• Posted August 18, 2026

Constipation In Space: A New Study Provides Answers

Constipation is a well-known problem for astronauts in space, and researchers now think they know why it occurs. 

Weightlessness appears to affect the gut microbiome, the billions of healthy bacteria that reside in a person’s digestive tract, researchers reported recently in Nature Communications.

"We see changes in astronauts’ blood samples that indicate that the gut bacteria begin to ferment protein to a greater extent than usual within weeks after the astronauts arrive in space, and this change continues until they are back on Earth," co-lead researcher Giorgia La Barbera said in a news release. She is an associate professors of nutrition, exercise and sports at the University of Copenhagen in Denmark. 

Protein fermentation happens when gut bacteria break down protein instead of fiber because the fiber in a person’s diet has been used up, researchers said.

"The lack of gravity in space probably causes food to move more slowly through the intestine, and this fits with the fact that we are seeing signs of increased protein fermentation," co-researcher Henrik Roager, also an associate professor of nutrition, exercise and sports at the university, said in a news release. "This may also help explain constipation in astronauts."

For the new study, researchers analyzed blood samples of 52 astronauts who participated in monthslong missions to the International Space Station.

Astronauts tend to develop constipation soon after entering orbit, researchers said in background notes.

Results showed signs of increased protein fermentation that could not be explained by changes in the astronauts’ diets.

"We know that products of protein fermentation are often associated with negative health consequences, such as kidney damage, potential effects on mood or reduced ability to focus, among others," said senior researcher Lars Ove Dragsted, a professor of nutrition, exercise and sports at the University of Copenhagen. 

The study highlights yet another health concern that will need to be taken into account as mankind aims for missions to Mars, researchers said.

“When we plan for longer journeys in space — for example, to Mars — countermeasures may be needed to mitigate negative effects of space travel on the intestines,” La Barbera said.

The study could point the way to interventions, another researcher said.

"Our findings can inform potential interventions — either directly through increasing dietary fiber, supplementation with prebiotics or other types of treatments that promote peristalsis and decrease gut transit time," Roager said. "This would help reduce protein fermentation and thus provide a healthy gut environment and avoid negative health consequences."

Prebiotics are supplements that provide fiber to feed healthy gut bacteria.

These results also might be used to promote health among the Earthbound, researchers added.

“You can use this knowledge to help bedridden patients,” Dragsted said. “They also experience constipation and likely also have increased protein fermentation that may aggerate health conditions.”

More information

The National Aeronautics and Space Administration has more on the risks of human spaceflight.

SOURCE: University of Copenhagen, news release, Aug. 14, 2026