Astronauts in space often suffer constipation, and scientists have now found why: their gut bacteria switch from fermenting fibre to fermenting protein within weeks of reaching orbit. Researchers at the University of Copenhagen and NASA analysed 488 blood samples from 52 astronauts who flew International Space Station missions between 2006 and 2018, staying two to nine months. Microgravity slows the transit of food through the gut, so bacteria exhaust the available fibre and turn to protein, producing metabolites linked to kidney damage and effects on mood and cognition. The findings, published in Nature Communications in mid-August, explain a long-running puzzle of space medicine. The team suggests countermeasures including extra fibre, probiotics and treatments that enhance peristalsis. Keeping astronauts' digestion healthy matters for longer missions, including planned journeys to the Moon and Mars, where medical help is far away. The study is the largest analysis of astronaut gut health ever conducted, drawing on two decades of mission data.