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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchBlood tests from astronauts show changes consistent with gut bacteria fermenting more protein during spaceflight. Researchers think slower intestinal movement in microgravity may contribute to that shift—and could help explain constipation—but the study did not directly measure transit time or bowel symptoms. The proposed explanation is plausible, not proven.
What the astronaut blood tests found
A study published in Nature Communications on 29 June 2026 analyzed 488 plasma samples from 52 astronauts who flew on the International Space Station for missions lasting 2–9 months between 2006 and 2018. Using untargeted liquid chromatography–high-resolution mass spectrometry, researchers found that spaceflight affected around 40 circulating compounds.
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Several compounds associated with gut microbial protein fermentation increased during flight. The highlighted markers included p-cresol sulfate, phenylacetylglutamine and 4-ethylphenol sulfate. The findings are evidence of a change in substances circulating in the blood; they are not a direct record of what happened inside the intestine.
The study did not measure intestinal transit time, bowel-movement frequency or constipation severity. It therefore cannot establish how often astronauts become constipated or prove that the metabolite changes caused constipation. The source does not provide a prevalence figure for astronaut constipation.
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How slower gut movement might be involved
Peristalsis is the wave-like contraction of intestinal muscles that moves food through the digestive tract. NASA’s STEMonstration: Digestion explains that in microgravity, motility can slow when the body relies on peristalsis alone.
The study authors propose that slower movement could give gut microbes less access to fiber and carbohydrates, prompting them to ferment more protein. Some products of that microbial activity can be absorbed into the bloodstream, which is consistent with the metabolite pattern the researchers observed. But because transit was not directly measured, this chain of events remains an interpretation rather than a demonstrated mechanism.
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“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,” study co-author Henrik Roager said in the University of Copenhagen’s release.
When the changes appeared—and what diet may explain
The metabolite changes appeared shortly after launch and subsided within a few days after landing. The researchers found no clear increase in the pattern as mission time in space lengthened. That timing supports an association with spaceflight, but does not identify whether microgravity, altered routines or another factor caused the changes.
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Clear out junk files and repair common Windows errorsFree Scan →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →The study also noted minor dietary shifts involving caffeine, fish and some fats. According to the study authors, these changes affected less than one third of the metabolites that responded to spaceflight. Diet could be part of the picture, but the reported pattern was not explained solely by those dietary differences.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Could fiber or prebiotics help?
The authors suggest that slowly fermented carbohydrates, including fiber, could be studied as a way to make more fermentable material available to gut microbes and reduce protein fermentation. The University of Copenhagen release also discusses prebiotic supplementation and treatments intended to promote peristalsis as possible countermeasures for future missions.
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These options were proposed, not tested as treatments in this study. The researchers did not show that fiber, a prebiotic supplement or a motility-promoting treatment prevents astronaut constipation. The findings also do not establish a treatment for constipation on Earth; astronaut results may inform questions about other settings, such as prolonged bed rest, but do not demonstrate clinical benefit there.
As La Barbera, joint first author and associate professor at the University of Copenhagen’s Department of Nutrition, Exercise and Sports, put it: “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.”
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