Why Astronauts Get Constipated — and What It Teaches Us

2026-08-31 · By the Diet for Constipation data editorial team · 1,528 words

Here is a sentence we did not expect to write this month: NASA has a constipation problem, and a new study published in Nature Communications explains why. Fifty-two astronauts who spent months aboard the International Space Station showed measurable changes in their gut bacteria — changes that look a lot like what happens when the human gut runs out of fiber. The research, led by the University of Copenhagen in collaboration with NASA, was reported by HealthDay on August 18, 2026, and it is one of the most useful constipation stories of the year. Not because most of us are going to space. But because the mechanism it uncovered is the same mechanism that backs up millions of people down here.

What the researchers actually found

The study team analyzed blood samples from 52 astronauts on monthslong ISS missions. Astronauts tend to develop constipation soon after reaching orbit — this has been documented for decades and was never fully explained. The Copenhagen group looked for molecular traces in the blood that reveal what gut bacteria are eating.

Gut microbes are flexible eaters. Their preferred menu is dietary fiber — complex carbohydrates your own body cannot digest. When fiber is plentiful, bacteria ferment it into short-chain fatty acids, which are broadly beneficial and keep the gut environment healthy. But when fiber runs out, bacteria switch fuels. They start fermenting protein instead, producing a different set of compounds — and several of those compounds are associated with negative health effects.

That switch is exactly what the researchers observed. Within weeks of arriving in space, astronauts' blood samples showed signs of increased protein fermentation, and the shift continued for the entire mission. Crucially, the change could not be explained by the astronauts' diets alone.

Why space slows the gut down

So if the astronauts were eating enough fiber, why did their bacteria run out? The answer is transit time. In microgravity, food moves through the intestine more slowly than it does on Earth. Co-lead researcher Giorgia La Barbera, an associate professor of nutrition, exercise and sports at the University of Copenhagen, explained in a news release that gut bacteria begin fermenting protein to a greater extent than usual within weeks of arrival in space — and that the change persists until return to Earth.

The mechanism is worth spelling out, because it is surprisingly intuitive. Imagine a conveyor belt carrying a steady supply of fiber to the bacteria that line the gut. If the belt speeds up, the bacteria see more fiber than they can process, and everyone eats well. If the belt slows down — or if the fiber supply thins out — the bacteria finish everything edible early in the journey and spend the rest of the trip breaking down protein. Slower transit plus the same diet equals a gut that behaves as if it were fiber-deprived, even when it is not.

Co-researcher Henrik Roager, also an associate professor at Copenhagen, put it plainly: the lack of gravity probably causes food to move more slowly through the intestine, which fits with the observed increase in protein fermentation — and, he noted, this may help explain constipation in astronauts.

The health stakes are bigger than discomfort

Constipation sounds minor, and for most people a single sluggish day is trivial. But the study's senior researcher, Lars Ove Dragsted, a professor at the University of Copenhagen, pointed out that products of protein fermentation are often associated with negative health consequences — including potential kidney effects and even mood-related changes observed in animal models. For astronauts bound for Mars, a seven-to-nine-month journey, the research team argues that dietary countermeasures will need to be built into mission planning.

And the relevance does not stop at the launchpad. Dragsted made the connection explicitly: the same knowledge can be applied to bedridden people, who also experience constipation and likely also show increased protein fermentation that may aggravate existing health conditions. In other words, anything that slows transit — immobility, dehydration, low-fiber eating patterns, certain medications — pushes the gut toward the same protein-fermenting state the researchers measured in orbit.

The fix, according to the researchers themselves

Here is where the story gets satisfying. The interventions the researchers propose for astronauts are almost word-for-word the ones nutrition clinicians have been recommending on Earth for decades. Roager suggested that eating whole foods rich in dietary fiber — vegetables, lentils, nuts — would be important to keep gut motility up to speed and to give gut bacteria additional substrate during missions. He also pointed to prebiotic supplementation and other options that promote peristalsis and decrease gut transit time, keeping microbes busy fermenting fiber instead of protein.

Translate that from spaceflight language into grocery language:

• Feed the bacteria continuously. Fiber should arrive throughout the day, not in a single heroic salad. The bacteria "run out" when supply stops — so breakfast, lunch and dinner each need a source.
• Choose slow-fermented carbohydrates. The researchers specifically highlighted carbohydrates that take longer to process — the kind found in whole grains, legumes, lentils and nuts — because they give microbes something to work on across the full length of the gut.
• Keep transit moving. Movement, hydration and adequate fiber all shorten transit time. Shorter transit means less time for the fiber supply to be exhausted.
• Do not let the diet collapse. The astronauts were not starving. Their problem was a shift in what was available to their bacteria. A week of white bread, cheese and processed snacks does the same thing at slower speed.

How this connects to our food ratings

This study is, in effect, a mechanistic endorsement of the rating system we use across this site. Our engine rates foods green primarily for fiber density, adjusts for water and fat, and flags refined and fried products — because those are the factors that determine whether the gut's conveyor belt carries enough fiber to keep bacteria on their preferred fuel. The NASA-funded work adds a vivid data point: when transit slows and fiber availability drops, measurable blood chemistry shifts within weeks. The same logic, at Earth gravity, underlies every green and red light on this site.

It also validates a point we make on food pages repeatedly: fiber needs water, and fiber needs movement. An astronaut on the ISS cannot go for a walk, which is one reason transit slows. On Earth, a twenty-minute walk after meals is one of the cheapest transit-time interventions available.

Three reasons to read it carefully

Three things to keep in mind before over-reading this study. First, the sample was 52 astronauts — a small, highly selected, extremely healthy population, followed with remarkable precision but not representative of the general public. Second, the study measured fermentation byproducts in blood, not bowel movements directly; the constipation connection is consistent with the data but partly inferential. Third, diet in orbit is controlled but not identical to free-living eating, and the researchers themselves note that changes could not be fully explained by diet — which is precisely why transit time is implicated.

None of this weakens the practical takeaway. If anything, the space setting is a cleaner experiment than most nutrition studies: it isolated the effect of slowed transit on gut bacterial behavior, in a way no Earth-based trial could.

What we would do with this information

If you are currently constipated, the study's implicit advice matches our green-rated food lists almost exactly. Build each day around two or three high-fiber anchors — oatmeal at breakfast, lentils or beans at lunch, a pear or kiwifruit as a snack — drink water steadily, and move your body daily. If you are caring for someone bedridden or recovering from surgery, where transit is slowed by circumstance rather than gravity, the same logic applies with extra emphasis on gentle, well-cooked fiber sources and careful hydration.

And if you are simply fascinated by the idea that constipation research now involves astronauts, blood metabolites and Mars mission planning: same. It is a good reminder that the most ordinary digestive complaint in the world is being studied with some of the most extraordinary tools available.

Sources

• Nature Communications (2026): gut microbiome and protein fermentation in 52 astronauts on ISS missions — University of Copenhagen / NASA collaboration.
• HealthDay, "Constipation In Space: A New Study Provides Answers," Aug. 18, 2026 (reporting by Dennis Thompson; quotes from G. La Barbera, H. Roager, L. O. Dragsted).
• NIDDK: Eating, Diet & Nutrition for Constipation.

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This article is informational only, based on published research and USDA data. It is maintained by our data editorial team — not medical professionals. Nothing here is medical advice.