According to data collected by NASA, six out of ten astronauts experience significant intestinal issues while in orbit. A study supported by the American space agency, published in the journal Nature Communications, has identified a clear biological explanation for this condition. The digestive problems often begin as early as the first week of a space mission and resolve only after returning to Earth. Common symptoms include bloating, indigestion, and acid reflux. Laxatives are routinely included in medical kits for missions such as Artemis II, in preparation for these potential issues. Researchers analyzed over 400 blood samples from 52 astronauts who spent more than two months aboard the International Space Station (ISS) between 2006 and 2018. This data helped identify 40 metabolites—small molecules produced by the body that indicate internal processes. The most significant findings pointed to an increase in bacterial fermentation of proteins in the intestine. On Earth, intestinal bacteria typically feed on complex carbohydrates like the fibers found in vegetables and grains. However, in the microgravity of space, these bacteria appear to shift their diet toward proteins, a change that begins early in a mission and continues until the astronauts return to Earth. One of the most compelling explanations involves the slowdown of intestinal transit in microgravity. Without gravity to help move food through the digestive system, the content of the intestine moves more slowly. This delay allows more time for fibers to be digested higher up in the digestive tract, leaving fewer fibers available for the bacteria further down. With fewer fibers to consume, these bacteria turn to proteins instead, as a sort of backup option. This mechanism mirrors a principle NASA explains: digestion on Earth relies on peristalsis, wave-like muscle contractions that move food independently of gravity. In space, however, body fluids shift toward the upper body, causing the stomach to float and the intestines to become less compact, leading to a slower digestive process. A prolonged imbalance in the intestinal microbiome could affect nutrient absorption, immunity, and overall well-being over time. As future space missions become longer—such as a round trip to Mars, which could take several years—this digestive issue could become more serious. The effects of such a long-term imbalance are still difficult to predict. This discovery highlights the importance of considering this hidden risk when planning future long-duration missions to Mars and beyond. Researchers are now working to understand and mitigate these challenges to ensure the health and safety of astronauts on extended space journeys.