Space travel can be hard on the gut

Space travel is often associated with dramatic challenges such as radiation exposure, muscle loss and the effects of living without gravity. But there is also a far less glamorous problem that astronauts know well – constipation.

Scientists have long known that digestion can become disrupted in space, but new findings out of Copenhagen, conducted with Nasa, suggest one possible reason lies in the way gut bacteria behave once the body enters microgravity.

For their research, the experts analysed blood samples from 52 astronauts who spent months aboard the International Space Station (ISS). They found signs that “indicate their gut bacteria begin to ferment protein to a greater extent than usual within weeks after the astronauts arrive in space”, said Giorgia La Barbera, an associate professor at the University of Copenhagen.

Protein fermentation is not unique to astronauts – it happens even on Earth, when gut bacteria have already used much of the available dietary fibre and begin breaking down protein instead.

The researchers suspect microgravity may slow the movement of food through the intestines, giving bacteria more time to switch towards protein fermentation. “This may also help explain constipation in astronauts,” said co-author Henrik Roager.

The body normally moves food and waste along the digestive tract through coordinated muscular contractions known as peristalsis. If that process slows, digestion can change in ways that go beyond simple discomfort.

To further understand what was happening, the team studied metabolites – small molecules in the blood that can reveal processes taking place throughout the body, including inside the gut. Some products created during protein fermentation can enter the bloodstream.

Senior author Lars Ove Dragsted pointed out that previous research has associated certain compounds with negative health effects, including kidney damage and possible changes in mood or concentration.

That raises questions about the gut-brain axis, the two-way communication system linking the digestive tract and the brain, and whether altered gut activity in space could have broader effects.

For astronauts spending months on the ISS, constipation may be inconvenient. On longer missions to the Moon or Mars, prolonged changes in gut function could become a more crucial health concern.

The researchers say possible countermeasures may include increasing dietary fibre, using prebiotics or finding other ways to encourage normal intestinal movement. The aim would be to shorten the time food remains in the gut, reducing excessive protein fermentation and helping maintain a healthier digestive environment.

The findings may also be relevant to patients on Earth who experience constipation and slowed intestinal movement. “You could, for example, use this knowledge to help bedridden patients,” Dragsted said.

The study is further notable for its unusually broad dataset: although the samples came from 52 astronauts across different missions and years, the scientists still detected a consistent pattern of increased protein fermentation.

These findings offer another reminder that the human body is finely tuned to life on Earth. Remove gravity, and even something as ordinary as moving food through the intestines may no longer work quite the same way.

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