One of the biggest problems facing astronauts on a future trip to Mars may have nothing to do with rockets or radiation. It may begin the moment they stop going to the bathroom normally. Scientists studying 52 astronauts have found that microgravity can slow the gut and change what its bacteria do with food. For a few months, that may mean constipation. On a much longer journey, researchers are asking whether the same change could create a far more serious health problem.
The Problem Starts When Food Stops Moving Normally
Astronauts have long reported digestive problems in space, including constipation, bloating, indigestion and acid reflux. Constipation can begin within the first week of a mission and, for some astronauts, persist until they return to Earth; laxatives are routinely carried aboard space missions because of it.
Scientists have suspected microgravity is responsible, but proving exactly what changes inside the gut has been difficult. The new study, published in Nature Communications on June 29, examined more than 400 blood samples from 52 astronauts who spent more than two months at a time aboard the International Space Station between 2006 and 2018.
Instead of trying to measure every digestive movement directly, researchers looked for metabolites—chemical byproducts created as the body and its microbes process food. Among 40 metabolites identified, the researchers found clues that pointed toward a change in how the gut microbiome was using nutrients.
Before worrying about the hidden health risks of constipation in space, there is an even more basic question: how does an astronaut use a toilet when gravity is gone? Watch the video below to see how astronauts stay in position and how suction and airflow keep an ordinary bathroom trip from becoming a floating disaster.
In Microgravity, the Gut Bacteria Change Their Menu
The key finding was an increase in protein fermentation. Under normal conditions, gut bacteria preferentially ferment complex carbohydrates such as dietary fiber, but the astronauts’ blood chemistry suggested that in space, the bacteria were increasingly turning to protein instead.
Researchers believe the chain may begin with the lack of gravity itself. Without normal gravity, food appears to move more slowly through the intestines, giving bacteria more time to break down fiber before shifting toward protein as another source of nutrients. That helps explain why constipation becomes so common in orbit.
The important point is that this change was not just a matter of diet. Astronauts’ food choices—including differences in caffeine, fish and fat intake—may have influenced the gut, but those dietary factors accounted for less than one-third of the observed metabolite changes. The researchers therefore identified microgravity as the primary driver of the effect.
The Bigger Concern Is What the Gut Produces
This is where an embarrassing bathroom problem becomes a spaceflight health question. When gut bacteria ferment more protein, they generate different metabolic byproducts, including compounds such as ammonia and hydrogen sulfide.
The researchers are concerned because products of protein fermentation have been associated with negative health effects, including possible kidney damage, effects on mood and concentration, and changes to intestinal health. Other research has linked these compounds with inflammation and weakening of the gut lining.
That does not mean the study showed astronauts are being harmed in these ways. The researchers specifically note that over short periods, the potentially harmful metabolites are likely to pose limited health risks; the concern is what could happen if those compounds continue to accumulate during much longer missions.
Mars Is Where the Problem Gets Serious
A stay on the International Space Station is one thing. A human journey to Mars would involve a much longer period away from Earth, making every small physiological change more important. The current study covered astronauts during relatively brief periods on the ISS, so researchers cannot yet say what months or years of altered gut function would ultimately do.
That uncertainty is exactly why scientists are already thinking about countermeasures. The study authors suggest that future missions could use more fiber-rich diets, probiotic supplements or methods to stimulate peristalsis, the wave-like muscle contractions that move food and waste through the digestive tract.
The goal would be to keep the gut from shifting so strongly toward protein fermentation. In other words, future Mars missions may need to treat digestion as part of the spacecraft’s life-support problem—not merely a matter of astronaut comfort.
What Astronaut Poop Could Teach Us on Earth
There is another reason the discovery matters. The same biological process may help scientists understand what happens to people on Earth whose intestines move poorly because they are immobile for long periods, including patients confined to bed.
That gives the research a useful connection to medicine beyond space exploration. Studying astronauts provides an unusually controlled way to observe what happens when normal movement is disrupted, and the resulting changes in gut bacteria and metabolites may offer clues for people experiencing similar gastrointestinal problems on Earth.
Still, scientists are not finished with the puzzle. The study provides strong evidence linking spaceflight, slower intestinal movement and increased protein fermentation, but the exact long-term health consequences remain uncertain and will require further research.
The strange reality of spaceflight is that a problem as ordinary as constipation may reveal a much deeper biological challenge. Researchers studying 52 astronauts found that microgravity appears to slow the movement of food through the intestine, changing gut bacteria from fermenting mostly fiber to fermenting more protein. That shift produces metabolites that could become harmful during very long missions, although the study has not shown that this will happen to astronauts. For a future trip to Mars, even digestion may become a mission-critical problem.
