New research from the University of Copenhagen, published in collaboration with NASA, explains why astronauts get constipated in space. The cause, according to blood tests, is straightforward: without gravity, food moves more slowly through the intestines, giving gut bacteria more time to digest protein rather than fiber. The study examined blood samples from 52 astronauts who spent months aboard the International Space Station.
The research followed the makeup of circulating metabolites — small molecules in the bloodstream — throughout the course of the study. Within weeks of the astronauts’ arrival in space, evidence emerged that the gut bacteria had started breaking down protein to a greater degree, and that change persisted until their return to Earth.
What the Blood Samples Showed
Scientists examined blood drawn from the 52 crew members at three points: before launch, while they were in orbit, and after they came back down. The investigators discovered that the microbes living inside the astronauts’ digestive tracts began processing protein at a higher rate within weeks of reaching space, and this alteration continued all the way through until the crew’s return to Earth.
Giorgia La Barbera is an associate professor and joint first author at the University of Copenhagen’s Department of Nutrition, Exercise and Sports. She said, “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.”
Why Protein Fermentation Happens
When the intestinal bacteria switch from digesting fiber to protein, a condition known as protein fermentation arises. This shift occurs after all available dietary fiber has been consumed. It is not an event confined to space travelers alone; it can occur on Earth whenever food passes through the digestive system at a pace that is too slow.
Henrik Roager, a co-author and associate professor at the Department of Nutrition, Exercise and Sports, believes that food moves more slowly through the digestive tract in microgravity. That delay gives bacteria extra time to break down protein, which is what “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. This may also help explain constipation in astronauts,” suggests.
The Broader Health Picture
The findings could matter beyond digestion. Changes in the gut can influence other parts of the body, including the brain. “We know that products of protein fermentation are often associated with negative health consequences, such as kidney damage, potential effects on mood or reduced ability to focus, among others,” says senior author Lars Ove Dragsted, professor at the Department of Nutrition, Exercise and Sports.
What Comes Next for Space Travel
These results carry particular weight for future space travel, since longer missions to the Moon and eventually Mars are being planned by space agencies. Astronauts undertaking such voyages would be spending far more time outside Earth’s gravity than the crews now serving aboard the International Space Station.
According to La Barbera, “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,” is the name attached to the statement.
How to Fix It
Several approaches might help reduce these digestive changes. “Our findings can inform potential interventions — either directly through increasing dietary fiber, supplementation with prebiotics or other types of treatments that promote peristalsis and decrease gut transit time. This would help reduce protein fermentation and thus provide a healthy gut environment and avoid negative health consequences,” adds Roager.
The coordinated muscle contractions that move food and waste through the digestive system are known as peristalsis. Helping that movement along could shorten how long material stays in the intestines and reduce the chance of too much protein breaking down.
The Bed-Ridden Connection
Beyond its uses for spaceflight, this research might apply to people confined to bed for long periods, who often suffer from constipation. Scientists propose that slowed intestinal motion in such patients could generate comparable shifts in gut fermentation.
“You can use this knowledge to help bedridden patients. They also experience constipation and likely also have increased protein fermentation that may aggravate health conditions,” concludes Dragsted.
What Protein Fermentation Actually Is
When there isn’t enough fiber left for bacteria to feed on, those same bacteria start breaking down protein instead. This occurs inside the gut, where the protein gets fermented. It tends to happen when food stays in the intestines for a long time, even when a person doesn’t notice any real trouble moving their bowels.
When bacteria break down protein, they give rise to a number of metabolic products. Those substances may enter the blood and circulate across the body. Certain ones may further affect the brain via the gut-brain axis, the pathway that connects the digestive tract and the brain in both directions.
Why the Results Were So Clear
Interpreting research on blood metabolites is often challenging, since metabolism varies widely from person to person. The distinctive character of the astronaut data helped researchers get around some of that individual variation and made them more confident in the pattern they found.
The material derived from 52 distinct astronauts provided the investigators with a sufficiently sizable collection to detect steady patterns even amid individual variations.
The Bottom Line
A plausible account of why constipation runs so high in space is offered by the study. The research further implies that those kept confined to bed may suffer from the same trouble.
The scientists lay out specific suggested fixes, among them adding more fiber and improving transit time.
The research points to three main areas for future work:
- Increasing dietary fiber to give bacteria more fiber to break down.
- Supplementing with prebiotics to support beneficial gut populations.
- Treatments that promote peristalsis and shorten gut transit time.
Each method addresses the core problem — food remaining in the intestines for an extended period — without depending on harsh approaches. What gives the study its power is its scale: 52 astronauts across multiple missions, a number large enough to reveal trends that would likely have been lost amid personal differences otherwise.
Source material: “Blood tests reveal why astronauts get constipated in space,” ScienceDaily.
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