Don’t be fooled by space operas that show humans spanning vast galaxies or by social media chatter that we can quickly and effortlessly reach and colonize Mars. Space is vast and terrestrial animals like humans have not evolved to deal with the physiological and psychological hardships of prolonged spaceflight.

Prolonged travel in deep space comes with the hazard of cosmic and solar radiation, which will be fatal, unless we successfully shield spaceships from it. The Earth’s magnetosphere protects us from these harmful high-energy particles while we are on the planet. It also protects astronauts who are on the International Space Station (ISS).

Don’t be fooled by space operas that show humans spanning vast galaxies or by social media chatter that we can quickly and effortlessly reach and colonize Mars. Space is vast and terrestrial animals like humans have not evolved to deal with the physiological and psychological hardships of prolonged spaceflight.

Prolonged travel in deep space comes with the hazard of cosmic and solar radiation, which will be fatal, unless we successfully shield spaceships from it. The Earth’s magnetosphere protects us from these harmful high-energy particles while we are on the planet. It also protects astronauts who are on the International Space Station (ISS).

But radiation isn’t the only worry. Adapting to weightlessness is another challenge of spaceflight. The conditions of microgravity during prolonged stay at the ISS can have detrimental effects on health. Some of the well-known physical issues that can occur are loss of bone density, muscle tissue loss, improper heart and vessel function, and alterations to the microbiome – the vast collection of microorganisms on and inside our bodies that are necessary for good health.

Astronaut Scott Kelly, who spent 340 consecutive days in space, gained about two inches of height because of a lack of compression on his spinal column while in space. Upon return to Earth, Kelly experienced many issues with readjusting to gravity which he recounted in his eminently readable book, Endurance: A Year in Space, a Lifetime of Discovery.

It is estimated that astronauts lose around 1-1.5% of bone density for every month spent in space compared to being on Earth. And the loss of bone density in osteoporosis is also a significant problem (although on a less rapid timescale) in elderly adults who spend their entire lives tied to the planet. Clearly, this is a problem worth studying.

NASA has been sending laboratory mice up to the ISS since 2014 to test the effects of microgravity on various aspects of mammalian health, including bone density loss. And now, research from the first “mice-astronauts” that returned to Earth alive after 4.5 weeks and 9 weeks has been published in the scientific journal, Cell Reports.

Material is added to existing bones throughout life. External factors like diet – be sure to get calcium and sunlight to make vitamin D – certainly play a role. There are also internal factors. Gut microbes are thought to be involved through their interactions with hormones and the immune system. These beneficial gut microbes play a positive role by making small chemicals (called short-chain fatty acids) which are utilised by the body. But the picture is far from clear.

In this new study, researchers tested metabolites in the blood and the gut microbe composition of mice that returned from space with those from mice that never left. They found that the gut microbes of mice that spent time at the ISS were more diverse than the ones on the ground. This parallels what has been seen in humans that returned from the ISS in earlier studies too. Incidentally, in both mice and humans, the changes to gut microbes seem to reverse shortly after returning to Earth.

Perhaps the most fascinating observation of the study is that some microbes that belong to Lactobacillus and Dorea were enhanced in mice that returned from the ISS. These gut microbes also seem to make metabolites that are associated with greater bone mineral density.

At first, this result seems a bit counterintuitive. Why would mice that might have lower bone density while they’re in space have more of the microbes that might help actually increase bone density?

The answer, according to the lead author, Joseph Bedree is, because the body is trying to make up for the harsh conditions of microgravity. He notes, “When someone’s in microgravity and experiencing bone loss, it would make sense that their body would try to compensate and that the biological systems within would be doing that as well, but we need to do more mechanistic studies to truly validate these hypotheses.”

Future studies are indeed needed to firmly establish the roles of these microbes in increasing bone density (both in space and on the ground). These microbes can be tested as probiotics to see if they change gut microbiome composition and make a difference in bone density. And if the answer is affirmative on both counts, a logical next step would be to see if the research on mice extends to humans.

The research adds to the growing body of work showing gut microbes are involved in both normal and diseased states. Around twenty years ago, the effects of the gut microbiome on metabolism, immune function, neurological disorders, heart health, and cancer progression were unknown. These days, groundbreaking research showing the effects of microbes and their products on health is published every week.

Anirban Mahapatra is a scientist by training and the author of a book on COVID-19

The views expressed are personal