Blood from 52 astronauts shows their gut bacteria change diet within weeks of orbit

Blood drawn from 52 astronauts on International Space Station missions carries a chemical trace of a shift in what their gut bacteria eat. Within weeks of reaching orbit, microbes in the intestine began fermenting protein to a greater extent than usual, and the pattern held until the crews came home.

“The gut bacteria begin to ferment protein to a greater extent than usual within weeks after the astronauts arrive in space,” said Giorgia La Barbera, an associate professor at the University of Copenhagen, describing work done in collaboration with NASA. The findings were published in Nature Communications, and the university’s account of them reached ScienceDaily on September 27.

Fiber runs out and protein becomes fuel

The explanation offered by the Copenhagen group runs through transit time. Henrik Roager, a co-author, said the lack of gravity probably causes food to move more slowly through the intestine. When food lingers, bacteria use up the dietary fiber that normally feeds them, and then turn to protein. Fermenting protein produces a different set of byproducts than fermenting carbohydrate, and those molecules cross into the bloodstream, where the researchers could measure them. Roager added that the same slowing may also help explain the constipation astronauts commonly report.

The slower transit is an inference, not a measurement, and the ScienceDaily release itself words it as what gravity “probably” does. The team did not track food moving through astronauts’ intestines; it read the downstream chemistry in blood, sampled at several points before, during and after each mission, and the gravity explanation is the researchers’ best reading of what that chemistry implies.

488 plasma samples, 40 shifted compounds

The paper in Nature Communications describes a longitudinal, untargeted metabolomics analysis of 52 astronauts, 41 men and 11 women, who flew 2 to 9 month missions to the station between 2006 and 2018. Metabolomics reads the small molecules in blood that reflect what tissues, food and microbes are doing at the moment of the draw, which is why it can hint at gut activity without a single stool sample or scan. Researchers analysed 488 plasma samples collected before flight, during flight and after landing, using liquid chromatography with high-resolution mass spectrometry to profile roughly 1,390 molecular features. Spaceflight affected around 40 circulating compounds.

Among the markers that rose were p-cresol sulfate and phenylacetylglutamine, both products of microbial protein breakdown. Compounds tied to caffeine and to fish intake fell, which points to changed eating in orbit, but those diet-related shifts accounted for less than a third of the observed changes. Plasma uric acid dropped by about 10 percent in flight, which the authors attribute to fluid redistribution rather than food, a reminder that not every change in the blood panel traces back to the gut or to diet.

Many astronauts across many missions means the result is not one crew’s quirk. The authors call it the largest longitudinal metabolomics study of astronauts with in-flight blood sampling, and they note it agrees with microbiome work in both astronauts and space-flown mice, including earlier sequencing of astronaut microbiomes.

Health stakes and the limits of the data

Lars Ove Dragsted, the senior author, said products of protein fermentation are often associated with negative health consequences such as kidney damage, potential effects on mood and a reduced ability to focus. Such links come from associations seen in other settings, and the study does not show those outcomes in astronauts. The Copenhagen researchers frame the work as a basis for nutrition on exploration missions to the Moon and Mars, where crews will spend far longer in orbit or in transit than the 2 to 9 month stays studied here. The authors suggest increasing slowly fermented carbohydrates at the station, and the Copenhagen team points to more dietary fiber, prebiotics or treatments that promote peristalsis for longer missions, an idea Futurity’s account extends to bedridden patients on Earth who may show similar digestive changes.

The caveats are the authors’ own. Only 11 of the 52 astronauts were women, which limits sex-specific analysis, and only one compound showed a difference between the sexes, so conclusions about how women’s microbial chemistry responds to flight rest on a small group. The design also cannot separate microgravity from confinement, workload and the station’s food system, all of which change at once on launch.

Older work fits the picture. NASA’s year-long Twins Study found that Scott Kelly’s gut flora was profoundly different during flight than before it and returned to its preflight state after landing, with the packaged food supply suspected as one contributor. The Copenhagen blood data add a candidate mechanism, slowed transit, without ruling the menu out, and they cover 52 people rather than one twin.

Two tests remain undone in this cohort: whether the protein signal grows over a Mars-length mission, and whether added fiber changes the blood chemistry the way the authors predict. Both would need astronauts to be sampled under a controlled diet, which the 2006 to 2018 data were not designed to provide, and neither has been run yet.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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