Blood tests from astronauts show changes consistent with gut bacteria fermenting more protein during spaceflight. Researchers think slower movement of food through the intestine in microgravity may contribute to that pattern—and could help explain constipation. But the study did not directly measure transit time or constipation, so it offers a plausible explanation, not proof of the cause.
What the astronaut blood tests found
A study published in Nature Communications on 29 June 2026 analyzed 488 plasma samples from 52 astronauts on International Space Station missions lasting 2–9 months between 2006 and 2018. Using untargeted liquid chromatography–high-resolution mass spectrometry, the researchers found that spaceflight affected around 40 circulating compounds. Several metabolites associated with gut microbes fermenting protein increased during flight. Read the study in Nature Communications.
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Among the highlighted metabolites were p-cresol sulfate, phenylacetylglutamine and 4-ethylphenol sulfate. Their presence is consistent with increased microbial protein fermentation; the blood results do not by themselves show how quickly food moved through the intestine or whether an astronaut was constipated.
Why slower transit could affect gut bacteria
Peristalsis is the wave-like contraction of intestinal muscles that moves contents through the digestive tract. NASA explains that motility can slow when the body relies on peristalsis alone in microgravity. NASA’s STEMonstration: Digestion describes how microgravity can change digestion.
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The study authors propose that if intestinal transit slows, gut bacteria may have less fiber and carbohydrate available and ferment more protein instead. Products of that microbial activity can enter the bloodstream, which offers a possible link between the blood pattern and bowel habits. The study did not directly test this sequence: it inferred the possibility from the metabolites rather than measuring transit or constipation symptoms.
When the changes appeared—and what diet explains
The metabolite changes appeared shortly after launch and subsided within a few days after landing. The authors did not find a clear increase in the pattern the longer astronauts remained in space.
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They also observed minor dietary changes involving caffeine, fish and some fats. Those changes affected less than one third of the metabolites that responded to spaceflight, according to the study authors. This does not rule out dietary contributions, but it suggests that the observed metabolite pattern was not explained solely by those measured changes.
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What might help, and what remains untested
The researchers suggest that slowly fermented carbohydrates, including dietary fiber, might reduce protein fermentation and support bowel habits during future long missions. University of Copenhagen researchers also raised prebiotic supplementation and treatments intended to promote peristalsis as possible countermeasures. These interventions were not tested in the study, and it did not establish that they prevent or treat constipation.
As joint first author Giorgia La Barbera 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.” Co-author Henrik Roager said slower movement through the intestine “may also help explain constipation in astronauts.” The University of Copenhagen release discusses the proposed mechanism and possible interventions.
A prebiotic fiber supplement is one possible category of intervention, not a treatment endorsed or proven by this astronaut study. The findings also do not establish a constipation treatment for people on Earth, including patients who are bedridden.
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What the study does—and does not—show
- It shows: blood-metabolite evidence consistent with increased gut microbial protein fermentation during spaceflight.
- It suggests: slower intestinal transit in microgravity may be part of the explanation.
- It does not show: that transit slowed in these astronauts, how many were constipated, or that fiber or prebiotics improved their symptoms.
The study therefore provides a biochemical clue to a possible cause of astronaut constipation, not a definitive causal answer. An explanatory report from Euronews also describes the findings and their limits.
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