Try and try and try as they may, scientists have yet to explain one of spaceflight’s most painfully enduring mysteries: Why do astronauts get constipated? After examining blood samples from 52 astronauts, they may have found an answer.
To learn more about how spaceflight affects digestion, researchers from the University of Copenhagen, working with NASA, tracked blood metabolites – molecules in the bloodstream that can indicate what’s happening inside the body – from astronauts who stayed aboard the International Space Station (ISS) between 2006 and 2018.Their paper, published June 29 in Nature Communications,found that within weeks of reaching orbit, the astronauts’ blood contained a heightened number of metabolites associated with a process called protein fermentation, typically caused by sluggish intestines, which can create a number of health complications.
“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. This may also help explain constipation in astronauts,” Henrik Roager, head of the Nutrition, Microbiome & Metabolomics research group at the University of Copenhagen and study co-author, said in a press release.
Metabolite tracking
The researchers drew multiple blood samples from each of the 52 astronauts before launch, during the mission, and after landing, then spun the samples down to isolate plasma.
Floating in that plasma are metabolites. These molecules are left behind by chemical reactions in the body, triggered by food, medicine, sunlight, and even stress. Metabolism is the sum of those reactions; metabolites are its signature, offering a chemical snapshot of the metabolism at a given moment. By tracking how astronauts’ metabolites shifted, the team could see how spaceflight and microgravity impact human metabolism.
Intestines slow down in space
On Earth, bacteria in the large intestine survive by breaking down fiber and other carbohydrates that the small intestine isn’t fully able to digest. That process is called fermentation, and the microbes in our guts do it daily. But when fiber and carbohydrates run out and the bacteria still need to feed to survive, they start breaking down protein instead in a process known as protein fermentation.
The team found elevated levels of the metabolites tied to protein fermentation in astronauts’ plasma from during their missions. The pattern showed up within the first 50 days of a mission and persisted through the length of the mission. In general, it faded within about 10 days of returning to Earth.
Previous terrestrial research on the topic has found that protein fermentation might occur as a result of slower “transit times” — the time it takes for food to travel through the digestive system. Without gravity’s pull, the team thinks, food may work its way through the intestines more slowly than it does on Earth, giving gut bacteria sufficient time to chow down on all the available fiber and then turn its appetite to protein. The result is fewer regular bowel movements and subsequently constipation.
The consequences of enhanced protein fermentation go beyond constipation, though. “We know that products of protein fermentation [are] often associated with negative health consequences, such as kidney damage, potential effects on mood or reduced ability to focus, among others,” said senior author Lars Ove Dragsted in the press release.
Other findings
The astronauts’ blood also showed dips in caffeine and a caffeine byproduct, possibly reflecting the switch to instant coffee aboard the station. Uric acid levels fell too, a pattern the researchers argue is most likely tied to shifting fluid balance in microgravity. Prior research has found that overall plasma levels drop during spaceflight as a response to fluid shifting throughout the body in unusual ways. So while the concentration of uric acid in the plasma remains unchanged, the overall volume likely drops along with the drop in plasma.
Implications for future missions
The findings arrive as NASA and other space agencies plan long-term crewed journeys to the Moon, and eventually to Mars, where astronauts could spend years away from Earth’s gravity. Therefore, understanding the long-term effects of spaceflight on gut function is essential to keeping crews healthy for those voyages.
One solution the researchers propose is increasing consumption of slow-fermenting carbohydrates in space food. They suggest this dietary change could keep gut bacteria supplied with their preferred fuel for longer, delaying protein fermentation and its negative side effects. Prebiotics or treatments that speed up gut motility could offer another route to the same goal.
“Our findings can inform potential interventions – either directly through increasing dietary fiber, supplementation with prebiotics or other types of treatments that promote peristalsis and decrease gut transit time,” Roager said. “This would help reduce protein fermentation and thus provide a healthy gut environment and avoid negative health consequences.”
The team also sees a use for the findings closer to home. Bedridden patients on Earth face many of the same problems as astronauts — reduced activity, slowed gut transit, and constipation. Studying digestion in space could inform care for patients at home, the authors argue.
Brooks Mendenhall is a staff writer at Astronomy, based in Chattanooga, Tennessee, fueled by an unending curiosity about the universe. A former classroom teacher, he has a knack for breaking down complex concepts for a wide audience.
