Astronauts on space missions lasting longer than six months suffer decades' worth of bone loss, much of which could be irreversible, a new study has found. The finding may present a serious challenge to future crewed missions to Mars.
For missions that last six months or longer, astronauts' exposure to the microgravity of space causes them to experience bone loss equivalent to two decades of aging. And only half of the lost bone recovers 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 they have evolved to reshape themselves under the constant mechanical strain caused by Earth's gravity. And, just like muscles, if weight-bearing bones are not used — such as during a long, low-gravity stint in space — they can be weakened irreversibly.
"We found that weight-bearing bones only partially recovered in most astronauts one year after spaceflight," lead author Leigh Gabel, an assistant professor in Kinesiology at the University of Calgary in Canada, said in a statement (opens in new tab). "This suggests the permanent bone loss due to spaceflight is about the same as a decade worth of age-related bone loss on Earth."
The researchers assessed the bones of 17 astronauts who had stayed on the International Space Station (ISS). The astronauts — 14 men and three women — had an average age of 47. Their stays aboard the ISS ranged from four to seven months.
To track the astronauts' bone deterioration and recovery, the researchers scanned specific regions of the astronauts' bodies — such as the wrists, ankles and shins — before they traveled to the ISS and as soon as they returned. Scientists then conducted two follow-up scans six and 12 months after the astronauts stepped back on solid ground.
The scans were taken 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 figured out the astronauts' bone mineral content and bone density — key indicators of how susceptible bones are to fracturing.
The results showed that, of the 17 astronauts, 16 had not regained their pre-space tibia strength after one year of recovery. Additionally, after the recovery year, the eight astronauts who spent longer than six months in space had tibia bones that had experienced the equivalent of a decade of aging and could sustain 75 pounds (334 Newtons) of force less than they could before their space missions. In contrast, the bones of the spacefarers' lower arms (radii) had barely deteriorated at all, likely because these bones are not weight-bearing.
Bones can be divided into roughly two layers: the cortical and the trabecular. The cortical part of bone accounts for roughly 80% of a human's bone mass and is the outer shaft of the bone that gives it its shape. The remaining 20% of bone mass is made up by the trabecular component, which is the trellis-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 bones’ strength and making them far more vulnerable to snapping.
"We've seen that many of those connections are lost during space flight, and so it is very likely that although new bone is be formed upon return to Earth, the ability of the body to replace those missing rods is highly unlikely," Steven Boyd, a Radiology professor at the Cumming School of Medicine in Calgary, Canada, told Live Science.
Previous research has predicted that, over a three-year round trip to Mars, 33% of astronauts would return at risk of osteoporosis, a progressive condition that sees the holes and spaces of the bone’s honeycomb grow larger, making them more susceptible to breaking.
And it's not just bone that deteriorates in low gravity. Prior studies have also shown that muscles, eyes, brains, hearts, spines, and even cells can all be damaged by prolonged stays in space — all of which present unique challenges to long-duration spaceflight. The silver lining from the new study is that in-flight deadlift training provided by the ISS's Advanced Resistive Exercise Device (ARED) slowed the rate of bone loss and boosted recovery — meaning that specific training regimens, equipment and targeted nutrition could be vital in 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 on future exploration-class missions, exercise equipment will need to be optimized for a smaller footprint," the scientists wrote in the study. "Resistance exercise training (particularly deadlifts and other lower-body exercises) will remain a mainstay for mitigating bone loss; however, adding a jumping exercise to on-orbit regimens may further prevent bone loss and reduce daily exercise time."
The scientists are now planning a follow-up study to research the impacts that journeys longer than seven months have 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 may mean further bone loss," Boyd said. "This is obviously a concern for missions that may take years (e.g., Mars). But, what we don't know is whether the human body reaches a plateau of bone loss at some point. It doesn't seem likely that the bones would entirely "melt" away, but we don't know at what level of bone loss equilibrium may be reached."
Beyond helping astronauts to stay healthy across long flights, the research also offers insights into how to help them to adjust to another shock to their systems: their return to Earth.
"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," Robert Thirsk, a former University of Calgary chancellor and astronaut, said in the statement. "Fatigue, light-headedness, and imbalance were immediate challenges for me on my return. Bones and muscles take the longest to recover following spaceflight. But within a day of landing, I felt comfortable again as an Earthling."
Originally published on Live Science.