Plate Nº 40 · recorded September 29, 2026
Space & AstronomyReported finding
Why Astronauts Get Constipated: Blood Tests Point to Gut Bacteria
Blood samples from 52 ISS astronauts show gut bacteria shift toward protein fermentation within weeks in microgravity, possibly explaining constipation in space.
By Elena Vasquez5 min read1,033 words
In brief
- Blood samples from 52 ISS astronauts showed increased gut bacterial protein fermentation within weeks of arriving in space, persisting until return to Earth.
- Researchers suspect microgravity slows food transit through the intestines, giving bacteria time to exhaust dietary fiber and switch to breaking down protein.
- Possible countermeasures include more dietary fiber, prebiotics, or treatments promoting peristalsis; findings may also apply to bedridden patients with constipation.
Astronauts who spend months aboard the International Space Station show measurable changes in their gut bacteria within weeks of arriving in orbit, according to new research from the University of Copenhagen conducted with NASA. The shift could explain a familiar complaint of space travel: constipation.
The study, published in Nature Communications on September 28, 2026, analyzed blood samples from 52 astronauts who spent extended periods on the space station. The researchers found signs that bacteria in the intestines begin breaking down more protein than usual — a process called protein fermentation — soon after astronauts leave Earth, and that this change persists until they return.
"We see changes in astronauts' blood samples that indicate that the gut bacteria begin to ferment protein to a greater extent than usual within weeks after the astronauts arrive in space, and this change continues until they are back on Earth," says Giorgia La Barbera, joint first author of the study and associate professor at the University of Copenhagen's Department of Nutrition, Exercise and Sports.
A gut without gravity
Protein fermentation happens when gut bacteria run out of dietary fiber to digest and switch to breaking down protein instead. The process is not unique to space travelers; it can occur in people on Earth too, particularly when food lingers in the intestines for a long time.
That detail points to what the researchers suspect is the underlying mechanism. In microgravity, food likely moves more slowly through the digestive tract. The longer it stays put, the more time bacteria have to exhaust the available fiber and turn to protein.
"The lack of gravity in space probably causes food to move more slowly through the intestine, and this fits with the fact that we are seeing signs of increased protein fermentation. This may also help explain constipation in astronauts," says co-author Henrik Roager, also an associate professor at the same department.
The word "probably" is doing real work here. The study measured chemical signatures of fermentation in the blood, not intestinal movement directly, so the link between slowed digestion and the bacterial shift remains a well-supported hypothesis rather than a proven fact.
Small molecules, wide effects
The findings came from analyzing metabolites — small molecules circulating in the bloodstream that offer clues about how the body is functioning. By tracking which metabolites changed during spaceflight, the researchers could infer what was happening inside the intestines.
The results may matter beyond digestion. As bacteria break down protein, they produce metabolites that can enter the bloodstream and travel throughout the body, including to the brain via the gut-brain axis, the two-way communication system linking the digestive tract and the brain.
"We know that products of protein fermentation [are] often associated with negative health consequences, such as kidney damage, potential effects on mood or reduced ability to focus, among others," says Lars Ove Dragsted, senior author of the study and professor at the Department of Nutrition, Exercise and Sports.
Previous research has linked protein fermentation metabolites to anxiety and reduced concentration. Those associations come from other studies, though, and the current work did not directly measure cognitive or kidney effects in the astronauts.
Why Mars planners should care
The findings take on added weight as space agencies prepare for longer missions to the Moon and eventually Mars. Crews on those journeys will spend far more time away from Earth's gravity than astronauts currently do on the International Space Station, meaning any digestive disruption could compound over months or years.
"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," says La Barbera.
Several approaches might help, according to the researchers. "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. This would help reduce protein fermentation and thus provide a healthy gut environment and avoid negative health consequences," adds Roager.
Peristalsis refers to the coordinated muscle contractions that move food and waste through the digestive system. Speeding up that movement would leave less time for bacteria to ferment protein.
A lesson for Earthbound patients
The research may also benefit people who never leave the planet. Patients confined to bed for extended periods often experience constipation, and the researchers suggest that slowed intestinal movement in these patients could produce similar changes in gut fermentation.
"You can use this knowledge to help bedridden patients. They also experience constipation and likely also have increased protein fermentation that may aggravate health conditions," says Dragsted.
Why the signal stood out
Blood metabolite research is often noisy because metabolism varies considerably from person to person. The astronaut data helped the researchers cut through that variability.
"The samples we have analyzed come from a total of 52 different astronauts, on different missions and across many years. This is very different from how you normally do a study. So, it is quite impressive that our method has been able to show so clearly that there is consistently more protein fermentation," says Jan Stanstrup, joint first author and assistant professor.
The team used a non-targeted approach, meaning they scanned the broad range of metabolites in the blood rather than searching only for predetermined substances. That let them identify which molecules changed during spaceflight. The analysis also picked up smaller changes associated with fish and caffeine consumption.
The samples came from three studies conducted aboard the International Space Station. Although only small quantities of blood were available, the researchers extracted additional information from the existing material. NASA researchers Sara R. Zwart and Scott M. Smith contributed alongside the Copenhagen team of La Barbera, Stanstrup, Roager and Dragsted.
The study has limits worth keeping in mind. It reports associations observed in blood chemistry, not a direct measurement of gut contents or digestion speed, and the proposed countermeasures remain untested in space. Still, the consistency of the signal across 52 astronauts over multiple missions gives researchers a solid starting point as they plan for journeys far beyond low Earth orbit — and for patients whose bodies, in some ways, behave as if they too were floating in space.
via dx.doi.org (Original)
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