Copenhagen: Scientists may finally have solved one of spaceflight's most uncomfortable mysteries — why astronauts get constipated. A University of Copenhagen study with NASA, published in Nature Communications, analysed blood samples from 52 astronauts who flew long missions to the International Space Station between 2006 and 2018.

The researchers found that within weeks of reaching orbit, astronauts' blood showed heightened levels of metabolites linked to protein fermentation — a process that occurs when gut bacteria shift from breaking down fibre to metabolising protein because fibre has run out. The change persisted until the astronauts returned to Earth.

'The lack of gravity in space probably causes food to move more slowly through the intestine,' said co-author Henrik Roager. That sluggish transit gives bacteria more time to ferment protein, likely contributing to constipation.

The findings matter beyond comfort: protein-fermentation metabolites are linked to kidney damage, mood changes and reduced focus. As agencies plan longer Moon and Mars missions, researchers suggest more dietary fibre, probiotics or treatments that speed up gut transit could help.