Why Astronauts Get Constipated in Space: New Study Reveals Cause

Astronauts frequently experience constipation during long-duration spaceflight aboard the International Space Station. According to a study published in Nature Communications by researchers from the University of Copenhagen and NASA, blood biomarker analysis of 52 astronauts reveals that microgravity may slow intestinal transit, prompting gut bacteria to ferment proteins at elevated rates.

Astronauts have complained about constipation for as long as people have flown to space. While researchers have long suspected that living without gravity changes how quickly food moves through the gut, the precise physiological mechanism remained unclear. By evaluating frozen blood samples collected across multiple missions over several years, investigators identified metabolic signatures that link weightlessness to altered microbial processing within the human colon.

Metabolic Shifts and Gut Microbiome Alterations in Microgravity

The human gastrointestinal tract relies on gravity and normal intestinal muscle movements to move digesting food forward. When astronauts enter the microgravity environment of the International Space Station, the lack of gravitational pull may cause food to move more slowly through the intestine. According to Henrik Roager, an associate professor at the University of Copenhagen’s Department of Nutrition, Exercise and Sports, this deceleration changes how microbial communities interact with ingested nutrients.

As food travels more slowly through the intestine, gut bacteria may exhaust available dietary fiber before the next meal arrives. Once fermentable carbohydrates are depleted, these microbes can rely more heavily on metabolizing available proteins. Giorgia La Barbera, a joint first author of the study and an associate professor at the same department, noted that blood samples displayed biochemical indicators of intensified protein fermentation within weeks of crew members arriving in space, persisting until their return to Earth.

This metabolic pathway generates molecules that pass into the bloodstream. While these microbial metabolites circulate throughout the body, researchers note that some substances produced when gut bacteria ferment protein have been associated with negative health effects, including kidney problems and potential influence on mood and the ability to concentrate.

In Plain English: The Clinical Takeaway

• Intestinal Slowdown: Microgravity may remove the natural gravitational assist that helps human digestion, causing meals to move more slowly in the gut.
• Microbial Adaptation: When available fiber is used up due to slow transit, gut bacteria can switch to breaking down proteins instead.
• Systemic Reach: The chemical byproducts of this protein breakdown enter the bloodstream, offering researchers a blood biomarker to track space digestion issues.

Translational Implications for Terrestrial Medicine

Understanding these spaceflight-induced shifts offers relevance for patient populations on Earth. Bedridden patients often experience slow digestion and constipation because of prolonged inactivity, and similar changes in gut bacteria may occur in these patients.

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Researchers suggest that interventions designed to optimize gut health could benefit both space explorers and terrestrial patients. Potential therapeutic avenues include increasing dietary fiber, using prebiotics to encourage beneficial gut bacteria, or treatments that help food move through the intestines more quickly.

As space agencies prepare for longer missions to the Moon and Mars, mitigating gastrointestinal complications becomes important, as relatively small health problems might become more serious over time during months or years away from Earth.

Summary of Gastrointestinal Research Findings in Spaceflight

Parameter	Terrestrial Baseline	Spaceflight Observation (Microgravity)
Intestinal Transit Speed	Assisted by gravity and normal intestinal muscle movements	May move more slowly
Primary Microbial Fuel	Dietary fiber fermentation	Increased protein fermentation
Biomarker Detection Method	Standard metabolic profiling	Metabolite analysis of astronaut blood
Cohort Sample Size	Varies by clinical trial	52 astronauts across multiple missions

Funding Transparency and Methodological Rigor

Conducting human physiological research in Earth orbit presents logistical hurdles, particularly given that blood is scarce on the space station. To achieve their results, the research team analyzed biological samples previously collected and frozen for three earlier studies run on different missions across several years.

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The investigation was led by academic researchers at the University of Copenhagen in collaboration with NASA.

Contraindications & When to Consult a Doctor

References

• La Barbera, G., Stanstrup, J., Roager, H., et al. (2026). Altered microbial protein fermentation indicated by blood metabolites in astronauts during long-duration spaceflight. Nature Communications.