Bone Density: Space has devastating effects


Space affects bone density, and astronauts lose decades of this tissue in the absence of gravity. Many do not even recover it after a year upon returning to Earth. A new study (ref.), published last Thursday in Nature, warns international space agencies of the possible risks that astronauts would face in future long-term missions.

This is the second study that highlights serious health risks for astronauts. Earlier this year, researchers at the University of Ottawa found serious effects on the cardiovascular system. The absence of gravity induces a 54% reduction in red blood cells.

The bone density study

One year after returning from long space flights, most astronauts show incomplete recovery of bone density, strength, and trabecular thickness of the tibia. In particular, incomplete recovery of bone density and strength is more pronounced in astronauts returning from space engaged in longer missions. The duration of the mission for skeletal health is increasingly relevant as longer missions on the ISS become more frequent and space exploration missions are planned for the near future.

To discover how astronauts recover once they return to Earth, the study scanned the wrists and ankles of 17 astronauts before, during, and after their stay on the International Space Station (ISS). The bone density lost by astronauts was equivalent to what they would have lost over several decades on Earth, said co-author Steven Boyd of the Canadian University of Calgary. Researchers found that the tibial density of 9 astronauts had not fully recovered. After a year on Earth, they were still missing about a decade of bone mass. The astronauts who undertook the longest missions, from four to seven months on the ISS, were the slowest to recover. “The more time you spend in space, the more bones you lose” Steven Boyd said clearly in an interview.

This research focused on astronauts on missions for months on the ISS, but what would happen to those engaged for years in future missions to Mars? “Will it continue to worsen over time or not? We still don’t know. It’s possible that a steady state will be reached after a while, or that bone loss will be progressive. But I can’t imagine the phenomenon going on indefinitely until there’s nothing left” Steven Boyd reiterated.

Pointing finger at the absence of gravity.

A previous modeling study from 2020 predicted that during a three-year space flight to Mars, 33% of astronauts would be at risk of osteoporosis. Steven Boyd stated that some confirmations on this matter could come from ongoing analyses of astronauts who have spent at least one year aboard the ISS. Guillemette Gauquelin-Koch, head of medical research at the French space agency CNES, said that the absence of gravity experienced in space is “the most drastic physical inactivity there is. Even with two hours of exercise a day, it’s like being bedridden for the remaining 22 hours“. “It won’t be easy for a future crew to set foot on Martian soil upon their arrival; the journey would be very disabling” she reiterated.

The astronauts' gym on the ISS. Exercises help mitigate the effects of gravity loss on bone and muscle tissue.
The astronauts’ gym on the ISS. Exercises help mitigate the effects of gravity loss on bone and muscle tissue. Credit: NASA

The study also showed how spaceflight alters the structure of bones themselves. Steven Boyd compared the skeletal system to the Eiffel Tower. It would be as if some of the metal beams supporting the structure were lost. “And when we return to Earth, we densify what remains of the bone, but we don’t actually create new structures” he said. Some exercises performed in space can mitigate the problem and are more effective than others in maintaining bone density. The study’s analysis suggests that in the future, heavier and more lower-body-focused exercises should be used.

The conclusions

It is almost certain that humans are not biologically “built” for long-duration space missions. Our biology has evolved over thousands of years under the constant presence and active influence of the force of gravity. The functioning of many tissues is closely related to the presence of this force.

If we want to embark on the path to becoming a multi-planetary species, it is imperative to identify preventive measures and improve countermeasures in flight. From a biological standpoint, identifying factors that influence skeletal recovery will provide us with information on optimizing strategies to mitigate loss and improve recovery of bone tissue. But steps must also be taken in the technological field, seeking to reproduce in-flight the force that has shaped our and other species on the planet: gravity.”

Stefano Gallotta
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