Constipation is a familiar problem for astronauts, but scientists have not fully understood why spending time in space can disrupt normal digestion. New research from the University of Copenhagen, conducted in collaboration with NASA, offers a possible explanation. Scientists analyzed blood samples from 52 astronauts who spent months aboard the International Space Station and found signs that the digestive system begins changing relatively soon after astronauts leave 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," says joint first author and associate professor at the Department of Nutrition, Exercise and Sports, Giorgia La Barbera. Protein fermentation occurs when bacteria in the intestines begin breaking down protein rather than fiber because available dietary fiber has already been used. The process is not unique to astronauts and can also occur in people on Earth.
The researchers suspect that microgravity may slow the movement of food through the digestive tract. If food remains in the intestines longer, bacteria have more time to turn to protein fermentation. "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 of the article Henrik Roager, who is also an associate professor at the Department of Nutrition, Exercise and Sports. The findings came from an investigation of metabolites in astronauts' blood. Metabolites are small molecules circulating through the bloodstream that can provide clues about how the body is functioning.
By analyzing these molecules, researchers can learn about processes ranging from diet and metabolism to what is happening inside the intestines. The results could matter beyond digestion because changes in the gut can influence other parts 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 professor at the Department of Nutrition, Exercise and Sports.
The findings could become especially important as space agencies prepare for longer missions to the Moon and eventually Mars. Astronauts on those journeys may spend far more time away from Earth's gravity than crews currently stationed aboard the International Space Station. "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 Giorgia La Barbera.
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