As soon as people started living in space, it became apparent that it was harder to go "number two" among the stars than down on Earth — and not just because of how complicated it is to use a space toilet. For most astronauts, this gastrointestinal change is just an inconvenience, but some have to take laxatives to overcome the problem. The drugs are routinely stocked on most space missions, including the recent Artemis II mission.
In a new NASA-backed study, published June 29 in the journal Nature Communications, researchers analyzed more than 400 blood samples from 52 astronauts who each spent more than two months at a time living on board the International Space Station (ISS). Their stays took place between 2006 and 2018.
NASA astronauts can experience constipation from the first week of the mission until they return to Earth. This photo shows astronauts Chris Williams (left), Sergey Kud-Sverchkov (center) and Sergei Mikaev (right) shortly after returning from a 241-day stay on the ISS in July 2026. (Image credit: Bill Ingalls/NASA via Getty Images)
"We see changes in astronauts' blood samples that indicate that the gut bacteria begin to ferment protein to a greater extent than usual," study first author Giorgia La Barbera, an associate professor in the University of Copenhagen's Department of Nutrition, Exercise and Sports, said in a statement. This happens "within weeks after the astronauts arrive in space" and can persist "until they are back on Earth," she added.
"The lack of gravity in space probably causes food to move more slowly through the intestine," study co-author Henrik Roager, head of the University of Copenhagen's Microbiome & Metabolomics research group, said in the statement. That means fiber has more of a chance to be thoroughly broken down, and gut bacteria eventually turn to protein instead.
Going to the bathroom in space is already more complicated than it is on Earth. This photo shows the toilet, or Waste and Hygiene Compartment (WHC), on the ISS. (Image credit: NASA)
The researchers also found that changes to astronauts' diets — including altered caffeine, fish and fat intake — may influence constipation, Live Science's sister site Space.com reported. However, this dietary factor accounts for less than one-third of the observed metabolite changes, leaving microgravity as the primary cause, the researchers noted.
"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," study co-author Lars Ove Dragsted, head of preventive and clinical nutrition at the University of Copenhagen, said in the statement. When it comes to intestinal health, products of protein fermentation have also been linked to inflammation and weakening of the gut's lining.
Researchers are concerned that long-haul spaceflights could cause metabolites from protein fermentation to rise to dangerous levels. This photo shows a Russian Soyuz rocket carrying the most recent crew members to the ISS on July 14, 2026. (Image credit: Bill Ingalls/NASA via Getty Images)
It is also unclear how prolonged periods of reduced gravity, like those experienced by future residents of NASA's upcoming moon base, could affect protein fermentation in the gut. (The current study covered only brief stints on the ISS.)
RELATED STORIES"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.
"This would help reduce protein fermentation and thus provide a healthy gut environment and avoid negative health consequences," Roager said.
This article is for informational purposes only and is not meant to offer medical advice.
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