Between 2006 and 2018, plasma samples were drawn from 52 astronauts before, during and after stays on the International Space Station, and those tubes of blood have now yielded a specific answer to an old astronaut complaint. Roughly 40 circulating compounds shifted in flight, and a distinct group of them are the fingerprints of gut bacteria switching from digesting fiber to fermenting protein.
The work is not new. It was published on June 29, 2026, in Nature Communications, and the University of Copenhagen publicized it in August. Reposts such as ScienceDaily’s late-September version keep circulating it, which makes an explainer of what the study did and did not show more useful than a news account.
Fifty-two astronauts and 488 blood draws
The Nature Communications paper, titled “Longitudinal metabolomics profiles reveal increased gut microbial protein fermentation during Spaceflight,” describes 52 astronauts, 11 women and 41 men, contributing 488 plasma samples in total. Missions lasted between two and nine months. First author Giorgia La Barbera and Jan Stanstrup, together with senior author Lars Ove Dragsted and Henrik Roager, work at the University of Copenhagen’s Department of Nutrition, Exercise and Sports. Sara R. Zwart and Scott M. Smith of NASA are co-authors, and the paper carries the identifier 10.1038/s41467-026-74979-w.
Blood was analyzed by liquid chromatography coupled to high-resolution mass spectrometry, an untargeted approach that measures thousands of molecules without deciding in advance which to look for. The authors then tracked how each compound moved from the pre-flight baseline, through the mission, to the days after landing. Because the astronauts served as their own controls, the comparison does not depend on matching them to healthy people who stayed on the ground.
The fingerprints of fermenting protein
Among the roughly 40 compounds that changed, the ones that rose were p-cresol sulfate, phenylacetylglutamine, 3-indoxyl sulfate, phenol sulfate, 4-ethylphenol sulfate and pimelic acid. The authors state that several of these “originate from bacterial fermentation of proteins in the large intestine” and are then processed by the liver before appearing in blood. Other markers fell, including caffeine and its breakdown product paraxanthine, biomarkers of fish intake, and a class of molecules called acylcarnitines. Uric acid dropped by about 10 percent during flight.
The timing is what the Copenhagen team stressed. La Barbera said the blood samples indicate gut bacteria “begin to ferment protein to a greater extent than usual within weeks” of arrival in space, in the account republished by ScienceDaily. Per the paper, the changes emerged shortly after launch and resolved within days of landing, which points to the environment of spaceflight rather than a lasting change in the astronauts.
Slower transit as the proposed cause
The proposed mechanism starts with transit time. Roager said the lack of gravity “probably causes food to move more slowly through the intestine,” which fits the rise in protein fermentation. The paper spells out the chain: reduced gravitational pressure on the intestines may lengthen the time food stays in the colon, dietary fiber runs short, and microbes fall back on amino acids to keep producing energy and growing.
That is a hypothesis consistent with the data, not a demonstrated cause. The University of Copenhagen’s own news release says the study measured associations rather than proving that microgravity causes the digestive changes, even though results were consistent across diverse samples. It also says the same pattern may apply to bedridden patients on Earth, who also experience slowed intestines and protein fermentation.
Possible costs for long missions, and countermeasures
Protein fermentation produces compounds that can be harmful in quantity. Dragsted described them as “often associated with negative health consequences, such as kidney damage, potential effects on mood or reduced ability to focus.” Those are risks drawn from what fermentation products do in other settings, and the astronaut data show the compounds rising, not a measured kidney or cognitive outcome. The link to constipation, a familiar in-flight complaint, is the more direct connection, and Euronews reported the changes lasting throughout the missions before disappearing on return.
The Copenhagen team also points to a route beyond the gut. According to the university release, the metabolites produced by protein fermentation enter the bloodstream and can affect brain function through the gut-brain axis, which is why mood and focus appear on the list of possible effects alongside kidney damage. The paper itself does not test mood or focus in the astronauts, and the release describes those as potential consequences.
The authors suggest a diet-based response. Their conclusion reads: “Increasing the consumption of slow-fermented carbohydrates at ISS to reduce protein fermentation might improve gastrointestinal health.” The Copenhagen release also lists more fiber, prebiotics and treatments that promote intestinal movement as options.
What remains untested is whether any of those measures changes the blood signature during a mission, and whether it holds beyond low Earth orbit. All 52 astronauts flew to the ISS, and the longest stay in the dataset was nine months, far shorter than a round trip to Mars.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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