Scientists finally figured out why astronauts suffer from constipation

As soon as humans began staying in space for extended periods, it became clear that having a bowel movement there was harder than on Earth. And it's not just that using a space toilet is quite complex. For most astronauts, such changes in gastrointestinal tract function are merely an inconvenience, but some have to take laxatives to deal with the problem. These medications are regularly included in the medical kits of most space missions.

According to NASA, constipation is just one of several stomach problems astronauts struggle with; others include bloating, indigestion, and heartburn. As with most other health problems in space, researchers have long suspected that microgravity is the main cause.

However, proving this hypothesis or understanding other consequences that chronic constipation might have on astronauts' overall health has been very difficult.

During a new study, supported by NASA, whose findings are highlighted by Live Science, scientists analyzed over 400 blood samples from 52 astronauts who spent more than two months each on the International Space Station. This refers to missions between 2006 and 2018.

In the blood samples, the team looked for specific metabolites—byproducts of metabolic processes that occur during the breakdown of food, medicines, and tissues in the body. They identified 40 such compounds, but the most indicative proved to be those produced by gut bacteria that help break down proteins in the intestines.

Protein fermentation occurs when gut bacteria begin breaking down proteins instead of their more usual complex carbohydrates, such as fiber. Such a switch can also happen on Earth — due to dietary peculiarities or digestive problems. However, in space, scientists believe the reason is different.

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Due to the absence of gravity, food moves more slowly through the intestines. As a result, bacteria have time to break down available fiber and then switch to proteins. This same slowed movement of food through the intestines may also explain constipation in astronauts.

Researchers also found that changes in astronauts' diets could influence constipation. During space missions, for example, the consumption of caffeine, fish, and fats changes. But, according to researchers, dietary differences can explain less than a third of the observed changes in metabolites. Therefore, they consider the conditions of being in space to be the main factor.

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Why this may be important for a flight to Mars

Protein fermentation in itself does not necessarily mean that an astronaut's health is at risk. However, during this process, substances such as ammonia and hydrogen sulfide are formed.

Some protein fermentation products have been linked in previous studies to adverse health effects, including kidney damage, possible effects on mood, and the ability to concentrate. Regarding gut health, studies link protein breakdown products to the development of inflammation and the weakening of its mucous membrane.

During relatively short space missions, the amount of such substances is unlikely to pose a significant danger. But researchers point out that the situation may be different for flights lasting many months.

That is why the issue becomes relevant in the context of future expeditions to Mars and the long-term presence of people outside Earth.

However, the authors of the study do not claim that prolonged space flight will necessarily lead to a dangerous concentration of such substances. This is currently a hypothesis that needs to be tested. The study covered people staying on the ISS, and the consequences of significantly longer missions or life in partial gravity conditions, such as on the Moon, remain insufficiently studied.

In the future, the problem might be mitigated with a diet with more fiber, probiotics, or agents that stimulate intestinal peristalsis. But the effectiveness of such measures for long-duration space missions still needs to be investigated.

The authors of the work note that the results may also be useful on Earth. Similar problems with peristalsis and constipation arise, for example, in people who are forced to observe prolonged bed rest.

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