Astronauts Suffer Significant, Permanent Bone Density Loss in Space

For all the time humanity has existed, every single person has spent their lives under normal Earth gravity — 9.806 meters per second squared. And then, a few decades ago human beings started spending days, then weeks, and then months in freefall. We’re only now beginning to understand what living without gravity does to the body, but it’s not good. Kinesiologists Leigh Gabel and Steven Boyd from the University of Calgary have published a new study that examines the bones of astronauts, finding that extended time in space causes degradation in bones that never fully reverses. If our future is in space, this could be a major problem. 

The pair does not mince words in the study, which was published in the journal Scientific Reports. They call the effects of weightlessness on astronaut skeletons “profound.” Even after a year back on Earth, bone density does not fully recover. Essentially, being in space for even a few months causes your bones to age at an increased rate — as much as 10 years per flight. 

This study focused on people who had been on long-duration space missions, which means three months or longer, in this case. In partnership with NASA’s Johnson Space Center in Houston, Gabel and Boyd examined 17 astronauts (14 men and three women) both before and after a 6-12 month excursion in space. 

Only one of the astronauts in the study saw bone density return to normal after an extended time back on Earth. For the others, most of the density loss was concentrated in weight-bearing bones of the lower extremities, but the arms did return to normal after about a year. The team analyzed the mineral content and density of bones, allowing them to determine how the “failure load” had changed. On average, the failure load of the astronauts’ tibias before spaceflight was 10,579 newtons, but that dropped to 495 newtons after the mission. They partially recovered as time went on, but after a year were still down an average of 152 newtons. 

The researchers also note that those who were in space longer suffered greater drops in bone strength. Astronauts who were on the International Space Station for longer than six months recovered less of their bone density after returning. That suggests that long-term missions, like the ones planned for the Artemis Program, could have serious impacts on astronaut health down the road. It also raises more questions about how humans would fare long-term in an environment where there is less gravity than Earth, for example on the Moon or Mars. 

It may be possible to mitigate the effects of weightlessness to some degree. Astronauts undertake a rigorous routine of exercise under simulated gravity during missions to prevent muscle wasting. The researchers used data on exercise routines in the study, and they say added focus on the legs (deadlifts, for example) could reduce density loss. However, we may be years from fully understanding how the body responds to microgravity.

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