Previous research has shown astronauts lose
between one to two percent of bone density for every month spent in space, as
the lack of gravity takes the pressure off their legs when it comes to standing
and walking.
To find out how astronauts recover once
their feet are back on the ground, a new study scanned the wrists and ankles of
17 astronauts before, during and after a stay on the International Space
Station
The bone density lost by astronauts was
equivalent to how much they would shed in several decades if they were back on
Earth, said study co-author Steven Boyd of Canada's University of Calgary and
director of the McCaig Institute for Bone and Joint Health.
The researchers found that the shinbone
density of nine of the astronauts had not fully recovered after a year on Earth
— and were still lacking around a decade's worth of bone mass.
The astronauts who went on the longest
missions, which ranged from four to seven months on the ISS, were the slowest
to recover.
"The longer you spend in space, the
more bone you lose," Boyd told AFP.
Boyd said it is a "big concern"
for planned for future missions to Mars, which could see astronauts spend years
in space.
"Will it continue to get worse over time
or not? We don't know," he said.
"It's possible we hit a steady state
after a while, or it's possible that we continue to lose bone. But I can't
imagine that we'd continue to lose it until there's nothing left."
A 2020 modelling study predicted that over
a three-year spaceflight to Mars, 33 percent of astronauts would be at risk of
osteoporosis.
Boyd said some answers could come from
research currently being carried out on astronauts who spent at least a year
onboard the ISS.
Guillemette Gauquelin-Koch, the head of
medicine research at France's CNES space agency, said that the weightlessness
experienced in space is "most drastic physical inactivity there is".
"Even with two hours of sport a day,
it is like you are bedridden for the other 22 hours," said the doctor, who
was not part of the study.
"It will not be easy for the crew to
set foot on Martian soil when they arrive — it's very disabling."
The silent disease
The new study, which was published in
Scientific Reports, also showed how spaceflight alters the structure of bones
themselves.
Boyd said that if you thought of a body's
bones like the Eiffel Tower, it would as if some of the connecting metal rods
that hold the structure up were lost.
"And when we return to Earth, we
thicken up what's remaining, but we don't actually create new rods," he
said.
Some exercises are better for retaining
bone mass than others, the study found.
Deadlifting proved significantly more
effective than running or cycling, it said, suggesting more heavy lower-body
exercises in the future.
But the astronauts — who are mostly fit and
in their 40s — did not tend to notice the drastic bone loss, Boyd said,
pointing out that the Earth-bound equivalent osteoporosis is known as "the
silent disease".
Canadian astronaut Robert Thirsk, who has
spent the most time in space, said that for him bones and muscles took the
longest to recover after spaceflight.
"But within a day of landing, I felt
comfortable again as an Earthling," he said in a statement accompanying
the research.
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