Astronauts’ Blood Hints At Why Spaceflight Causes Constipation

Space Travel Slows Astronauts’ Digestion, and Their Gut Bacteria Are Adapting to It

Constipation is a well-documented problem among astronauts. Now, a new study points to a likely explanation. The bacteria in their gut appear to start feeding on protein instead of fiber once astronauts leave Earth’s gravity. A study tracking 52 astronauts before, during, and after stays on the International Space Station identified this shift as one of the clearest metabolic patterns in the astronauts’ blood, and the team suggests that adding more fiber to space diets could help address it.

Published in Nature Communications, the study analyzed blood samples from astronauts who flew missions lasting between two and nine months aboard the ISS between 2006 and 2018. Researchers used a highly sensitive chemical analysis technique to scan for hundreds of compounds circulating in the bloodstream, ultimately zeroing in on about 40 that changed during spaceflight. Several of the clearest changes pointed to altered activity by gut bacteria, particularly an increase in protein fermentation.

Slower Digestion in Orbit Pushes Gut Bacteria Toward Protein

Bacteria in the large intestine normally feed on fiber and carbohydrates, but when digestion slows, those preferred fuel sources can run out. The microbes may then switch to breaking down protein instead, a process that generates a different set of chemical byproducts, several of which researchers detected at elevated levels in astronauts’ blood during flight. When the astronauts returned to Earth, those levels came back down within days.

Some of those compounds carry baggage. One has been linked in recent research to accelerating cell aging. Others have been associated in animal studies with changes in brain activity and anxiety-like behavior. In people with compromised kidney function, elevated levels of these compounds are tied to increased disease risk. Whether those same risks apply to otherwise healthy astronauts on missions is not yet established, though researchers consider the pattern worth taking seriously.

One additional marker that rose during flight, a compound called pimelic acid, has also been linked in human studies to slower movement of food through the gut. Its appearance in the data reinforces the broader picture researchers are drawing.

Blood Chemistry Shifted Within Days of Launch and Reversed After Landing

Using blood samples collected at multiple time points, the research team tracked the metabolic arc of spaceflight in unusual detail. In total, 488 plasma samples were collected from 11 female and 41 male astronauts, with an average age of 48. Changes appeared shortly after launch, held roughly steady during the mission itself, then reversed within days of landing.

Previous spaceflight metabolism studies had largely relied on comparing samples only before and after flight, or had focused on just one or two subjects. This study’s size and its repeated sampling allowed the team to track which metabolic changes appeared during spaceflight and how they shifted after the astronauts returned to Earth. A statistical model built from the data correctly identified blood samples taken during spaceflight with an error rate of just 2.6%. Samples taken on Earth before launch were harder to classify by comparison, coming in at a 7.1% error rate, which researchers say may reflect the wider variety of diets and routines available on the ground.

Diet Explained Only 27.5 Percent of the Changes Seen

Not every metabolic change traced back to gut bacteria. About 27.5% of the changing features appeared to be related to what astronauts were eating, or not eating, during their missions. Caffeine-related compounds dropped during flight, which researchers suggest may reflect astronauts drinking less coffee or switching to instant coffee, which contains less caffeine than brewed. Markers tied to fish intake also declined, along with compounds typically associated with eating fatty fish or fish oils.

Researchers found no major sex-specific differences in how male and female astronauts’ metabolism responded to spaceflight, with one exception: the flight-related change in the fish intake marker was observed only in the female astronauts.

Despite established ISS dietary guidelines that include fish and seafood, the data suggest actual intake patterns shift in practice during missions. Prior NASA research had encouraged fish consumption partly for its potential protective effect against bone loss in space.

Researchers Say More Fiber in Space Diets Could Ease the Problem

Researchers propose that increasing the availability of slowly fermented carbohydrates, meaning dietary fiber, aboard the ISS could help reduce protein fermentation and support gut health during spaceflight. As the authors put it, “Increasing the consumption of slow-fermented carbohydrates at ISS to reduce protein fermentation might improve gastrointestinal health in astronauts in future human long-term spaceflights.”

Proving that microgravity itself causes this shift, rather than some other feature of life in orbit, will take further research, since this study could not isolate microgravity from every other aspect of spaceflight. Still, as space agencies plan future long-duration missions, gut health may deserve the same careful planning as the engineering challenges of getting there.

---