Space Changes Gut Bacteria, Explaining Astronaut Constipation

Astronauts aboard the International Space Station face a problem that rarely makes headlines but affects nearly everyone who spends time in orbit: constipation. Researchers from the University of Copenhagen and NASA have now identified what appears to be the culprit, and the answer lies not in the food itself but in how microgravity reshapes the bacteria living in the human gut.

Scientists analyzed blood samples from 52 astronauts who spent months aboard the station and discovered that within weeks of entering space, their digestive systems begin a measurable shift. The gut bacteria start fermenting protein at elevated levels—a process that normally occurs only when fiber has been depleted and bacteria turn to alternative food sources. The researchers believe microgravity slows the movement of food through the intestines, giving bacteria more time to break down protein rather than the fiber they would typically consume. This extended transit time, they argue, explains why astronauts experience constipation so frequently.

Giorgia La Barbera, an associate professor at the Department of Nutrition, Exercise and Sports at the University of Copenhagen, describes the finding as a window into how profoundly spaceflight alters basic bodily functions. "We see changes in astronauts' blood samples that indicate that the gut bacteria begin to ferment protein to a greater extent than usual within weeks after the astronauts arrive in space, and this change continues until they are back on Earth," she explains. The shift is not temporary or minor—it persists throughout their time away from Earth's gravity.

What makes this discovery significant extends beyond the discomfort of constipation. The byproducts of protein fermentation circulate through the bloodstream and can affect systems far removed from the digestive tract. Previous research has linked these metabolites to kidney damage, mood disturbances, and reduced ability to concentrate. For astronauts spending months aboard the station, these effects remain manageable. But as space agencies plan longer missions—to the Moon and eventually to Mars—the duration of exposure to microgravity could intensify these health risks. An astronaut on a journey to Mars might spend a year or more in weightlessness, potentially magnifying the digestive and neurological consequences.

Henrik Roager, a co-author and fellow associate professor, notes that the mechanism is straightforward once understood: "The lack of gravity in space probably causes food to move more slowly through the intestine, and this fits with the fact that we are seeing signs of increased protein fermentation." The researchers have identified several potential interventions. Increasing dietary fiber, supplementing with prebiotics, or using treatments that promote peristalsis—the coordinated muscle contractions that move material through the digestive system—could all reduce the time food spends in the intestines and thereby limit protein fermentation.

The study's strength lies partly in its unusual design. Metabolite research typically struggles with high variability between individuals, making patterns difficult to detect. But by pooling blood samples from 52 different astronauts across multiple missions and years, the researchers found a signal so consistent that it cut through that noise. Jan Stanstrup, a joint first author and assistant professor, emphasizes the rarity of this clarity: "It is quite impressive that our method has been able to show so clearly that there is consistently more protein fermentation."

Beyond spaceflight, the findings may help patients on Earth. People confined to bed for extended periods often experience constipation and likely undergo similar changes in gut fermentation. Lars Ove Dragsted, the study's senior author, sees potential application in hospitals and care facilities: "You can use this knowledge to help bedridden patients. They also experience constipation and likely also have increased protein fermentation that may aggravate health conditions." The research thus opens a door to understanding how immobility—whether caused by gravity's absence or by illness—reshapes the microbiome and its effects on human health.

As space agencies prepare for the next era of exploration, this work underscores a fundamental challenge: the human body evolved under Earth's gravity, and leaving it behind triggers cascading changes that science is only beginning to map. The constipation astronauts experience is not a minor inconvenience but a visible marker of deeper metabolic shifts that will require careful management on future deep-space missions.