How spaceflight affects astronauts’ gut flora

New research on intestinal flora is being carried out in an unusual location – in outer space. NASA is currently collecting samples from astronauts at the international space station, ISS, to find out how the extreme conditions affect their microbiomes. 


NASA taking samples of microbiomes

During their time in space, astronauts are exposed to a unique situation that entails many different stress factors – everything from G-forces, radiation, and microgravity to high noise levels and broken sleep patterns. A space expedition also involves mental stress, caused by isolation, homesickness and worry. What’s more, the dietary change that the astronauts experience can also affect their well-being.

Let’s look more closely at this particular example of the astronauts’ unusual situation during their sojourn in space: their diet. Specially adapted for consumption in space, the astronauts’ diet nevertheless has to meet certain criteria; food must be appetising, easy to prepare, and should have a nutritional content in line with the crew’s recommended intake. What can be offered, however, is very limited as the food has to have a long shelf life, and this often means freeze-drying or preservation. As a result, there is not much in terms of fresh fruit and vegetables.

All the same, food is a vital element in the planning of space travel as it affects the astronauts’ health, both socially and psychologically. It’s not unusual for astronauts to eat too little during their stay in space and to lose weight. It is probable that this may be accounted for by the food not ‘settling’ in the stomach in the same way in space as it would on earth. Astronauts can then experience a feeling of fullness despite not having eaten enough. That’s why it is so important that the food the astronauts eat is not only nutritious but also tasty enough to excite the appetite and avoid illnesses and psychological stress.

Both stress and diet have been shown to cause changes to intestinal flora, which means that astronauts and their unusual environment are of interest to scientists studying in this field.

In order to study the microbiome of the astronauts, NASA is collecting samples of their saliva, excrement and skin, and taking nasal swabs before, during, and after their stay in space. Samples taken onboard the space station are frozen and sent with the first available spacecraft to the Johnson Space Center in Houston, Texas. All samples are taken in accordance with NASA Spaceflight Standard Measures to ensure that they are consistent and can contribute to future research on the effects of spending time in space.


Space expeditions affect intestinal flora

A study published in 2019 researched how the microbiome of astronauts changes during extended periods of space travel. By analysing the composition of microorganisms before lift-off (when the astronauts were in their peak physical condition) as well as later on during their time in space, it was possible to study the effects on the astronauts’ health.

Nine astronauts took part in the study and samples were taken on multiple occasions. As well as gathering information on their microbiomes, the astronauts’ immune systems and stress levels were also measured by analysing saliva and blood samples. The results showed that their microbiomes had changed during the time in space. The amount of a particular bacteria that is linked to chronic intestinal inflammation in patients with IBD, Parasutterella, had increased,. At the same time, reductions in Fusicatenibacter, Pseudobutyrivibrio and Akkermansia were observed, all three of which are bacteria with anti-inflammatory properties. The levels of stress hormones had also changed. Analysis of the intestinal flora also revealed that the composition of the astronauts’ bacteria had become more like each other’s, even if they hadn’t spent time together at the space station. The researchers think this may be owing to the astronauts eating a more similar diet in space than they would on earth. Further tests on more astronauts are needed in order to make generalisations based on the study.

In another study, published in 1976, there were indications that the levels of harmful bacteria increased noticeably among astronauts who had been under stressful circumstances in space. Another study showed that the amounts of the friendly bacteria, Bifidobacterium and Lactobacilli decreased, while the amounts of harmful bacteria, E. coli and Enterobacter increased during the preparations for space travel. This increase was particularly noticeable during the period immediately after the spacecraft had left Earth. Another interesting observation was that the changes to occur to the stomach flora’s composition during space travel appeared to endure over several weeks – in some cases to several months or even years.


Stress factors on earth affect the intestinal flora in a similar way

Studies of groups here on Earth have also shown that mental and physical stress affects intestinal flora. One example is a study carried out on 23 students over two separate periods: in the beginning of a term when stress levels were low, and during the first week of exams when stress levels were much higher. The result showed that the amount of beneficial bacteria in the intestinal flora was reduced during the exam period. Beneficial bacteria of the intestinal flora have a positive effect on our health, and reduction of such bacteria may lead to digestive changes, weakening of the immune system and an increase in the amount of harmful bacteria. The study also showed that the intestinal flora had not yet recovered after five days.


Lessons from the studies

Despite astronauts being exposed to extreme and extraordinary conditions, the research team hoped that the insights from the study would be able to generate usable information for those of us on Earth. One relevant factor, for example, is the link between stress and the intestinal flora. as stress is something which can affect almost anyone. Professor Stig Bengmark, in relation to previous studies on astronauts, has commented that ‘We’ve been able to see for a long time, both clinically and through experiments, that mental stress can cause a lot of problems for our digestive health’. The knowledge of how a limited diet influences the intestinal flora could also be important in other contexts than in space.

NASA’s current data collection is expected to continue up to and including March 2023 and is thus set to provide more exciting information about intestinal flora in the future.



Bengmark, Stig. (2017). Inflammation: From Molecular and Cellular Mechanisms to the Clinic, First Edition. John Wiley & Sons, Ltd.

Gilles, C. R. (2022) Spaceflight Standard Measures. NASA.

Gonzales-Juarbe, N., Ott, M., Pierson, D. (2019). Astronaut Microbiome. JCVI.

Knowles, S.R., Nelson, E.A., and Palombo, E.A. (2008) Investigating the role of perceived stress on bacterial flora activity and salivary cortisol secretion: a possible mechanism underlying susceptibility to illness. Biol. Psychol., 77, 132–137.

Lorenzi, H., Mark Ott, C., Pierson, D. L. (2021). Study of the Impact of Long-Term Space Travel on the Astronauts’ Microbiome.

Tesei, D., Jewczynko, A. Lynch, A. M., Urbaniak, C. (2022). Understanding the Complexities and Changes of the Astronaut Microbiome for Successful Long-Duration Space Missions. Life.

Voorhies, A.A., Mark Ott, C., Mehta, S. et al. Study of the impact of long-duration space missions at the International Space Station on the astronaut microbiome. Sci Rep 9, 9911 (2019).


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