A new study from the University of Copenhagen, conducted in collaboration with NASA, sheds light on why astronauts so commonly experience constipation in space. By analyzing blood samples from 52 astronauts across multiple International Space Station missions, researchers found that gut bacteria begin fermenting protein at higher-than-normal rates within weeks of entering microgravity — and the change persists until astronauts return to Earth.

The leading theory: microgravity slows the movement of food through the digestive tract. When food lingers longer in the intestines, gut bacteria exhaust available fiber and turn to protein as a fuel source instead. The byproducts of this protein fermentation — metabolites that enter the bloodstream — have been linked to kidney damage, mood changes, and reduced concentration, raising health concerns that extend well beyond digestive discomfort.

Key Takeaways:

• Increased dietary fiber, prebiotic supplementation, or treatments that promote gut motility (peristalsis) could help counter these effects during long-duration spaceflight
• The findings may apply to bedridden patients on Earth, who experience similar constipation and likely comparable gut fermentation changes
• A non-targeted metabolomics approach allowed researchers to detect broad metabolic shifts from limited blood sample volumes

Why it matters: With missions to the Moon and Mars on the horizon, understanding and mitigating gut health disruptions in astronauts is increasingly urgent. But the science doesn't stop at the stratosphere — these findings could reshape how clinicians manage digestive health in immobile or bedridden patients right here on Earth.