Space travel harms astronauts’ bone density, finds a new study

Floating around in space might seem funny, but it actually causes a significant loss in bone density that won’t recover even a year after they return to Earth, according to a new study from the University of Calgary.

Bone loss in space

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

“It’s incredibly rare to understand what happens to astronauts and how they recover. It lets us look at the processes going on in the body in such a short time frame. We’d 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 astronauts’ wrists and ankles before they went into space, on their return to Earth, and then six and 12 months after their return.

“We found that a year after spaceflight in most astronauts, the load-bearing bones only partially recovered,” said Gabel. “This suggests that permanent bone loss due to space travel is roughly equivalent to a decade of age-related bone loss on Earth.”

This loss occurs because bones that would normally support on Earth do not have to support weight in the microgravity environment of space.

“We’ve seen astronauts who had trouble walking due to weakness and balance issues after returning from space, and others happily riding their bikes around the Johnson Space Center campus to meet us for a study visit. Astronauts react very differently to their return to Earth,” said Dr. Steven Boyd, Ph.D., director of the McCaig Institute for Bone and Joint Health, professor at the Cumming School of Medicine and leader of the new study.

A testimony of an astronaut

Former Chancellor and astronaut of UCalgary, Dr. Robert Thirsk testified that he witnessed this phenomenon on his return from space.

“Just as the body has to adjust to spaceflight at the start of a mission, it also has to adjust back to Earth’s gravitational field at the end,” Thirsk said. “Fatigue, drowsiness and imbalance were immediate challenges for me upon my return. Bones and muscles take the longest to recover after a space flight.”

The study further found that astronauts who had flown on shorter missions (less than six months) were better able to restore bone density after their return. As we prepare for missions to Mars and beyond, it’s important to consider how this type of travel will affect our bodies.

In addition to examining how we react to alien atmospheres, we also need to consider how weightlessness affects our bodies in the short and long term to avoid significant lasting after-effects.

The study is published in the journal Scientific Reports.

Determining the extent of bone recovery after an extended space flight is important for understanding the risks to astronauts’ long-term skeletal health. We assessed 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 space flight, upon return to Earth, and after 6 and 12 months of recovery, and assessed biomarkers of bone turnover and movement. Twelve months post-flight, group median tibial bone strength (F.Load), total, cortical, and trabecular bone mineral density (BMD), volume fraction, and trabecular bone thickness remained reduced by -0.9% to -2.1% im Compared to before flight (p ≤ 0.001). Astronauts on longer missions (> 6 months) had poorer bone regeneration. For example, F.Load recovered 12 months post-flight in astronauts on shorter (< 6 months; −0.4% mean deficit) but not longer (−3.9%) missions. Similar differences were found for total, trabecular, and cortical BMD. Overall, nine of 17 astronauts did not fully recover from total tibial BMD at 12 months. Astronauts with incomplete recovery had higher bone turnover biomarkers compared to astronauts whose bones recovered. Study results indicate incomplete recovery of bone strength, density, and trabecular microarchitecture at the weight-bearing tibia, consistent with at least a decade of age-related bone loss.