Astronauts on space missions lasting more than six months suffer decades of bone loss, much of which may be irreversible, a new study has found. The find could pose a serious challenge to future manned missions to Mars.
For missions that last six months or more, exposing astronauts to the microgravity of space causes them to experience bone loss equivalent to two decades of aging. And only half of the lost bone regenerates after a year back on Earth, leaving them with a decade of age to their bone structure, researchers wrote in a study published June 30 in the journal Scientific reports (opens in new tab).
Bones, like muscles, are always growing and have evolved to change under the constant mechanical stress caused by the Earth gravity. And just like muscles, if weight-bearing bones aren’t used—for example, during a long stay in low-gravity space—they can be irreversibly weakened.
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“We found that the weight-bearing bones were only partially restored in most astronauts one year after spaceflight,” lead author Lee Gable, assistant professor of kinesiology at the University of Calgary in Canada, said. it said in a statement. “This suggests that the permanent bone loss due to spaceflight is about the same as the bone loss associated with the age of the Earth in one decade.”
Researchers evaluated the bones of 17 astronauts who remained on the International Space Station (ISS). The astronauts – 14 men and three women – had an average age of 47. Their stay aboard the ISS varies from four to seven months.
To track the deterioration and repair of the astronauts’ bones, the researchers scanned specific areas of the astronauts’ bodies — such as the wrists, ankles and shins — before they traveled to the ISS and immediately after they returned. The scientists then performed two follow-up scans six and 12 months after the astronauts set foot on solid ground.
The scans were made using a technique called high-resolution peripheral quantitative computed tomography (HR-pQCT), which builds 3D images of human bone structure at scales finer than the width of a human hair. Using these scans, the researchers understood the mineral content of the astronauts’ bones and bone density—key indicators of how susceptible bones are to fracture.
The results showed that of the 17 astronauts, 16 had not regained their pre-space tibial strength after one year of recovery. Also, after the recovery year, the eight astronauts who spent more than six months in space had tibia bones that experienced the equivalent of a decade of aging and could withstand 75 pounds (334 newtons) less force , than before their space missions. In contrast, the bones of the lower arms (radii) of the space travelers were almost undamaged, probably because these bones were not weight-bearing.
Bone can be roughly divided into two layers: cortical and trabecular. The cortical part of the bone makes up approximately 80% of the human bone mass and is the outer part of the bone that gives it its shape. The remaining 20% of bone mass is made up of the trabecular component, which is a lattice-like structure of microscopic beams and struts that reinforce the cortical bone from within. When people lose bone density, some of this trabecular honeycomb disappears, reducing bone strength and making them much more vulnerable to fracture.
“We’ve seen that many of these connections are lost during spaceflight, so it’s very likely that, although new bone is formed on return to Earth, the body’s ability to replace the missing rods is very unlikely,” Stephen Boyd, professor of of radiology at the Cummings School of Medicine in Calgary, Canada, told Live Science.
Previous research predicted that after a three-year trip to Mars, 33 percent of astronauts would return at risk of osteoporosis, a progressive condition in which holes and honeycomb spaces in the bone grow larger, making them more prone to fracture.
And it’s not just bone that breaks down in low gravity. Previous studies have also shown that muscles, eyes, brains, hearts, spines, and even cells can be damaged by extended stays in space—all of which pose unique challenges to long spaceflight. The silver lining from the new study is that in-flight deadlift training provided by the ISS Advanced Resistance Exercise Device (ARED) slows the rate of bone loss and enhances recovery—meaning that specific training regimens, equipment and targeted nutrition can be vital to keeping astronauts fit during long journeys such as a future three-year round-trip mission to Mars.
“Since cramped quarters will be a limiting factor for future exploration-class missions, exercise equipment will need to be optimized for a smaller footprint,” the scientists wrote in the study. “Resistance exercise training (especially deadlifts and other lower body exercises) will remain a mainstay in mitigating bone loss; however, adding jumping exercise to orbit regimens may further prevent bone loss and reduce daily exercise time.”
The scientists are now planning a follow-up study to examine the impact of trips longer than seven months on bones. This research is planned as part of a NASA project to study the long-term effects of space on more than a dozen vital parts of the human body.
“Those who spent more time in space lost more bone. So it would be reasonable to assume that spending even longer time in space could mean additional bone loss,” Boyd said. “This is clearly a problem for missions that may take years (eg Mars). But what we don’t know is whether the human body reaches a plateau in bone loss at some point. It doesn’t seem likely that bone would ‘melt’ completely, but we don’t know at what level of bone loss equilibrium might be reached.”
In addition to helping astronauts stay healthy during long flights, the research also offers insights into how to help them adjust to another shock to their systems: their return to Earth.
“Just as the body must adapt to spaceflight at the beginning of a mission, it must also adapt back to Earth’s gravitational field at the end,” Robert Thirsk, former chancellor of the University of Calgary and an astronaut, said in the statement. “Fatigue, dizziness and imbalance were immediate challenges for me upon my return. Bones and muscles take the longest to recover from spaceflight. But within a day of landing, I felt comfortable as an Earthling again.”
Originally published on Live Science.
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