The study found that the 14 members of ISS Expeditions One through Six suffered the same amount of bone loss as cosmonauts aboard the now decommissioned space station Mir, suggesting the need for more even more conditioning exercises and countermeasures than those currently in place.
"This is one of our top priorities," said Guy Fogleman, director of bioastronautics research for NASA's Office of Biological and Physical Research in Washington, D.C. "And even though it's a problem, it's not intractable."
NASA has long known the dangers of accumulated bone loss in humans due to long periods in space. The new study, though, offers researchers a deeper view inside the bones of ISS crewmembers using a method called three-dimensional X-ray computer tomography (CT).
"The CT allows you to derive a three-dimensional picture of bone loss," said Thomas Lang, lead author of the study and an associate professor of radiology at the University of California San Francisco. "You can sort of peel away the outer layers and look inside."
Lang and his colleagues, which included researchers from Baylor College of Medicine in Houston, Texas, used computer tomography to map the bone density and structure for ISS crewmembers' hips both before and after their flights, which lasted between four and six months. Fogleman told SPACE.com the experiment was the first time reseachers have been able to take subsurface bone measurements in ISS crewmembers.
Researchers combined that data with ultrasound measurements of the heel bone, as well as x-rays of crewmember spines, and found that on the average ISS crewmembers lost interior bone material at a rate of up to 2.7 percent for each month they spent in space. Outer hipbone material was lost at up to 1.7 percent each month. That's about on par with a separate study by Russian scientists on Mir cosmonauts. However, Lang's team did not detect additional bone loss areas of the spine that connect with back muscles, a difference that may be due to the space station's resistive exercise device to maintain lower back muscles.
"I think that some of the similarities [to the Mir study] may be due to the fact that NASA was just getting the station up to speed," Lang told SPACE.com. "It will be interesting to see what happens with the next seven missions."
Rebuilding bone
The need to keep bones healthy is imperative for any long-duration human spaceflight, including any future trips to Mars or long-term stays on the moon. If astronauts can't regain bone material they lose in space, it could lead to fractures, if not upon their Earthly return then during old age when bone strength naturally declines.
"We're not only looking to understanding the effects of bone loss, but also to develop countermeasures that limit those effects," Fogleman said. "Even though this study shows that we don't have the answer, we do have a number of projects underway."
ISS crewmembers currently use a treadmill, a stationary bicycle and resistive, elastic piece of equipment that resembles a Bowflex to keep their bones and muscles strong. There are also current investigations into the use of particular frequencies of vibrations, a lower-body negative pressure device and a class of drugs called bisphosphonates, which are commonly used to treat the bone disease osteoporosis here on Earth.
The ultimate extent of bone loss an astronaut can experience, such as whether it increases with flight time or eventually levels off, is still unknown. Lang's study was limited by the fact that all six of the ISS missions had a maximum duration of up to six months. But an eventual mission to Mars, which is part of NASA's new space vision announced in January, could take years.
"The nightmare scenario would be a bone fracture on Mars," Lang said. "Or in a person returning from an extremely long space flight."
Lang is planning a follow-up study that would determine changes in bone strength over time in space.
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