Gut Bacteria Shift Explains Astronaut Constipation, Study Suggests

Constipation is a well-known problem for astronauts, but the biological reason behind it has been unclear. Researchers from the University of Copenhagen, working with NASA, have now found evidence of what changes in the digestive system when gravity disappears, by analyzing blood samples from 52 astronauts who spent months aboard the International Space Station.

The team found gastrointestinal changes that appeared within weeks of the astronauts leaving Earth. "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," said joint first author Giorgia La Barbera, associate professor at the university's Department of Nutrition, Exercise and Sports. Protein fermentation happens when gut bacteria break down protein instead of fiber once dietary fiber runs low — a process that also occurs in many people on Earth, not just in space.

Co-author Henrik Roager, also an associate professor at the department, said the lack of gravity in space probably slows the movement of food through the intestine, which fits with the increased protein fermentation the team observed and may help explain astronaut constipation. Senior author Lars Ove Dragsted, a professor at the department, said products of protein fermentation are often associated with negative health effects such as kidney damage and potential effects on mood or the ability to focus.

As space agencies plan longer crewed missions to the Moon and eventually Mars, La Barbera said countermeasures may be needed to offset the effects of spaceflight on the intestines. Roager said the findings could inform interventions such as increasing dietary fiber or using prebiotic supplements to promote gut movement and reduce protein fermentation.

Dragsted said the same knowledge could help bedridden patients on Earth, who also experience constipation and likely increased protein fermentation that may worsen their health conditions.