Constipation is a surprisingly common problem for astronauts living in space.
Scientists have long known that digestion can change when people leave Earth, but exactly what happens inside the gut has remained unclear.
Now, blood samples from astronauts are providing new clues.
Researchers from the University of Copenhagen, working with NASA, studied blood samples from 52 astronauts who spent months aboard the International Space Station. They found signs that the way gut bacteria process food begins to change within just a few weeks of entering space.
The study, published in Nature Communications, suggests that weightlessness may slow the movement of food through the intestines. This could encourage gut bacteria to break down more protein than usual.
Normally, many gut bacteria prefer to feed on dietary fiber. But when available fiber has been used up, bacteria can begin breaking down protein instead. This process, known as protein fermentation, also happens in people on Earth and is not necessarily unusual by itself.
However, researchers found evidence that protein fermentation increased after astronauts arrived in space and remained elevated until they returned to Earth.
One possible explanation is the lack of gravity. On Earth, gravity and normal intestinal muscle movements help food travel through the digestive system. In microgravity, food may move more slowly. A longer journey through the intestines gives bacteria more time to ferment protein, and the slower movement could also contribute to constipation.
Researchers discovered these changes by studying metabolites in the astronauts’ blood. Metabolites are small molecules produced as the body processes food and carries out other biological functions. Their levels can provide scientists with clues about diet, metabolism and what is happening inside the gut.
The findings could have implications beyond digestive discomfort. Some substances produced when gut bacteria ferment protein have been associated with negative health effects, including kidney problems. Researchers say they could potentially also influence mood and the ability to concentrate.
That makes understanding astronauts’ digestive health particularly important as space agencies prepare for much longer missions. Future crews traveling to the moon or Mars could spend months or years away from Earth, meaning relatively small health problems might become more serious over time.
The researchers suggest several possible ways to protect astronauts’ gut health. Increasing dietary fiber or using prebiotics could encourage beneficial gut bacteria. Other treatments might help food move through the intestines more quickly, reducing the opportunity for excessive protein fermentation.
The discovery could also benefit people who never leave Earth. Bedridden patients often experience slow digestion and constipation because of prolonged inactivity. Similar changes in gut bacteria may occur in these patients.
By studying what happens to the human body in the unusual environment of space, scientists may therefore find better ways to keep both astronauts and people on Earth healthier.
