Astronaut Guts Turn to Protein Fermentation in Weeks
Blood analysis of 52 astronauts aboard the International Space Station shows gut bacteria switch to breaking down protein instead of fiber within the first weeks of weightlessness.
Researchers at the University of Copenhagen, working with NASA, analyzed blood samples from 52 astronauts who each spent several months aboard the International Space Station, and found that gut function starts changing within the first weeks of weightlessness. The work was published in Nature Communications. Constipation has long been a known complaint among crews, but until now it was discussed mostly in general terms. There is now a chemical trace in the blood that shows exactly what changes, and when.
The change is simple to describe and unpleasant in its consequences. Gut bacteria start breaking down protein more heavily instead of fiber, because the fiber from meals runs out. This is not unique to orbit: the same pattern shows up in people on Earth, and it is not a disease in itself.
“We see changes in the blood samples from astronauts showing that gut bacteria start to ferment protein more than usual already within a few weeks of the astronauts arriving in space, and that change continues until they return to Earth,” said Djordja la Barbera, associate professor at the Department of Nutrition, Exercise and Sports at the University of Copenhagen and the study’s first author, in a university statement.
What protein breakdown leaves behind is not harmless residue. The bacteria produce a range of metabolites that get absorbed into the blood and travel through the body, and some of them reach the brain through the so called gut-brain axis.
Why does food move more slowly through the gut in orbit?
The explanation the authors offer has to do with the physics of spaceflight itself. Without gravity, the contents of the gut move more slowly, so bacteria work on the same material for longer, and once the fiber runs out they switch to whatever is left.
“The lack of gravity in space likely makes food move more slowly through the gut, and that fits with the fact that we see signs of increased protein fermentation,” said Henrik Roager, associate professor in the same department and a co-author of the study.
The word “likely” is not decoration. The slower passage of food through the gut was not measured directly in this study, it is an assumption that fits what was measured in the blood.
How the measurement was done
The samples come from three earlier studies conducted aboard the International Space Station, across different missions and several years. The researchers used an untargeted method, meaning they did not look for specific substances in advance but analyzed every metabolite in the blood to see which ones changed. Metabolites are small molecules in the bloodstream that reveal how the body is functioning, including what a person has eaten and how their gut is working. Besides the protein related shift, the tests also picked up smaller changes linked to fish and caffeine intake.
“The samples we analyzed come from a total of 52 different astronauts, on different missions and across many years. That is quite different from how a study is usually done. So it is quite impressive that our method managed to show so clearly that there is consistently more protein fermentation,” said Jan Stanstrup, assistant professor and also a first author of the study.
On the NASA side, the study involved Sara R. Zvart and Scott M. Smith, while the University of Copenhagen team, besides la Barbera and Stanstrup, also included Roager and Lars Ove Dragsted.
What this means for a mission to Mars
When a trip is planned to last months, the state of an astronaut’s gut stops being a minor detail.
“We know that the products of protein fermentation are often linked to negative health effects, such as kidney damage, a possible impact on mood, or a reduced ability to concentrate, among other things,” said Dragsted, professor at the Department of Nutrition, Exercise and Sports and the study’s senior author.
The measures the authors propose are as ordinary as the problem itself: more fiber in meals, prebiotics, and therapies that promote gut motility and shorten transit time. The goal is to reduce protein fermentation, and with it the risk of the consequences linked to it.
The authors tie the same finding to bedridden patients as well. They also suffer from constipation, so it is possible that they too have increased protein fermentation that worsens their condition.
Limits of the study
The study is observational and shows an association, not a cause. It does not prove that protein fermentation causes constipation in astronauts, nor that the kidneys, mood, or concentration of crew members were actually impaired, since none of that was measured here. The health consequences Dragsted refers to come from earlier research into the products of this type of fermentation.
What was measured were small molecules in the blood, not the composition of the gut flora itself, so the conclusion about what the bacteria are doing is indirect. The samples come from three separate studies, with different crews and different missions, which helped the authors keep individual differences from blurring the finding, but it was not an experiment with predetermined conditions. Whether the proposed measures actually help will have to be verified on crews in orbit.
