The astronauts' blood showed an increase in substances that indicate protein fermentation by gut bacteria. The researchers suspect that the slower transit of food through the gut in microgravity may explain the phenomenon—but transit time and constipation themselves were not measured in the study.
Constipation is a common complaint among Astronauts, but the reasons for it are not fully understood. A study conducted in collaboration with University of Copenhagen וNASA Found changes in the blood of astronauts indicating increased protein fermentation by Intestinal bacteria During the stay in space.
The researchers speculate that Microgravity Slows down the passage of food inDigestive SystemWhen intestinal contents remain in the gut longer, the bacteria consume a greater proportion of the carbohydrates and fiber available to them and switch to using amino acids derived from proteins. However, the study did not directly measure intestinal transit time, bacterial population composition, or constipation frequency, so the mechanism remains a possible explanation.
The study was published in the journal Nature Communications..
488 blood samples from missions lasting up to nine months
The researchers analyzed 488 plasma samples collected from 52 astronauts before their flight, during their stay inThe International Space Station and after their return to Earth. The sample included 41 men and 11 women, whose average age was 48. The missions took place between 2006 and 2018 and lasted between two and nine months.
Using liquid chromatography and mass spectrometry, the researchers examined hundreds of small molecules in the blood. This field, known as Metabolomics, allows you to identify changes in metabolism and the effect of nutrition and intestinal bacteria on the body.
About 40 blood compounds changed as a result of the space flight. The changes appeared shortly after launch, persisted during the stay on the space station, and subsided within a few days of landing. The researchers did not detect a trend of the changes becoming more pronounced as the mission lasted longer.
Chemical signature of protein breakdown in the intestine
During the flight, levels of several metabolites formed by the breakdown of amino acids by bacteria in the large intestine increased. Among the substances found were p-cresol sulfate, phenylacetylglutamine, and 4-ethylphenol sulfate. Other substances related to protein fermentation, including indoxyl sulfate and phenol sulfate, also showed an upward trend.
The bacteria first produce compounds from the breakdown of proteins. After they are absorbed into the bloodstream and reach the liver, the body changes them and produces the metabolites measured in the plasma.
At the same time, levels of tyrosine, one of the amino acids used as a starting material for part of the process, decreased. There was also an increase in fimalic acid, a compound that in previous studies has been linked to slower transit of intestinal contents.
According to the researchers, the combination of changes fits the expected pattern as intestinal transit time lengthens and bacteria switch from carbohydrate fermentation to increased protein fermentation.
Why does slow transit time change bacterial activity?
Gut bacteria generally prefer to use carbohydrates and dietary fiber as a source of energy. As the contents of the gut move slowly, the carbohydrates available to the bacteria become less available. At this point, they switch to using a larger amount of proteins and amino acids.
Protein fermentation is not an unusual process and occurs in people on Earth as well. However, its prolonged amplification can lead to the production of compounds such as ammonia and hydrogen sulfide, along with the metabolites found in the astronauts' blood.
Some of these compounds have been linked to health harms in other studies. Phenylacetylglutamine, for example, has been linked to cellular aging and possible damage to mitochondrial function. High levels of indoxyl sulfate and p-cresol sulfate have been linked to increased risk in patients whose kidney function is already compromised.
Studies in mice have also suggested a possible link between some of the metabolites and changes in brain activity and anxiety-like behavior. However, the current study did not show that the metabolites caused health damage, cognitive impairment, or changes in the astronauts' mood.
Not just microgravity: food in space has also changed
The researchers also found signs of dietary changes during the missions. Levels of caffeine and one of its breakdown products decreased, likely due to lower coffee consumption or differences between coffee on the space station and coffee brewed on Earth.
There were also signs of a decrease in the consumption of fish and certain oils. Overall, less than a third of the altered metabolites were linked to diet. The researchers therefore believe that differences in diet alone do not explain the increase in protein fermentation products.
The study does not have a parallel control group that remained on Earth under similar isolation, nutrition, and activity conditions. Therefore, it is difficult to completely separate the effects of microgravity from changes in diet, stress, sleep, exercise, and the unique routine of life on the space station.
Fiber and prebiotics in long missions
The researchers suggest looking into increasing the amount of fiber and slowly fermentable carbohydrates in astronauts' diets. These substances reach the colon and serve as a food source for bacteria, thus potentially reducing their need to break down proteins.
Other options include prebiotic supplements or treatments that encourage bowel movements. However, the study did not examine dietary or therapeutic interventions, so it is not yet known which measures would be effective and safe in space.
The question becomes especially important in preparation for long missions to the Moon and Mars. The space station can return a sick astronaut to Earth or send new supplies. On a journey to Mars, which will take many months in each direction, it will be necessary to prevent cumulative changes in the digestive system and metabolism in advance.
The researchers note that the findings may also be relevant to people who are bedridden. In them, lack of movement and constipation may also prolong intestinal transit time and increase protein fermentation, but this possibility was not examined in the current study.
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Questions and Answers
What did the researchers find in the astronauts' blood? An increase in several metabolites produced when gut bacteria break down proteins and amino acids was found. The changes appeared shortly after launch and subsided after return to Earth.
Has the study proven that microgravity causes constipation? No. The researchers did not directly measure intestinal transit time or constipation frequency. They suggest that slowing transit time in microgravity is a possible explanation for the metabolite pattern found.
Is protein fermentation dangerous? It is a normal part of gut bacterial activity, but its prolonged enhancement may increase the production of compounds that have been linked to negative health effects in other studies. The study did not show any harm to the astronauts.
How can this phenomenon be reduced in long-term tasks? The researchers suggest testing fiber supplementation, slowly fermentable carbohydrates, and prebiotics, as well as measures to shorten intestinal transit time. The effectiveness of these measures in space still requires research.
Original article: Longitudinal metabolomics profiles reveal increased gut microbial protein fermentation during spaceflight, Nature Communications.
For the scientific article: Opening the scientific article
