Constipation is common in astronauts—blood tests provide new answers

For astronauts in space, constipation is a well-known problem. But why this happens has long been unclear. Now, a new study from the University of Copenhagen in collaboration with NASA provides an idea of what happens in the stomach without the influence of gravity.

The researchers examined blood samples from 52 astronauts who participated in months-long missions to the International Space Station. The study identified gastrointestinal changes that occur shortly after the astronauts leave Earth and gravity behind. The findings are published in the journal Nature Communications.

“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,” says joint first author and an associate professor at the Department of Nutrition, Exercise and Sports, Giorgia La Barbera.

Protein fermentation occurs when gut bacteria begin to break down protein instead of fiber because the fiber in the diet has been used up. This is not abnormal, and it also happens to many people on Earth.

“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,” says co-author Henrik Roager, who is also an associate professor at the Department of Nutrition, Exercise and Sports.

The intestines are connected to the brain

These findings were part of an experiment to analyze astronauts’ blood metabolites, which are small molecules in circulation that reveal how the body’s metabolism works. Metabolites can be used as clues to many aspects of human health, including what people have eaten and how the intestines are functioning.

These findings are important because changes in the gut not only affect digestion but also can have consequences for the rest of the body—including the brain.

“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,” says Lars Ove Dragsted, senior author of the study and a professor at the Department of Nutrition, Exercise and Sports.

Longer journeys and bedridden patients

With plans to send crews on extended space missions to the moon and beyond, the researchers point out that the current findings may be important.

“When we plan for longer journeys in space—for example, to Mars—countermeasures may be needed to mitigate negative effects of space travel on the intestines,” says La Barbera.

“Our findings can inform potential interventions—either directly through increased dietary fiber, supplementation with prebiotics or other types of treatments that promote peristalsis and decrease gut transit time. This would help reduce protein fermentation and thus provide a healthy gut environment and avoid negative health consequences,” adds Roager.

The authors point out that the research is not only relevant for space travelers but also has implications on Earth.

“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,” concludes Dragsted, senior author of the paper.

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Gaby Clark

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