What Happens to Your Gut in Space? A Study of 52 Astronauts Offers Clues

Your gut in space may behave differently, according to a 2026 study of 52 astronauts. During International Space Station missions, blood samples showed higher levels of compounds consistent with gut microbes breaking down more protein. The researchers propose that slower intestinal movement in microgravity could help explain the pattern. They did not directly measure gut bacteria, transit time or constipation, however, and did not demonstrate harm to the astronauts. Many changes subsided after landing. For readers on Earth, this offers clues about how bowel habits and microbial activity may interact, rather than proof that everyone needs a new diet.

It is an unusually relatable space question. We can admire a rocket launch and still wonder what happens after lunch. Following Shubhanshu Shukla’s 2025 Axiom-4 mission, which made him the first Indian to visit the International Space Station (ISS), the ordinary details of life in orbit feel a little closer to home. Eating, drinking and using the loo still have to happen up there.

The study in Nature Communications, published on 29 June 2026, looks at what may be happening inside the body during those daily routines. It provides an intriguing lead, with an important distinction: the researchers studied chemical clues in blood, not a direct recording of what the gut bacteria were doing.

How did researchers study the gut in space?

Researchers at the University of Copenhagen, the University of Houston and NASA’s Johnson Space Center analysed 488 blood-plasma samples from 52 astronauts. The participants included 11 women and 41 men, with an average age of 48. Their ISS missions lasted two to nine months and took place between 2006 and 2018, so Shukla was not part of this group.

Samples were collected before launch, during spaceflight and after landing. That allowed the team to compare changes within the same people, rather than simply compare astronauts with a different group on Earth.

The gut microbiome is the community of microbes living in the digestive tract. To look for clues to their activity, the researchers used metabolomics: measuring many small chemical compounds, or metabolites, in a sample. These compounds can come from food, the body’s own processes or microbes. Some microbial products enter the blood after absorption and further processing by the body.

Think of these as traces left behind by a process. A trace can suggest what happened, but it does not show every step that produced it.

What changed in the astronauts’ blood?

The analysis identified around 40 compounds that changed during spaceflight. Several were linked to microbial protein breakdown, including p-cresol sulfate and phenylacetylglutamine. Their higher levels were consistent with increased protein fermentation by gut microbes.

The pattern appeared early in flight and persisted during the missions. The researchers did not see it steadily intensifying the longer astronauts stayed in space. Many of the changes subsided within days of returning to Earth.

Other changes suggested differences in dietary exposures, including lower caffeine exposure. The team estimated that fewer than one-third of the altered metabolic features were related to food intake. This is not the same as proving that diet played no role: these were interpretations of chemical signals, not a complete accounting of everything each astronaut ate.

What was measured, and what remains a hypothesis?

Why might slower digestion change microbial activity?

Two terms help make sense of the proposed explanation.

Transit time is how long material takes to pass through the digestive tract, or through a particular part of it. Here, the researchers were interested in whether intestinal contents might remain in place for longer during spaceflight.

Microbial fermentation is the breakdown of substances by gut microbes. In the large intestine, bacteria can use carbohydrates that escaped digestion, including fermentable fibres. They can also break down proteins and amino acids, producing a different mix of compounds.

The researchers propose that, if material stays in the intestine longer, microbes may use up more of the available carbohydrates and increasingly rely on protein breakdown. That could explain the blood pattern they observed. The study did not establish that the astronauts ran out of fibre or that all their bacteria abruptly changed fuel.

There is also a basic physiology point worth clearing up: food does not simply fall through the gut. Muscular contractions called peristalsis push and mix digestive contents. The spaceflight paper proposes that microgravity may alter transit; it does not show that digestion stops working without Earth’s usual gravitational conditions.

Does this explain constipation in astronauts?

Constipation is a recognised issue in spaceflight, as the paper and the University of Copenhagen’s account of the research note. Slower transit is one possible contributor, and the blood findings are compatible with that explanation.

But the study did not record constipation rates, individual symptoms or intestinal transit time. It therefore offers a possible mechanism to investigate, not proof of why astronauts become constipated.

Nor does a raised metabolite automatically mean someone has been harmed. The paper discusses adverse effects associated with some protein-fermentation products in other research settings. It did not test whether these compounds caused health problems in the astronauts.

How do astronauts eat, drink and use the toilet?

The microbial question sits inside a much bigger practical challenge: keeping people fed, hydrated and comfortable away from Earth.

Space food has to survive the journey

NASA’s space-food teams prepare freeze-dried foods, powdered drinks and heat-stabilised meals in pouches. Food must be safe, nutritious, acceptable to the crew and workable within limited storage and preparation space.

Space toilets use airflow

NASA explains that space toilets use moving air to guide urine and solid waste into collection systems. Its Universal Waste Management System uses a funnel and hose for urine, and a seat for bowel movements. Restraints help the user stay in position.

Designs and waste handling differ between spacecraft.

Recycled water is treated water

In 2023, NASA reported that the ISS life-support system had demonstrated 98 per cent water recovery. That includes reclaiming water from urine and moisture in cabin air, including crew breath and sweat.

The recovered water is filtered, processed and checked for purity; water that fails the checks is processed again. The 98 per cent figure describes system recovery, not how much of any glass is urine.

What might this mean for longer missions?

The researchers suggest testing ways to keep more slowly fermented carbohydrates available to gut microbes, potentially through changes to fibre intake. The idea is to provide substrates that remain available further along the intestine and may reduce reliance on protein fermentation.

That is a research proposal, not a demonstrated solution. This study did not test a fibre supplement, a new space menu or a treatment to speed intestinal movement. It cannot tell us which approach would work, at what dose or with what health benefit.

For longer missions, including possible journeys to Mars, those are practical questions worth answering before recommending a particular countermeasure.

What can readers on Earth take from the study?

The connection to Earth is plausible, but it needs its own evidence. In a 2016 study of adults on Earth, longer colonic transit was associated with higher urinary levels of microbial protein-breakdown products. That supports the spaceflight team’s explanation without proving that the same mechanism caused the astronauts’ results.

The new study is not a reason to stop eating protein, buy prebiotic supplements or diagnose your microbiome from how you feel after a meal.

For everyday bowel habits, general health guidance supports adequate fibre and fluids, regular physical activity and making time to use the toilet. Familiar meals with dal, whole-grain roti, sabzi and fruit can help you include fibre without adopting a special menu. Increase fibre gradually if you are not used to it, and follow any medically prescribed diet or fluid limits.

These are general measures, not interventions shown to reverse the astronaut findings. If your own bowel pattern has changed, a space study cannot tell you why. Seek medical advice if symptoms persist; blood in stool, ongoing abdominal pain, vomiting or unexplained weight loss warrant prompt attention.

For more everyday context, read why sitting all day can leave you feeling heavy.

The bottom line

In 52 astronauts, blood-metabolite changes were consistent with increased microbial protein fermentation during spaceflight. Slower intestinal transit is a possible explanation, and many changes subsided after return to Earth.

The study gives researchers a useful question to test: could different fibre sources or support for bowel regularity help on long missions? For the rest of us, it is a reminder that what gut microbes do may depend on how long material remains in the intestine as well as what we eat. It is not yet a new set of dietary instructions.

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