Astronauts often have stomach problems in space. Scientists studied blood from 52 astronauts who stayed on the International Space Station for two to nine months. Soon after launch, some chemicals in their blood went up. These chemicals are made when gut bacteria break down protein.

Food may move more slowly through the body in space. After the astronauts came back to Earth, their blood returned to normal within a few days.

Constipation is a common problem for astronauts, and microgravity may affect how the gut works. A team from the University of Copenhagen and NASA's Johnson Space Center analyzed 488 blood samples from 52 astronauts who had spent two to nine months on the International Space Station. About 40 compounds changed during spaceflight.

Many of those that increased are produced when gut bacteria ferment protein, a process that becomes more common when food moves slowly through the intestines. The changes appeared soon after launch and faded within days after landing. Changes in diet explained less than one third of them. The researchers suggest that adding slowly fermented carbohydrates to space food might help protect astronauts' gut health.

Constipation is frequently reported among astronauts, and microgravity may affect gastrointestinal function during spaceflight. In a study published in Nature Communications, researchers from the University of Copenhagen, NASA's Johnson Space Center and other institutions conducted a longitudinal metabolomic analysis of 488 blood samples from 52 astronauts who spent two to nine months aboard the International Space Station between 2006 and 2018. Roughly 40 compounds responded to spaceflight, shifting soon after launch and recovering within days of landing. Several that increased, such as p-cresol sulfate and phenylacetylglutamine, are derived from protein fermentation by gut bacteria and are known markers of longer intestinal transit time.

The authors therefore propose that microgravity slows the passage of food through the gut, giving bacteria more time to ferment protein. Dietary changes accounted for fewer than a third of the responsive compounds, and no major differences between men and women were found. However, transit time was inferred from blood chemistry rather than measured directly, and astronauts are a highly selected group whose diets and fitness differ from those of the general population. Testing slowly fermentable carbohydrates in space food could be a practical next step.