Space travel hurts astronauts' bone density, finds a new study

Missions that last longer than six months are especially hard to recover from.
Loukia Papadopoulos
4 stages osteoporosis.CreVis2/iStock

Floating around in space may seem fun, but it actually leads to a significant loss of bone density which is not recovered even a year after their return to Earth, according to a new study by the University of Calgary. 

Bone loss in space

“Bone loss happens in humans — as we age, get injured, or any scenario where we can’t move the body, we lose bone,” said Dr. Leigh Gabel, Ph.D., assistant professor in kinesiology, and lead author of the study.

“Understanding what happens to astronauts and how they recover is incredibly rare. It lets us look at the processes happening in the body in such a short time frame. We would have to follow someone for decades on Earth to see the same amount of bone loss."

Gabel and his team traveled to the Johnson Space Center in Houston, Texas to measure the wrists and ankles of the astronauts before they left for space, on their return to Earth, and then at six and 12 months post-return.

"We found that weight-bearing bones only partially recovered in most astronauts one year after spaceflight," said Gabel. "This suggests the permanent bone loss due to spaceflight is about the same as a decade worth of age-related bone loss on Earth."

This loss occurs as a result of bones that would normally be weight-bearing on Earth not having to carry weight in space's microgravity environment.

“We’ve seen astronauts who had trouble walking due to weakness and lack of balance after returning from spaceflight to others who cheerfully rode their bikes on the Johnson Space Center campus to meet us for a study visit. There is quite a variety of responses among astronauts when they return to Earth," said Dr. Steven Boyd, Ph.D., director of the McCaig Institute for Bone and Joint Health, professor in the Cumming School of Medicine, and leader of the new study.

A testimony from an astronaut

Former UCalgary chancellor and astronaut Dr. Robert Thirsk testified to experiencing this phenomenon when returning from space.

“Just as the body must adapt to spaceflight at the start of a mission, it must also readapt back to Earth’s gravity field at the end,” said Thirsk. “Fatigue, light-headedness, and imbalance were immediate challenges for me on my return. Bones and muscles take the longest to recover following spaceflight.”

The study further noted that astronauts who flew on shorter missions (less than six months) were better able to recover bone density after their return. As we prepare to venture on missions on Mars and beyond, it is crucial that we consider how this kind of travel will impact our bodies.

Beyond studying how we will respond to foreign atmospheres, we must also consider how weightlessness will affect our bodies in the near and long term to avoid any significant lasting after-effects. 

The study is published in the journal Scientific Reports.

Determining the extent of bone recovery after prolonged spaceflight is important for understanding risks to astronaut long-term skeletal health. We examined bone strength, density, and microarchitecture in seventeen astronauts (14 males; mean 47 years) using high-resolution peripheral quantitative computed tomography (HR-pQCT; 61 μm). We imaged the tibia and radius before spaceflight, at return to Earth, and after 6- and 12-months recovery and assessed biomarkers of bone turnover and exercise. Twelve months after flight, group median tibia bone strength (F.Load), total, cortical, and trabecular bone mineral density (BMD), trabecular bone volume fraction and thickness remained − 0.9% to − 2.1% reduced compared with pre-flight (p ≤ 0.001). Astronauts on longer missions (> 6-months) had poorer bone recovery. For example, F.Load recovered by 12-months post-flight in astronauts on shorter (< 6-months; − 0.4% median deficit) but not longer (− 3.9%) missions. Similar disparities were noted for total, trabecular, and cortical BMD. Altogether, nine of 17 astronauts did not fully recover tibia total BMD after 12-months. Astronauts with incomplete recovery had higher biomarkers of bone turnover compared with astronauts whose bone recovered. Study findings suggest incomplete recovery of bone strength, density, and trabecular microarchitecture at the weight-bearing tibia, commensurate with a decade or more of terrestrial age-related bone loss.

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