NASA and researchers identify cause of astronaut constipation

Researchers from the University of Copenhagen and NASA have identified why astronauts frequently experience constipation and gastrointestinal issues in space. A study published in Nature Communications analyzed blood samples from 52 astronauts who spent more than two months on the International Space Station between 2006 and 2018.

The findings suggest that microgravity causes food to move more slowly through the digestive tract. This delay leads to a shift in the gut microbiome; once available carbohydrates and fibers are fully degraded, gut bacteria switch to fermenting proteins for energy. This process releases specific metabolites into the bloodstream.

While dietary changes such as altered caffeine or fat intake contribute to metabolic shifts, they account for less than one-third of the observed changes, indicating that microgravity is the primary driver. The shift to protein fermentation produces compounds like ammonia and hydrogen sulfide, which may pose health risks during long-duration space missions.