In the weightless corridors of the International Space Station, the human body quietly unravels habits it took millions of years to form. A joint study from the University of Copenhagen and NASA, drawing on blood samples from 52 astronauts, has found that microgravity slows the gut's muscular rhythms so profoundly that resident bacteria abandon their usual diet of fiber and turn instead to fermenting protein — a metabolic shift with consequences that reach from the intestines into the bloodstream, the kidneys, and perhaps the mind itself. As humanity contemplates journeys to the Moon and Mars,
Microgravity alters gut bacteria, causing digestive disruption in astronauts
Remove gravity, and even the most routine biological process becomes something the body must relearn.
Mark
So the astronauts' guts are fermenting protein instead of carbs. Why does that matter beyond the obvious discomfort?
Mimi
Because the byproducts of that fermentation enter the bloodstream. Some of those compounds have been linked to kidney damage and changes in mood or concentration. We're not saying it's happening to every astronaut, but the mechanism is there.
Luke
Wait—has anyone actually documented kidney damage or mood changes in the astronauts they studied? Or is that a theoretical concern based on other research?
Mimi
It's theoretical at this point. The study shows the metabolites are present in the blood. The connection to actual harm is based on previous research, not on what happened to these 52 astronauts.
Mark
And the cause is that food moves slower in microgravity?
Mimi
That's the hypothesis. Peristalsis—the muscular contractions that move food along—appears to slow without gravity. So food sits longer, bacteria have more time to switch their metabolism, and you get this shift toward protein fermentation.
Luke
But they didn't directly measure peristalsis in these astronauts, right? They measured metabolites in blood and inferred the mechanism?
Mimi
Correct. The mechanism is inferred from the metabolite data and what we know about how gut bacteria work.
Mark
For someone going to Mars, how long would they be exposed to this?
Mimi
Months, possibly longer depending on the mission profile. That's why the researchers are already thinking about countermeasures—more fiber, prebiotics, ways to keep things moving.
Luke
And those countermeasures—have any of them been tested in space yet?
Mimi
Not that I can see from this study. They're proposing them as possibilities based on what we know works on Earth.
Mark
The study also mentions this could help bedridden patients. That's interesting—so immobility on Earth creates a similar problem?
Mimi
Apparently. The digestive slowdown isn't unique to weightlessness; it's what happens when the body isn't moving normally. The research gives us a mechanism to work with.
Luke
So the real value here is understanding the mechanism, not yet having a solution.
Mimi
Exactly. They've identified what's happening. Now comes the work of figuring out how to prevent it.
Le Pouls
- Astronauts have long endured constipation and bloating in orbit, but a new study finally names the mechanism: microgravity slows peristalsis, leaving food to linger and bacteria to turn protein into their fuel of last resort.
- Blood samples from 52 ISS astronauts showed the same unsettling pattern within weeks of arrival — a consistent, reproducible rewiring of gut microbial behavior that no individual diet or mission profile could explain away.
- The byproducts of protein fermentation don't stay in the gut — they enter the bloodstream, where prior research links them to kidney damage and disruptions along the gut-brain axis that could cloud mood and concentration on missions where mental clarity is survival.
- With crewed missions to Mars potentially lasting years, researchers warn that what is manageable discomfort on the ISS could become a sustained physiological crisis in deep space.
- Proposed countermeasures — higher fiber diets, prebiotics, and interventions to restore intestinal motility — offer a navigable path forward, and the findings may equally benefit bedridden patients on Earth whose immobility mirrors the stillness of weightlessness.
In the weightless corridors of the International Space Station, the human body quietly unravels habits it took millions of years to form. A joint study from the University of Copenhagen and NASA, drawing on blood samples from 52 astronauts, has found that microgravity slows the gut's muscular rhythms so profoundly that resident bacteria abandon their usual diet of fiber and turn instead to fermenting protein — a metabolic shift with consequences that reach from the intestines into the bloodstream, the kidneys, and perhaps the mind itself. As humanity contemplates journeys to the Moon and Mars, this research asks a humbling question: how well do we truly understand the body we intend to carry there?
Astronauts aboard the International Space Station have long complained of constipation and bloating, chalking it up to the indignities of weightlessness. A new study from the University of Copenhagen, conducted with NASA, now offers a precise explanation — and a warning that reaches well beyond digestive discomfort.
