Why Constipation Is So Common in Astronauts: New Gut Study

Astronauts spending months aboard the International Space Station frequently experience gastrointestinal disruptions, specifically constipation. A new study published in Nature Communications and conducted in collaboration with NASA examines blood samples from 52 astronauts to uncover the physiological mechanisms driving these digestive changes in microgravity.

• Researchers analyzed blood metabolites from 52 astronauts across multiple long-duration missions to track gastrointestinal metabolism in space.
• Blood markers reveal that gut bacteria ferment dietary protein to a higher degree shortly after leaving Earth, a shift linked to slower intestinal motility.
• Study authors suggest that countermeasures such as increased dietary fiber, prebiotics, or treatments promoting peristalsis may be necessary for future deep-space missions to the Moon and Mars.

The physiological stress of spaceflight impacts human biology in complex ways, with the gastrointestinal tract proving particularly sensitive to the absence of gravity. To understand how microgravity alters digestion, investigators from the University of Copenhagen turned to biochemical traces left in circulation.

Metabolic Traces and Protein Fermentation in Space

The research team examined blood metabolites—small molecules circulating in the bloodstream that reflect the body’s metabolic state and intestinal function—to determine how astronauts’ bodies process nutrients away from Earth. By analyzing samples from 52 different astronauts collected over multiple years, the researchers reduced individual variability and uncovered consistent metabolic shifts.

According to Giorgia La Barbera, associate professor at the University of Copenhagen’s nutrition, exercise, and sports department and joint first author of the study, blood samples showed changes indicating that gut bacteria begin fermenting protein to a greater extent within weeks of arriving in space. This heightened fermentation persists throughout the mission until the astronauts return to Earth.

Normally, gut bacteria ferment dietary fiber. When fiber is depleted, microbes break down protein instead. Henrik Roager, coauthor and associate professor at the department, notes that the lack of gravity likely causes food to move more slowly through the intestine, aligning directly with the observed rise in protein fermentation and helping explain why astronauts frequently suffer from constipation.

Lars Ove Dragsted, senior author of the study and professor at the department, points out that products of protein fermentation are frequently associated with negative health consequences, including kidney damage, alterations in mood, and reduced cognitive focus.

Implications for Deep-Space Exploration and Terrestrial Medicine

As space agencies plan extended missions to the Moon and Mars, mitigating the physiological toll of microgravity on the digestive tract becomes a priority. La Barbera emphasizes that countermeasures will be required to protect crew health during prolonged interplanetary travel.

Roager adds that potential interventions could involve increasing dietary fiber, utilizing prebiotic supplementation, or administering treatments that promote peristalsis to decrease gut transit time. These targeted measures would reduce protein fermentation, maintain a healthy gut environment, and prevent subsequent health complications.

Beyond orbital flight, these findings carry direct relevance for immobile populations on Earth, such as bedridden patients who also experience constipation and heightened protein fermentation.

Disclaimer: The information provided in this article is for educational and scientific communication purposes only and does not constitute medical advice. Always consult with a qualified healthcare provider regarding any medical condition, diagnosis, or treatment plan.