Why Astronauts Get Constipated in Space: Gut Bacteria Switch to Protein

If you've ever wondered why astronaut "space poop" keeps coming up in NASA briefings, blood work from 52 astronauts now offers a biochemical explanation. A new study in Nature Communications found that astronauts' gut bacteria change what they eat once in orbit, shifting from fermenting fiber to fermenting protein, in a pattern that lines up with the constipation crews have reported for decades. If you manage a gut condition that flares when things slow down, like IBS with constipation, the findings are a reminder of how tightly bowel transit speed and microbial chemistry are linked, on Earth or off it.

Key takeaways

• Blood from 52 astronauts (11 women, 41 men, average age 48) on International Space Station missions lasting two to nine months between 2006 and 2018 showed shifts in roughly 40 circulating compounds tied to gut bacteria fermenting protein instead of fiber.
• The shift showed up within weeks of reaching orbit and lasted until the astronauts returned to Earth.
• Researchers think the pattern reflects slower gut transit time caused by microgravity, though the study cannot directly confirm transit time was the cause.
• The mechanism may also be relevant to people on Earth who experience slow-transit constipation, including bedridden patients.

Astronauts' Gut Bacteria Shift From Fiber to Protein Fermentation

Researchers from the University of Copenhagen, working with NASA, found that astronauts' gut microbiota (the community of bacteria living in the intestines) change their primary fuel source once in space. On Earth, gut bacteria mostly ferment fiber (the indigestible part of plant food) into short-chain fatty acids that keep the gut moving. In orbit, the same bacteria appear to switch toward fermenting protein instead, a process called proteolytic fermentation, which produces a different mix of byproducts, some of which are linked to a sluggish gut. The team, publishing in Nature Communications, didn't sample stool or gut tissue directly; instead they read this shift off the astronauts' blood chemistry using untargeted metabolomics, a technique that measures thousands of small molecules circulating in the body at once.

Why This Matters if You Deal With Slow-Transit Constipation

Constipation driven by slow intestinal transit isn't unique to astronauts. It's a common complaint in IBS with constipation (IBS-C), in people who are bedridden or have limited mobility, and in older adults generally. This study adds a plausible mechanism to that picture: when the gut moves food through more slowly, resident bacteria run out of their preferred fuel (fiber) faster and start breaking down protein instead, generating fermentation byproducts that show up in the bloodstream. That gives researchers a measurable blood signature for a gut problem that's otherwise hard to track without invasive testing, which is useful for anyone whose constipation is intermittent and doesn't line up neatly with a doctor's visit.

The Study Behind the Finding: 52 Astronauts, 488 Blood Samples, One Mass Spectrometer

The researchers analyzed 488 blood samples collected from the 52 astronauts before, during, and after their missions, using high-resolution liquid chromatography-mass spectrometry (LC-MS), a lab technique that separates and identifies chemical compounds in a fluid sample. This untargeted approach let them scan for changes across the full metabolome rather than testing a handful of pre-chosen markers, and it's what allowed them to catch a coordinated shift across roughly 40 compounds rather than one isolated signal. Because the same astronauts were sampled repeatedly across a mission (rather than comparing different people at a single time point), the design controls for a lot of the individual variation that normally muddies human studies, though it's still an observational design, not an experiment that manipulated gut transit directly.

What This Could Mean for You

None of this is a reason to start guessing at a "space diet" fix for your own constipation. What it does offer is a plausible biological story: if your gut is moving slowly, whether from limited mobility, an IBS-C flare, or another cause, your gut bacteria may be shifting fuel sources in a way that's detectable in blood chemistry, not just symptoms. If you deal with chronic or unexplained constipation, that's worth mentioning to a doctor as a possible angle for investigation, alongside standard fiber, hydration, and motility discussions, rather than as a standalone treatment plan. The study doesn't test any intervention, so nothing here should be read as an endorsement of a specific supplement or diet change.

What the Study Doesn't (Yet) Prove

The biggest caveat is causality: the researchers measured blood chemistry, not gut transit time itself, so the link between "protein fermentation went up" and "transit slowed down" is an inference, not a direct measurement. The authors themselves are explicit about this hedge in their own paper. It's also a specific population, healthy, highly screened astronauts under extreme physical conditions (microgravity, radiation, stress, altered diet) that don't map cleanly onto everyday constipation on Earth. Whether the same blood signature shows up in bedridden hospital patients or people with IBS-C hasn't been tested yet.

What Comes Next

The researchers frame this as a starting point for a blood-based way to monitor gut health during long-duration missions, which matters more as NASA and other agencies plan multi-year trips to the Moon and Mars, where in-flight diagnostic options are limited. On the Earth-side applications, testing whether this same metabolic signature tracks with constipation in bedridden or immobile patients would be the logical next step, though no such follow-up study has been announced yet.

Common Questions

Does this mean gut bacteria cause constipation, or is it the other way around?

The study can't fully separate the two. The leading interpretation is that slower gut transit (however it starts) gives bacteria more time with less fiber, pushing them toward protein fermentation, but the paper doesn't rule out other directions of cause and effect.

Could this research help people with IBS or other gut disorders on Earth?

Potentially, but it hasn't been tested in those populations yet. The astronaut study establishes a blood signature associated with a slow-transit state; whether it holds up in IBS-C, immobile patients, or other groups is an open question for future research.

Why did the researchers use blood instead of stool samples?

Blood samples were already being collected as part of astronauts' routine medical monitoring, which let the team analyze existing data across many missions and years without adding new procedures for the crews.