Space constipation explained: microgravity reshapes gut bacteria and risks on long missions

A NASA-backed Nature Communications study analyzed more than 400 blood samples from 52 astronauts who stayed on the International Space Station for 2+ months (2006–2018) and found microgravity slows gut movement and shifts gut bacteria to ferment protein more than usual, helping explain space constipation. While dietary changes (caffeine, fish, fat) influence metabolites, microgravity is the main driver; the resulting protein‑fermentation byproducts may be linked to kidney stress, mood changes, and gut inflammation—risks that could grow on longer missions to Mars. Researchers suggest countermeasures like higher‑fiber diets, probiotics, or ways to boost peristalsis, noting the findings could also illuminate bed‑rest effects on Earth.