When human beings leave the gravitational embrace of Earth, even the invisible communities living within them are transformed. Researchers at the University of Copenhagen, studying blood samples from 52 astronauts aboard the International Space Station, have traced a quiet but consequential shift: without gravity to move food efficiently through the gut, bacteria abandon their preferred diet of fiber and turn to fermenting protein instead — a change that ripples outward into mood, cognition, and organ health. The finding reframes constipation not as a minor inconvenience of spaceflight, but as
Space Study Links Astronaut Constipation to Altered Gut Bacteria
Gravity doesn't just affect how you move—it affects how fast food moves through you
Mark
So astronauts get constipated in space—that's been known for a while. What's new here?
Mimi
The new part is the why. They found that gravity doesn't just affect how you move around. It affects how fast food moves through your intestines. Slower transit means the bacteria in your gut start eating protein instead of fiber, and that shift happens consistently across all the astronauts they studied.
Mark
Why does that matter if it's just constipation?
Mimi
Because the byproducts of that protein fermentation don't stay in your gut. They enter your bloodstream and circulate everywhere—your kidneys, your brain. The research suggests these metabolites can damage kidneys, affect mood, even reduce focus.
Mark
But astronauts are only up there for months at a time, right?
Mimi
Right now, yes. But NASA and other agencies are planning missions to Mars that could last a year or more. On a journey that long, sustained protein fermentation becomes a real health risk, not just an inconvenience.
Mark
Can you fix it?
Mimi
Potentially. The researchers suggest increasing fiber in the diet, adding prebiotics, or using drugs that speed up intestinal movement. The key is reducing how long food sits in the gut.
Mark
And this applies to people on Earth too?
Mimi
Exactly. Bedridden patients experience the same constipation and likely the same bacterial shift. The same solutions could help them.
El Pulso
- Within weeks of arriving in microgravity, astronauts' gut bacteria switch from fermenting fiber to fermenting protein — a metabolic shift that persists for the entire mission.
- The byproducts of that protein fermentation don't stay in the gut: they enter the bloodstream and have been linked to kidney damage, mood disturbances, and reduced cognitive focus.
- Short ISS missions keep these effects manageable, but planned crewed missions to the Moon and Mars would expose astronauts to months of cumulative damage — raising the stakes considerably.
- The study's unusual strength comes from its scale: 52 astronauts across many years produced a consistent biological signature that cut through the individual variation that normally clouds this kind of research.
- Dietary fiber, prebiotics, and medications that accelerate intestinal movement are now being evaluated as countermeasures — and the same interventions could benefit bedridden patients on Earth facing identical gut changes.
When human beings leave the gravitational embrace of Earth, even the invisible communities living within them are transformed. Researchers at the University of Copenhagen, studying blood samples from 52 astronauts aboard the International Space Station, have traced a quiet but consequential shift: without gravity to move food efficiently through the gut, bacteria abandon their preferred diet of fiber and turn to fermenting protein instead — a change that ripples outward into mood, cognition, and organ health. The finding reframes constipation not as a minor inconvenience of spaceflight, but as a window into how profoundly the body's inner ecology depends on the physical world around it.
Constipation has long been accepted as an uncomfortable fact of spaceflight, but its underlying cause has remained poorly understood — until now. Researchers at the University of Copenhagen, collaborating with NASA, analyzed blood samples from 52 astronauts who spent extended periods aboard the International Space Station, and found a consistent, repeatable pattern: within weeks of entering microgravity, gut bacteria shift from fermenting fiber to fermenting protein.
The mechanism is elegantly simple. Without gravity's pull, food moves more slowly through the digestive tract. When fiber — the microbiome's preferred fuel — becomes scarce in transit, bacteria adapt by breaking down protein instead. On Earth, this happens occasionally; in space, it becomes the sustained default state of the gut.
What elevates this beyond a story about discomfort is where those fermentation byproducts go. They circulate through the bloodstream, reaching the kidneys and the brain, where they have been associated with organ damage, mood changes, and diminished focus. For short ISS missions, the effects remain tolerable. For future crewed missions to Mars lasting many months, the cumulative consequences represent a genuine threat to crew health and mission integrity.
The study's findings carry unusual credibility. Blood metabolite research is typically muddied by individual biological variation, but the breadth of the dataset — 52 astronauts across multiple missions and years — paradoxically sharpened the signal, producing a clear biological fingerprint of microgravity's effect on the gut.
The path forward involves dietary fiber, prebiotics, and medications that promote intestinal movement — interventions that could protect astronauts on long missions and, notably, bedridden patients on Earth who experience the same gravitational stillness and likely the same microbial shift. Understanding the mechanism, the researchers argue, is the first step toward preventing consequences that extend far beyond an uncomfortable flight.
Constipation in space has long been an accepted occupational hazard for astronauts—an uncomfortable but largely unexplained consequence of leaving Earth's gravity behind. Now researchers at the University of Copenhagen, working with NASA, have identified what actually happens inside the gut when gravity disappears, and the answer lies in how bacteria behave when food moves through the intestines more slowly than it should.
The study examined blood samples from 52 astronauts who spent months aboard the International Space Station. Within weeks of arriving in space, their blood chemistry revealed a consistent pattern: gut bacteria began fermenting protein at elevated rates. This shift persists throughout the mission and reverses only after astronauts return to Earth. The mechanism is straightforward in hindsight. Without gravity's pull, food travels more slowly through the digestive tract. When fiber—the bacteria's preferred fuel—runs low, the microbes switch to breaking down protein instead. On Earth, this happens to many people occasionally. In space, it becomes the default state.
What makes this finding significant extends beyond the discomfort of constipation. The byproducts of protein fermentation circulate through the bloodstream and reach organs throughout the body, including the brain. Researchers have linked these metabolites to kidney damage, mood disturbances, and diminished cognitive focus. For astronauts on brief missions to the International Space Station, these effects remain manageable. But as space agencies plan longer journeys—crewed missions to the Moon, eventual trips to Mars—the cumulative impact of sustained protein fermentation becomes a legitimate health concern.
The research itself carries unusual strength for this type of study. Blood metabolite analysis is typically difficult to interpret because individual variation obscures patterns. But the nature of spaceflight research—samples from 52 different astronauts across multiple missions and many years—paradoxically reduced that noise. The consistency of the findings across such diverse subjects surprised even the researchers. The method revealed a clear, repeatable biological signature of microgravity's effect on the gut.
The implications reach beyond astronauts. Bedridden patients on Earth experience similar constipation and likely undergo the same shift toward protein fermentation. The same interventions being considered for space travelers could help them: increasing dietary fiber, adding prebiotics, or using medications that promote intestinal movement and reduce the time food spends in transit. For space missions, these countermeasures become essential preparation. The researchers emphasize that understanding this mechanism opens a path not just to managing an uncomfortable symptom, but to preventing downstream health consequences that could compromise crew safety and mission success on journeys that may last months or years.
Citas Notables
When we plan for longer journeys in space—for example to Mars—countermeasures may be needed to mitigate negative effects of space travel on the intestines— Giorgia La Barbera, associate professor, University of Copenhagen
Our findings can inform potential interventions—either directly through increasing dietary fiber, supplementation with prebiotics or other types of treatments that promote peristalsis and decrease gut transit time— Henrik Roager, associate professor, University of Copenhagen
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