority area for follow-up investigation.
The researchers are careful to note that protein fermentation is not exclusive to spaceflight; it also occurs in terrestrial populations. What distinguishes the astronaut data is the consistently elevated magnitude of the effect, implying a genuine alteration of the intestinal milieu during orbital residence.
Looking Ahead: Lunar and Martian Missions
As space agencies design crewed campaigns to the Moon and, ultimately, Mars, the duration of microgravity exposure will grow substantially. If intestinal motility continues to lag over months or years, countermeasures will become operationally essential. The authors propose interventions such as augmented dietary fibre intake, prebiotic supplementation, or pharmacological agents that stimulate peristaltic activity.
The relevance of these findings may extend well beyond orbit. Patients confined to bed for prolonged periods face a comparable reduction in intestinal motility, and the same interplay of slowed transit and heightened protein fermentation could contribute to their gastrointestinal and systemic health problems.
In the end, the study offers a plausible biochemical account for one of space travel's less celebrated hazards: strip away gravity, and the gut follows suit into a state of sluggish, protein-fermenting stasis.
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