European Space Agency (ESA) astronaut Samantha Cristoforetti runs on a treadmill. The space station has low gravity, causing the astronaut's bone mass to decrease by 1.5% each month. To prevent this, she ties herself with elastic bands and runs regularly./Courtesy of ESA/NASA

A time is not far off when civilians can easily travel to space. Jared Isaacman, who has been nominated as the new head of the National Aeronautics and Space Administration (NASA), flew to space on a SpaceX spacecraft last September and became the first civilian to conduct a spacewalk.

However, space travel is not just a fun and enjoyable experience. The outer space beyond Earth has almost no gravity, which causes the muscles and bones that support the body to weaken. The human body system, accustomed to Earth's gravity, undergoes gradual changes in space. The microgravity of space has a particularly significant impact on bones.

An international joint research team led by Professor Matthias Walle from the University of Calgary in Canada conducted an in-depth investigation into the effects of space stay on bones, focusing on 17 astronauts who stayed at the International Space Station (ISS). The results of the study were published in the academic journal Science Advances on the 21st.

It is well known that microgravity in space reduces bone density. Living in an environment with almost no gravity causes calcium to leach from bones. Astronauts who stayed at the former Soviet Union's Mir space station reportedly lost an average of 1 to 2% of their bone mass per month.

The research team further investigated how much bone recovers after returning to Earth. Bones are constantly remodeled through the processes of bone resorption and bone formation in response to gravity. This study examined how the rates of bone resorption and bone formation change in space, as well as any differences in these rates after returning to Earth.

The 17 astronauts studied by the research team spent a short duration of 4 months to a maximum of 7 months in space. During this time, the bone resorption rate was approximately three times higher than the bone formation rate. This indicates that bones were disappearing faster than they were being remade.

The astronaut's tibia viewed through high-resolution peripheral quantitative computed tomography (HR-pQCT). Purple indicates bone resorption areas, yellow indicates bone formation areas, and green indicates areas that have recovered after returning to Earth. It can be seen that while staying in space, the bone resorption areas are widely distributed, and upon returning to Earth, the bone formation areas have somewhat increased./Courtesy of Matthias Walle

The research team continued to track the astronauts even after they returned to Earth. They investigated how much of the bone lost in space was reformed in the year following their return to Earth. This comparison was made using high-resolution peripheral quantitative computed tomography (HR-pQCT) to assess the areas of bone resorption and formation. The results showed that only 35% of the lost bone was reformed.

In particular, the first six months were found to be the golden time for bone reformation. Of the bones that underwent resorption during space stay, 31.8% were reformed in the first six months after returning to Earth. Only 2.7% were reformed in the subsequent six months. Professor Walle noted, "This indicates that there is a limited timeframe in which factors for human bone reformation can be activated," and added, "This study first confirmed that there may be an expiration period for the recovery process of bones."

The research team stated that as new findings on bone reformation accumulate, it could help find ways to address health threats that arise when astronauts stay in space for extended periods.

Reference materials

Science Advances (2024), DOI: https://doi.org/10.1126/sciadv.adq3632

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