Why Astronauts Struggle to Poop in Space: New Study Reveals Cause and Risks

Astronauts frequently struggle with constipation and other digestive issues during spaceflight, and a new NASA-backed study published June 29 in the journal Nature Communications reveals that microgravity slows the movement of food through the intestine, leading to increased protein fermentation by gut bacteria. According to researchers at the University of Copenhagen, analyzing more than 400 blood samples from 52 astronauts who lived on the International Space Station between 2006 and 2018 showed that this microbial shift begins within weeks of arriving in space and persists until return, raising potential long-term health concerns for future long-haul missions to Mars.

Going to the bathroom in orbit has presented challenges since humans first started living in space, stretching far beyond the mechanical complications of using specialized space toilets. NASA notes that astronauts regularly contend with bloating, indigestion, and acid reflux alongside chronic constipation. While missions routinely stock laxatives—including the Artemis II mission—researchers have long suspected that microgravity is the primary culprit behind these gastrointestinal hurdles, though proving that hypothesis has historically proved difficult.

Metabolic Analysis Reveals Gut Bacteria Shifts in Microgravity

To explore the digestive tract’s internal biochemical alterations, scientists analyzed blood gathered from astronauts who each spent upwards of two months residing on the International Space Station. Out of 40 circulating compounds identified during the analysis, the most instructive were metabolites produced when microbes in the colon break down proteins. Giorgia La Barbera, an associate professor in the University of Copenhagen’s Department of Nutrition, Exercise and Sports and first author of the study, stated that blood samples show gut bacteria begin to ferment protein to a greater extent than usual shortly after arrival in space.

Normally, bacteria in the colon rely on complex carbohydrates and fiber for energy. However, study co-author Henrik Roager, head of the University of Copenhagen’s Microbiome & Metabolomics research group, explained that the lack of gravity likely causes food to move more slowly through the intestine. This deceleration gives fiber more time to break down thoroughly, causing microbes to eventually switch to fermenting proteins instead. While dietary adjustments involving caffeine, fish, and fat intake influence these metabolite levels, researchers noted that food factors account for less than one-third of the observed changes, cementing microgravity as the primary driver.

Health Implications and Countermeasures for Long-Duration Spaceflight

Although short-term space missions involve gastrointestinal inconveniences that are managed with standard remedies, extended travel deeper into the solar system could amplify these risks. Lars Ove Dragsted, head of preventive and clinical nutrition at the University of Copenhagen, noted that products of protein fermentation are frequently associated with negative health consequences such as kidney damage, inflammation, weakened gut linings, and potential effects on mood or focus. Compounds such as ammonia and hydrogen sulfide could accumulate to harmful levels during long-haul trips to Mars. To prepare for prolonged exploration, researchers emphasize that countermeasures will likely be necessary to maintain astronaut intestinal health. By reducing excessive protein fermentation, future space travelers can maintain a healthier gut environment during extended voyages away from Earth.