When human beings leave the gravitational embrace of Earth, even the invisible communities living within them are transformed. Researchers at the University of Copenhagen, studying blood samples from 52 astronauts aboard the International Space Station, have traced a quiet but consequential shift: without gravity to move food efficiently through the gut, bacteria abandon their preferred diet of fiber and turn to fermenting protein instead — a change that ripples outward into mood, cognition, and organ health. The finding reframes constipation not as a minor inconvenience of spaceflight, but as