Astronaut constipation has long puzzled scientists, and new research now points to a clear explanation. A study published in Nature by researchers at the University of Copenhagen, working alongside NASA, suggests that the prolonged periods astronauts spend in microgravity cause food to move more slowly through the gut, leading to digestive difficulties.
Constipation is a common experience for most people at some stage of life, but for astronauts it occurs frequently. Investigations into how low- and zero-gravity environments affect human digestion date back nearly three decades, yet the precise cause had remained unclear until now.
How microgravity affects the gut
To conduct the study, researchers analysed samples from 52 astronauts who had spent months aboard the International Space Station. They found that time in space significantly altered their gut microbiomes, a change likely to trigger constipation and other gastrointestinal issues.
The gut microbiome typically breaks down carbohydrates and fibre first as its primary energy source. In the near weightlessness of space, however, food travels far more slowly through the digestive tract. This delay means gut microbes exhaust the available carbohydrates early and shift to fermenting protein for energy instead. That protein fermentation releases specific compounds into the bloodstream, which allowed researchers to demonstrate that a shortage of gut-accessible carbohydrates leads to slower bowel movements.
“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,” said Giorgia La Barbera, one of the study’s authors and an associate professor in the Department of Nutrition, Exercise and Sports at the University of Copenhagen.
A dietary solution for space travel
Researchers propose that a diet richer in slow-fermented carbohydrates could ease the problem during off-planet travel. These carbohydrates take longer for the body to process, giving the gut microbiome material to work on as it moves through the intestines.
“Eating whole foods, rich in dietary fiber, such as vegetables, lentils and nuts, would be important to keep the gut motility up to speed and to provide additional substrates for the bacteria during space missions,” said Henrik Roager, study co-author and associate professor at the University of Copenhagen. “This way, the gut microbes would be kept busy fermenting dietary fiber instead of protein.”
Roager added that supplementation with prebiotics and other treatments may also help reduce gut transit time, assisting astronauts in avoiding constipation. The recommendations mirror the advice doctors routinely give to patients on Earth who wish to prevent the same condition.
The study drew on samples from 52 astronauts who had spent months aboard the International Space Station.
