NASA Researchers Find Why Astronauts Struggle with Constipation in Space

Space travel comes with iconic imagery: weightless astronauts floating through modules, dehydrated meals consumed from pouches, and the endless black void of the cosmos. Yet beneath the heroism of exploration lie persistent, unglamorous biological hurdles. Among the most common yet rarely discussed issues on orbit is persistent constipation.

That clinical ambiguity has finally lifted. By examining biological samples collected from dozens of crew members who lived and worked on the International Space Station, researchers have mapped out exactly how microgravity rewires human digestion and alters the microscopic ecosystem inside the human gut.

Tracking Digestion in 52 Astronauts Aboard the Space Station

To pinpoint why bowel movements become difficult in orbit, an international team of scientists analyzed plasma samples gathered from astronauts between 2006 and 2018. The research cohort included 52 space station residents who spent anywhere from two to nine months in orbit.

Out of the hundreds of circulating compounds tracked in the blood, roughly 40 distinct metabolites shifted noticeably during spaceflight. These molecular changes began almost immediately after launch and reliably subsided within days of returning to Earth. Crucially, the research team investigated whether differences in the astronauts’ diets—such as their intake of caffeine, fish, and dietary fats—accounted for these metabolic shifts. The data revealed that dietary variations explained less than one-third of the changes, pointing directly to microgravity itself as the primary driver of digestive disruption.

How Microgravity Forces Gut Bacteria to Shift From Fiber to Protein

The core issue comes down to transit time and microbial hunger. Under normal conditions on Earth, gravity assists the muscular contractions of the gastrointestinal tract, keeping food moving steadily through the intestines. Without gravity pulling contents downward, digestion slows down significantly. This traffic jam inside the intestinal tract starves the resident gut bacteria of their preferred fuel source: carbohydrates and dietary fiber.

Faced with a depleted supply of carbohydrates, the microbes do not stop consuming. Instead, they switch tactics, breaking down proteins in the colon to sustain themselves. This widespread protein fermentation releases distinct chemical byproducts—including compounds like ammonia and hydrogen sulfide—which cross into the bloodstream and signal that digestion has slowed to a crawl.

The metabolic evidence confirms that food lingers much longer in the intestines of spacefarers. According to study co-author Henrik Roager, the lack of gravity in space slows intestinal motility, perfectly aligning with the biological markers of heightened protein fermentation and providing a clear physiological explanation for chronic space constipation.

Dietary Countermeasures for Long-Duration Space Missions

Solving the digestive woes of space travel is not merely about comfort; it is a critical medical requirement for deep-space exploration. A crewed mission to Mars will take roughly seven to nine months each way.

Fortunately, the scientific team proposes a straightforward, non-technological countermeasure: modifying space diets to include significantly more fiber. Specifically, researchers recommend incorporating whole foods rich in dietary fiber, such as vegetables, lentils, and nuts, alongside slowly fermented carbohydrates and prebiotic supplements.

By supplying astronauts with slow-burning carbohydrates, space agencies can give gut microbes plenty of material to ferment, keeping bowel movements regular and preventing microbes from relying on protein breakdown. Because these digestive pathways are shared by all humans, researchers note that understanding microgravity’s effects on the microbiome may also yield valuable clues for treating bedridden patients and individuals suffering from chronic digestive disorders on Earth.