Why Astronauts Struggle to Poop in Space, According to Science

Microgravity Slows Digestion in Orbit

Astronauts living in low-Earth orbit experience a significant slowing of gastrointestinal motility driven primarily by microgravity. This shift leads to chronic constipation and unexpected metabolic changes within the body, according to a NASA-backed study published June 29 in the journal Nature Communications. Researchers examining more than 400 blood samples from 52 astronauts who spent over two months aboard the International Space Station (ISS) between 2006 and 2018 discovered that this cosmic constipation forces gut bacteria to ferment proteins instead of complex carbohydrates, releasing potentially harmful metabolites into the bloodstream.

• Microgravity slows intestinal movement, causing gut microbes to deplete dietary fiber and switch to fermenting proteins within weeks of spaceflight.
• Metabolomic analysis identified roughly 40 circulating compounds, noting significant elevations in protein fermentation byproducts like ammonia and hydrogen sulfide.
• Researchers suggest countermeasures such as increased fiber intake, probiotics, and targeted physical interventions for long-haul space missions to Mars.

Operational Hurdles on Long-Duration Flights

Constipation remains a persistent operational and clinical challenge during space missions, requiring routine stockpiling of laxatives on flights like the recent Artemis II mission. Alongside bloating, indigestion, and acid reflux, gastrointestinal distress impacts space crews from their first week on orbit until they return to Earth. While changes to caffeine, fish, and fat intake influence these digestive symptoms, dietary adjustments account for less than one-third of the observed metabolic changes, leaving microgravity as the primary driver behind the altered gut environment, as reported by Space.com.

Metabolomic Analysis Reveals Cellular Shifts

To evaluate the biological mechanisms at play, a research team led by scientists at the University of Copenhagen analyzed 488 plasma samples collected from ISS crew members. Using advanced metabolomics, the investigators tracked metabolic byproducts generated when intestinal microbes break down food, tissue, and drugs. The data revealed that lack of gravity causes food to move more slowly through the intestines. This extended transit time allows fiber to be thoroughly degraded early in the colon, leaving gut bacteria dependent on protein fermentation for survival.

“We see changes in astronauts’ blood samples that indicate that the gut bacteria begin to ferment protein to a greater extent than usual,” study first author Giorgia La Barbera, an associate professor in the University of Copenhagen’s Department of Nutrition, Exercise and Sports, stated in a release. This metabolic shift emerges within weeks of arrival and persists until crew members land back on Earth.

Health Risks and Proposed Countermeasures

The elevation of protein fermentation introduces distinct clinical concerns for extended-duration spaceflight. When gut microbes process proteins rather than complex carbohydrates, they produce metabolic byproducts including ammonia and hydrogen sulfide. According to study co-author Lars Ove Dragsted, head of preventive and clinical nutrition at the University of Copenhagen, these compounds associate with potential negative health impacts such as kidney damage, mood alterations, reduced cognitive focus, and localized inflammation or weakening of the intestinal mucosal lining.

“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,” La Barbera noted. Proposed preventive strategies include formulating diets rich in slowly fermented carbohydrates, administering targeted probiotic supplements, and stimulating peristalsis via physical massage or pharmacological agents to maintain regular bowel function and protect the gut microbiome. Patients experiencing chronic digestive slowdowns or managing lazy bowel syndrome on Earth, particularly those undergoing extended bed rest, may also benefit from these clinical insights into transit-related 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.*