Danish researchers studying constipation in space may have found why so many astronauts struggle with it. A study by the University of Copenhagen, conducted in collaboration with NASA, suggests that microgravity slows the passage of food through the intestine, changing how gut bacteria process nutrients. The findings could matter not only for future missions to Mars, but also for people with limited mobility on Earth.
What happens to the gut without gravity
Researchers analysed 488 blood samples from 52 astronauts who spent between two and nine months aboard the International Space Station (ISS). The samples were collected before, during and after missions carried out between 2006 and 2018.
The Danish study, published in Nature Communications, found changes in around 40 compounds circulating in the astronauts’ blood. Some of the clearest signs pointed towards increased protein fermentation by gut bacteria during spaceflight.
Normally, bacteria in the intestine mainly ferment carbohydrates and fibre. But when food moves slowly through the digestive system and these substrates are depleted, bacteria increasingly turn to proteins.
The researchers believe microgravity may be an important part of the explanation.
“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,” Henrik M. Roager, associate professor at the University of Copenhagen’s Department of Nutrition, Exercise and Sports, said in a university statement.
This, he added, may help explain why constipation is frequently reported by astronauts.
Why constipation in space could become a bigger problem
For a relatively short stay aboard the ISS, constipation may sound more inconvenient than dangerous. On much longer missions, however, the researchers say changes in gut activity could become more significant.
Protein fermentation can produce compounds including ammonia, hydrogen sulphide and other microbial metabolites. Previous research has associated some of these substances with adverse effects on kidney function, while others have been linked to changes involving the brain and nervous system.
Senior author Lars Ove Dragsted, professor at the University of Copenhagen, told DR that the possible consequences are unlikely to be substantial in the short term. The issue becomes more relevant when considering journeys lasting years, such as future human missions to Mars.
The study itself remains cautious. It identifies metabolic patterns consistent with slower intestinal transit rather than directly measuring how quickly food moved through each astronaut’s digestive system. The researchers therefore describe the connection with microgravity as a likely explanation that will require further experiments.
The Danish study points to a familiar solution
Creating normal gravity inside a spacecraft is difficult. Changing what astronauts eat is considerably easier.
One potential countermeasure is therefore a familiar one: more dietary fibre.
The researchers suggest increasing slowly fermented carbohydrates, giving intestinal bacteria something to process for longer and reducing the shift towards protein fermentation. Prebiotics or other treatments that promote intestinal movement could also be investigated.
As Dragsted put it in his interview with DR, “It isn’t rocket science, even though it is.”
The joke reflects a serious practical consideration. Nutrition is already carefully planned aboard the ISS, but missions lasting months or years could require diets designed not only around calories, vitamins and food preservation, but also around the behaviour of the gut microbiome in microgravity.
What Danish space research could teach doctors on Earth
The findings may also have implications much closer to home.
Reduced movement is associated with constipation on Earth, particularly among people who spend long periods confined to bed. The Copenhagen researchers say the mechanism identified in astronauts could therefore help investigate what happens to the gut of bedridden patients.
“You can use this knowledge to help bedridden patients,” Dragsted said in the University of Copenhagen statement, noting that they too frequently experience constipation and may have increased protein fermentation.
The research also raises broader questions about the role of physical movement in intestinal health. Dragsted told DR that the importance of gravity could provide another reason why exercise helps the digestive system, although this connection will need further study.
For now, the study offers an unusual example of how Danish space research can expose mechanisms that are difficult to isolate on Earth. Understanding why astronauts struggle to go to the toilet may seem like a minor problem compared with the technological challenges of space exploration. On a journey lasting years, however, keeping the digestive system working normally could become one more essential part of getting humans safely to Mars and back.