Researchers analyzed blood samples from 52 astronauts across multiple ISS missions and found a consistent pattern: within weeks of entering microgravity, gut bacteria shifted from fermenting dietary fiber to fermenting protein. On Earth, this metabolic pivot happens only when fiber runs out. In space, it appears to be triggered by something more fundamental — the slowing of peristalsis, the muscular contractions that normally move food through the intestines at a steady pace. Without gravity's assistance, food lingers, and bacteria adapt accordingly.
The consequences extend beyond the gut. Protein fermentation produces compounds that enter the bloodstream, and prior research has linked some of these to kidney damage and neurological effects via the gut-brain axis — the communication network connecting digestion to the central nervous system. Co-author Henrik Roager noted that altered gut chemistry in space could plausibly affect mood and concentration, faculties that matter enormously on long missions.
Senior author Lars Ove Dragsted stressed that the stakes rise sharply as agencies plan voyages to the Moon and Mars. Weeks of digestive disruption on the ISS is one thing; years of it in deep space is another. The team proposes dietary fiber, prebiotics, and motility interventions as countermeasures — approaches that could also benefit bedridden patients on Earth whose immobility produces strikingly similar digestive challenges.
What the study ultimately reveals is that the human body evolved in intimate partnership with gravity. Strip that away, and even the most routine biological process — moving a meal from one end of the body to the other — becomes something the body must, in a sense, relearn.
Astronauts aboard the International Space Station have long complained of digestive trouble—constipation, bloating, the ordinary discomforts of weightlessness made worse by months in orbit. A new study from Copenhagen, conducted in partnership with NASA, points to a specific culprit: microgravity fundamentally changes how the bacteria in an astronaut's gut behave, triggering a cascade of metabolic shifts that ripple through the bloodstream.
Researchers analyzed blood samples from 52 astronauts who spent extended periods on the ISS. Within weeks of arrival in space, the data revealed a consistent pattern. The gut bacteria began fermenting protein at elevated rates—a process that normally occurs on Earth only after bacteria have exhausted the available dietary fiber and turn to protein as an alternative fuel source. Giorgia La Barbera, an associate professor at the University of Copenhagen, described the findings as evidence of a fundamental change in how the microbial ecosystem responds to the absence of gravity.
The mechanism appears straightforward, though its implications are not. On Earth, food moves through the digestive tract via peristalsis—coordinated muscular contractions that propel material along the intestines at a predictable pace. In microgravity, that process slows. Food lingers longer in the gut, giving bacteria more time to shift their metabolic strategy. Instead of fermenting carbohydrates, they begin breaking down protein. The result is constipation and digestive disruption, but also something less visible: the production of metabolic byproducts that enter the bloodstream.
Henrik Roager, a co-author on the study, noted that this explanation accounts for the constipation astronauts experience. But the concern extends beyond simple discomfort. Previous research has linked certain compounds produced during protein fermentation to kidney damage and potential changes in mood and concentration. The gut-brain axis—the bidirectional communication system connecting the digestive tract and the central nervous system—raises the possibility that altered gut function in space could have broader neurological consequences.
Lars Ove Dragsted, the study's senior author, emphasized that these findings take on new urgency as space agencies plan longer missions. A few weeks of constipation on the ISS is manageable. But astronauts bound for the Moon or Mars, spending months or years in microgravity, could face sustained changes in gut function with unknown long-term health effects. The researchers propose several countermeasures: increasing dietary fiber intake, using prebiotics to support beneficial bacteria, or finding ways to stimulate normal intestinal movement and reduce the time food spends in the gut.
What makes this study particularly robust is its dataset. The 52 astronauts came from different missions spanning multiple years, yet the pattern of increased protein fermentation appeared consistently across the group. The findings suggest that microgravity does not merely inconvenience the human digestive system—it rewires it in measurable, reproducible ways.
The implications extend beyond orbit. Dragsted noted that the same knowledge could help patients on Earth who experience constipation and slowed intestinal movement—bedridden patients, for instance, whose immobility creates digestive challenges analogous to those faced by weightless astronauts. The study is a reminder that the human body evolved for a specific environment. Remove gravity, and even the most routine biological process—moving food from mouth to exit—becomes something the body must relearn.
Citations marquantes
Gut bacteria begin to ferment protein to a greater extent than usual within weeks after astronauts arrive in space— Giorgia La Barbera, associate professor at the University of Copenhagen
This may also help explain constipation in astronauts— Henrik Roager, co-author
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