Astronaut constipation: 52-person study reveals gut trouble
Living in space comes with plenty of well-known hazards, but researchers have now zeroed in on a less glamorous one: astronaut constipation, and what it may signal about health beyond the bathroom. A new study analyzed blood samples from 52 astronauts and found that microgravity appears to slow digestion, setting off a chain reaction inside the gut.
The research, a collaboration between the University of Copenhagen and NASA, was published in Nature Communications and announced by the university Sept. 28. Scientists studied blood drawn from astronauts who spent months aboard the International Space Station, pooling data from three separate ISS studies to see what changes inside the body once gravity drops out of the equation.
Why astronaut constipation happens
Astronaut constipation has long been a known nuisance for people living in space, but exactly why it happens hasn’t been well understood. This study offers an answer: Without gravity, food likely moves more slowly through the digestive tract.
“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,” said Henrik Roager of the University of Copenhagen.
That movement normally depends on peristalsis, the coordinated muscle contractions that push food and waste along. Researchers believe microgravity slows that process.
Astronaut constipation is only the most visible sign of that slowdown, which creates a cascade effect inside the gut. Within weeks of arriving in space, astronauts’ gut bacteria began fermenting protein more heavily. Normally, gut bacteria feast on fiber first. But once that fiber runs low, the bacteria turn to protein instead, a process that produces different byproducts than fiber fermentation.
Those byproducts, called metabolites, are small molecules that circulate in the bloodstream and can offer scientists a window into how the body is functioning. Instead of hunting for a short list of known markers, the team cast a wide net with what’s called a non-targeted approach, which is part of what allowed them to spot the protein fermentation shift in the first place.
More than just an inconvenience
The concern isn’t purely digestive. Lars Ove Dragsted, the study’s senior author, noted that the products of protein fermentation are often tied to health problems, such as kidney damage, possible effects on mood and trouble concentrating. That matters because of the gut-brain axis, the network of nerves and chemical signals that lets the gut and the brain influence each other. Changes happening in the gut don’t necessarily stay in the gut.
The study’s blood analysis also picked up smaller metabolite changes tied to things like fish and caffeine intake, a reminder of just how sensitive this kind of testing can be to everyday diet, not just spaceflight itself.
Looking ahead to Mars
The stakes of astronaut constipation get bigger the longer a mission runs. Dragsted, a professor in the University of Copenhagen’s Department of Nutrition, Exercise and Sports, pointed to future exploration as a key reason this research matters now.
“When we plan for longer journeys in space — for example, to Mars — countermeasures may be needed to mitigate negative effects of space travel on the intestines,” Dragsted said.
The research team, led by joint first authors Giorgia La Barbera and Jan Stanstrup, along with Roager and NASA researchers Sara R. Zwart and Scott M. Smith, suggests a few possible fixes for astronaut constipation worth exploring: boosting dietary fiber, adding prebiotics or developing treatments that help stimulate peristalsis directly.
What astronaut constipation could mean back on Earth
Interestingly, this isn’t just a space problem. People confined to bed for long stretches often deal with constipation too, and the researchers say the findings could help those patients. That overlap opens the door for research on astronaut constipation to eventually inform treatment far outside the astronaut corps.
Plenty remains unclear, including which specific interventions for astronaut constipation work best, how effects might differ between short missions and multiyear ones, and exactly how these metabolites influence mood and cognition. Anyone dealing with persistent digestive changes, in space or on the ground, should bring concerns to a doctor rather than self-treating. For more on everyday gut care, see our wellness coverage.
As space agencies look toward longer missions to the moon and eventually Mars, gut health is shaping up to be a quieter but very real engineering challenge, one measured in blood samples rather than rocket fuel.