52 astronauts left Earth for months in orbit, and within weeks their gut bacteria changed how they processed food, revealing why spaceflight can trigger constipation and potentially affect brain health

Constipation may seem like a minor inconvenience beside the dramatic challenges of spaceflight, but for astronauts it points to a deeper biological problem. Once the human body leaves Earth, the digestive system no longer operates under the gravitational conditions in which it evolved. A new study from the University of Copenhagen, conducted with NASA, offers fresh clues about what happens inside the intestine during months in orbit. Blood samples from 52 astronauts suggest that changes in gut metabolism begin within weeks of reaching the International Space Station and persist until they return home.

The finding helps explain why constipation is so common in space, while also raising a broader question about how the human microbiome responds when gravity disappears. Researchers found signs that bacteria in the intestine shift toward fermenting more protein than usual during spaceflight. That does not necessarily mean astronauts are eating more protein or developing an intestinal disease. Instead, it may indicate that food and its contents are moving differently through the digestive tract, leaving less readily available fiber for gut microbes to consume.

Why gravity matters to the human intestine

On Earth, digestion depends on a complicated combination of muscle contractions, nervous signals, fluid movement and the physical properties of food. Gravity is not the only force moving material through the digestive system, but it contributes to the environment in which the gastrointestinal tract normally operates. In microgravity, that familiar physical influence is largely removed, potentially changing how quickly material progresses through the intestine.


The intestine has its own muscular transport system called peristalsis, so food does not simply fall through the digestive tract under Earth's gravity. Rhythmic contractions push material forward, while the nervous system coordinates movement, secretion and absorption. Yet microgravity can alter fluid distribution, body physiology and gastrointestinal function, creating conditions in which intestinal transit may become slower or less predictable. The new findings fit with the possibility that these changes allow intestinal contents to remain available to microbes for longer.

That matters because the bacteria living in the gut constantly adjust their metabolism according to what reaches them. When sufficient dietary fiber is available, many microbes ferment complex carbohydrates and produce short-chain fatty acids and other compounds. When fermentable carbohydrates become scarce, microbial communities can increasingly turn to proteins and amino acids as alternative sources of energy. The Copenhagen researchers detected metabolic signatures consistent with this shift in astronauts during their time in orbit.

Blood reveals changes happening inside the gut

The researchers did not need to directly examine the astronauts' intestines to detect these changes. Instead, they analyzed metabolites in blood, small molecules produced or modified during metabolism that can provide clues about processes taking place throughout the body. Because substances generated by intestinal microbes can enter the bloodstream, blood chemistry can act as an indirect window into the gut. In this study, those molecular signals revealed a change that appeared soon after astronauts arrived in space.

Giorgia La Barbera, joint first author and associate professor at the University of Copenhagen's Department of Nutrition, Exercise and Sports, said the blood samples showed increased signs of protein fermentation within weeks of arrival. The pattern continued throughout the mission and changed again when astronauts returned to Earth. That timing is important because it suggests the effect is linked to the spaceflight environment rather than being simply a permanent characteristic of the individuals studied.

Protein fermentation itself is not unusual or automatically harmful. It occurs in people on Earth as well, particularly when microbes have less carbohydrate or fiber available in the colon. The concern comes from what prolonged changes in microbial metabolism can produce and how those compounds may interact with the rest of the body. For astronauts spending increasingly long periods away from Earth, even relatively subtle metabolic changes could become more significant over time.

The possible link between gut microbes and constipation

The researchers propose that slower movement of food through the intestine could help explain the metabolic shift. If intestinal contents remain in place for longer, bacteria have more time to act on whatever nutrients remain available. At the same time, dietary fiber may be consumed before material reaches other parts of the microbial community, potentially pushing bacteria toward protein as another substrate. The result could be a measurable increase in protein fermentation products circulating through the blood.

Henrik Roager, an associate professor at the same department, said the lack of gravity probably causes food to move more slowly through the intestine. That interpretation is consistent with the blood findings and with the longstanding observation that constipation occurs frequently during space missions. The study does not mean that protein fermentation has been proven to cause constipation, however. Instead, the results point toward a biological chain in which altered intestinal movement may influence the gut microbiome, which in turn leaves a recognizable metabolic signature.

That distinction is important because constipation in astronauts is unlikely to have a single cause. Spaceflight changes many aspects of human physiology, including fluid distribution, physical activity, diet, sleep and daily routines. Medications and the limited variety of foods available aboard spacecraft can also influence bowel function. The new research adds another piece to that complicated picture by showing that the microbial environment of the intestine appears to change alongside the symptoms.

Why the gut may matter beyond digestion

The significance of these findings extends beyond whether astronauts can maintain regular bowel movements. The intestine is a major metabolic interface between food, microbes and the rest of the body. Microorganisms living there produce thousands of chemical compounds, some of which remain in the digestive tract while others can enter circulation and interact with organs far from the intestine.

Lars Ove Dragsted, senior author of the study and professor at the University of Copenhagen, points out that some products associated with protein fermentation have been linked with unfavorable health effects. Previous research has connected certain microbial protein metabolites with processes involving kidney function, mood and cognitive performance, although the precise effects can vary considerably between compounds and individuals. The astronaut findings therefore raise questions about whether prolonged changes in intestinal metabolism could influence other aspects of human health during long missions.

The brain connection is especially relevant for future exploration missions. Astronauts traveling to the Moon or Mars will need to maintain concentration, decision-making and psychological resilience while operating in an environment that leaves little room for error. If changes in diet, intestinal transit and microbial metabolism can influence compounds reaching the circulation, gut health could become part of the larger strategy for protecting astronaut performance.

Mars missions could make the problem more important

The International Space Station provides a valuable laboratory for studying long-duration spaceflight, but missions to Mars would impose a substantially different challenge. A journey to Mars could keep crews away from Earth for years when travel, surface operations and the return journey are considered. During such missions, astronauts would have far less opportunity to rely on medical support or fresh supplies from Earth.

That makes seemingly ordinary problems such as constipation more important than they might appear. Researchers will need to understand whether the metabolic changes observed in astronauts remain stable, intensify over longer periods or reverse quickly after returning to normal gravity. They will also need to determine whether changes in diet, fiber intake, exercise or other countermeasures can prevent undesirable shifts in gut function.

The research may also have relevance beyond spaceflight. The human body experiences major changes in movement and metabolism when people spend long periods bedridden or physically inactive. Although bed rest and microgravity are not identical conditions, both can disrupt normal physiology and reduce the mechanical demands placed on the body. Understanding how the intestine responds to altered physical conditions could therefore provide useful clues for health problems on Earth as well.

A small digestive problem with a much bigger lesson

The new astronaut study illustrates how spaceflight can expose biological processes that are easy to overlook on Earth. Constipation is not simply an uncomfortable side effect of living in orbit; it may reflect a chain of changes involving intestinal movement, microbial metabolism and the chemicals released into the bloodstream. By analyzing metabolites rather than relying only on symptoms, researchers can begin to see that chain more clearly.

The next question is whether astronauts can be given practical ways to interrupt it. Diets richer in appropriate fiber, carefully designed foods, changes in exercise and other countermeasures could potentially help maintain healthier intestinal function during extended missions. But before those strategies can be tailored for a journey to Mars, scientists need to understand exactly which changes are beneficial, which are harmful and how quickly the gut can recover.

For now, the blood of astronauts is providing an unusual view of life without gravity. It shows that when humans leave Earth, the effects reach far beyond muscles, bones and balance. Even the microscopic ecosystem inside the intestine responds to the journey, and that response may help explain why one of spaceflight's most ordinary problems has been so difficult to understand.