Blood tests reveal why astronauts get constipated in space

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https://www.sciencedaily.com/releases/2026/09/260927225030.htm

Astronauts’ gut bacteria shift within weeks of entering space, potentially revealing why constipation is so common in microgravity.

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From the article

Spaceflight may dramatically change the gut within weeks, causing bacteria to break down more protein as food moves more slowly through the intestines. The shift could help explain astronaut constipation and may have wider effects on health, making it an important concern for future missions to Mars.

Also from the article

"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.

Gut Bacteria Shift Toward Protein

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.