New research from the University of Copenhagen, conducted with NASA, analyzed blood metabolites from 52 astronauts who spent months aboard the International Space Station. Within weeks of leaving Earth, the samples showed signs that gut bacteria were fermenting protein more than usual, a pattern that continued until the astronauts returned. The authors say this shift may help explain why constipation is so common in space.
The likely trigger is microgravity slowing the movement of food through the digestive tract, leaving bacteria more time to switch from fiber to protein. That fermentation produces metabolites that can enter the bloodstream, and the researchers note these products have been associated with negative health effects such as kidney damage, mood changes, or reduced ability to focus. The findings could matter for future long-duration missions to the Moon or Mars, where crews would spend far more time away from Earth's gravity.
The study's authors suggest possible countermeasures, including increasing dietary fiber, using prebiotics, or treatments that promote peristalsis and shorten gut transit time. They also point out that bedridden patients on Earth experience similar constipation and may show comparable protein fermentation, meaning the results could extend beyond spaceflight. The study is based on a single source, so no conflicting findings{