For astronauts, life in space is filled with challenges that are easy to imagine—radiation, muscle loss, bone weakening and the psychological stress of living far from Earth. But one surprisingly common problem is much more ordinary: constipation.
Now, new research from the University of Copenhagen, carried out in collaboration with NASA, may have uncovered an important clue about why digestion changes in space.
Scientists analyzed blood samples from 52 astronauts who spent months aboard the International Space Station (ISS). Their results suggest that within just a few weeks of entering space, the activity of gut bacteria changes significantly. In particular, bacteria appear to shift toward breaking down more protein.
This discovery could help explain why astronauts frequently experience constipation—and it may become increasingly important as humans prepare for longer missions to the Moon and Mars.
The Gut Changes Soon After Leaving Earth
The human digestive system depends on a complex community of trillions of microorganisms known collectively as the gut microbiome. These bacteria help digest food and produce numerous chemical compounds called metabolites.
On Earth, gravity helps food and waste move through the digestive tract, although the process is actually driven mainly by muscular contractions known as peristalsis.
In microgravity, however, normal intestinal movement may be altered.
The researchers found evidence that astronauts' gut bacteria begin fermenting protein more heavily within weeks after arriving in space. According to Giorgia La Barbera, associate professor at the University of Copenhagen and joint first author of the study, this change continues during the space mission and reverses after astronauts return to Earth.
The finding provides a possible connection between microgravity, slower intestinal movement and constipation.
Why Would Gut Bacteria Start Breaking Down More Protein?
To understand the discovery, it helps to look at what gut bacteria normally eat.
Dietary fiber is an important fuel source for many beneficial intestinal bacteria. When bacteria ferment fiber, they produce compounds that can support a healthy intestinal environment.
But when material moves slowly through the digestive system, bacteria have more time to interact with its contents.
The researchers suggest that slower movement through the intestines in microgravity may contribute to a shift in bacterial activity. Once easily available fiber is used, bacteria can increasingly turn toward proteins as another source of nutrients.
This process is called protein fermentation.
It is not unique to astronauts. Protein fermentation can also occur in people on Earth, particularly when intestinal transit is slow or dietary conditions change.
However, the spaceflight environment may create conditions that make this process more noticeable.
Henrik Roager, associate professor at the University of Copenhagen and a co-author of the study, says the lack of gravity probably causes food to move more slowly through the intestine. This fits with the researchers' observation of increased protein fermentation and could help explain constipation during spaceflight.
Blood Tests Provided a Window Into the Gut
One of the most interesting aspects of the research is that scientists did not need to directly examine astronauts' intestines.
Instead, they analyzed metabolites in blood samples.
Metabolites are small molecules produced during the body's normal chemical processes. Because some compounds produced by gut bacteria can enter the bloodstream, blood can provide indirect clues about what is happening inside the digestive system.
The researchers used a non-targeted metabolomics approach. Rather than looking only for a few specific chemicals, they examined a broad range of molecules and searched for patterns that changed during spaceflight.
The samples came from three different studies involving astronauts aboard the ISS.
Despite having only small amounts of blood available, researchers were able to extract additional information from the existing samples.
Could These Gut Changes Affect More Than Digestion?
Constipation itself may seem like a relatively minor inconvenience compared with the other dangers of spaceflight. But researchers say the consequences of altered gut metabolism could potentially extend beyond the digestive system.
Protein fermentation produces a variety of chemical compounds. Some metabolites associated with this process have been linked in previous research to potentially unfavorable health effects.
Lars Ove Dragsted, professor at the University of Copenhagen and senior author of the study, notes that products of protein fermentation have been associated with effects such as kidney damage, changes in mood and reduced ability to concentrate.
The new study does not establish that astronauts experience these health problems because of protein fermentation. Instead, it identifies a metabolic change that could potentially be relevant to health and deserves further investigation.
This distinction is important because the researchers are observing biological signals, not proving that those signals directly cause particular diseases or neurological effects.
Why the Gut-Brain Connection Matters
The gut and brain communicate continuously through what scientists call the gut-brain axis.
Signals can travel between the digestive system and the nervous system through several pathways, including chemical signals produced by gut microbes.
That means changes in the gut microbiome can potentially influence processes elsewhere in the body.
Some metabolites produced during protein fermentation have previously been investigated for possible connections with brain function, including mood and concentration.
For astronauts, maintaining cognitive performance is especially important. During a long-duration mission, astronauts need to make complex decisions while operating in an environment where medical assistance may be millions of kilometers away.
Still, more research is needed to determine whether the metabolic changes observed in astronauts actually produce measurable effects on brain function.
A Bigger Problem on the Journey to Mars
The discovery could become particularly important as space exploration moves toward longer missions.
Astronauts aboard the ISS typically spend months in microgravity. A future crew traveling to Mars could experience a much longer period of altered gravity conditions, followed by time on the Martian surface and eventually another long journey home.
That means even seemingly small changes in digestion could become important when they persist for months or years.
Scientists may therefore need to develop ways to protect the digestive system during long-duration missions.
One possible approach is increasing dietary fiber.
Another could involve prebiotics, substances that support beneficial gut bacteria. Researchers also mention treatments designed to encourage peristalsis and reduce the amount of time material spends inside the intestine.
The goal would be to maintain healthier intestinal movement, reduce excessive protein fermentation and support the overall gut environment.
The Discovery May Help People on Earth Too
Interestingly, the research could have applications far beyond astronauts.
People who are confined to bed for long periods can also experience constipation because their normal physical activity and intestinal movement may be reduced.
If similar changes in gut fermentation occur in these patients, understanding what happens during spaceflight could potentially help researchers develop better approaches for managing digestive problems on Earth.
This makes astronauts useful as an unusual model for studying how the human body responds when normal physical conditions are dramatically altered.
Why Studying 52 Astronauts Was Important
Human metabolism varies significantly from person to person. Diet, age, lifestyle and genetics can all influence the metabolites found in blood.
That makes it difficult to identify consistent biological changes in small studies.
The researchers analyzed samples from 52 astronauts across different missions and multiple years. According to joint first author Jan Stanstrup, finding a consistent increase in signs of protein fermentation across such varied samples makes the pattern particularly interesting.
The study also detected smaller metabolic changes associated with factors such as fish and caffeine consumption, demonstrating that the blood analysis could capture multiple influences on astronauts' metabolism.
A New Piece of the Spaceflight Puzzle
Astronaut constipation may sound like a simple side effect of living in space, but the new research suggests that something more complicated could be happening inside the body.
Within weeks of entering microgravity, astronauts appear to develop measurable changes in the way their gut bacteria process food. The increase in protein fermentation may be connected to slower intestinal movement and could help explain why constipation is so common during spaceflight.
The discovery does not provide the complete answer yet. Scientists still need to determine exactly how microgravity changes intestinal movement, how long these effects persist after returning to Earth and whether increased protein fermentation produces meaningful long-term health consequences.
But as humanity prepares for increasingly ambitious missions, understanding these hidden changes could be just as important as designing better rockets.
Because when humans travel to Mars, keeping the gut healthy may be another essential part of keeping the crew healthy.
Reference: La Barbera, G., Stanstrup, J., Zwart, S. R., Smith, S. M., Roager, H. M., & Dragsted, L. O. Longitudinal metabolomics profiles reveal increased gut microbial protein fermentation during Spaceflight. Nature Communications (2026), 17. DOI: 10.1038/s41467-026-74979-w.
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