SATURDAY, April 6, 2002 (HealthDayNews) -- What do astronauts and people in wheelchairs have in common?
Bone loss.
Bones, especially those in the lower spinal cord and legs, weaken without the force of gravity on the skeletal system. In space travelers and those with spinal cord injuries alike, the loss in bone can lead to fractures, says Dr. Jay Shapiro, bone loss team leader for the National Space Biomedical Research Institute.
However, a new drug may help slow or halt the decrease in bone density among astronauts and the wheelchair-bound.
Shapiro and his colleagues are testing a medication called Zoledronate, part of a class of medications known as bisphosphonates that inhibit the activity of cells responsible for reabsorbing bone. Currently, bisphosphonates are used to slow bone loss in people with certain types of cancer.
"One of the major barriers to having astronauts in space for a long time is bone loss," Shapiro says.
Under normal conditions, bones are in a constant state of growth and re-absorption. In a healthy person, these mechanisms are pretty much in balance, Shapiro says.
But without the pull of gravity, the muscles quickly atrophy. Losing muscle leads to a decrease in stress on bone, which leads to an increase in bone re-absorption, he says.
This causes the bones to lose density, becoming become weak, frail and prone to fractures.
"Maintaining normal amounts of bone is directly dependent on gravity exposure," Shapiro says. "Without the active pull of muscles against bone, the bone senses it really doesn't have anything to do."
To stem this, astronauts have tried exercising in space, but it didn't do much to stop the bone loss, Shapiro says.
Nor can much be done to prevent bone loss in people with spinal cord or brain injuries, or those who have neuromuscular disorders or developmental disorders such as spina bifida and cerebral palsy, says Ken Baldwin, a professor of physiology and biophysics at University of California, Irvine.
For people who still have some mobility in the lower extremities, intensive physical therapy programs can help.
"Bone loss in spinal cord injury patients is a significant problem," Baldwin says. "But up until this point, there hasn't been a clear strategy to help them."
The bone density loss in outer space and among people in wheelchairs occurs quickly. People with a spinal cord injury lose bone at a rate of as much as 1 percent or 2 percent a month. The rate of bone loss stabilizes after several years, and little additional bone is lost, Baldwin says.
The rate is very similar to what occurs in astronauts. Previous research has shown the rate of bone loss among is between 0.5 percent and 2 percent a month, Shapiro says.
That means an astronaut who's in space for six months could lose as much as 10 percent to 15 percent of their bone mass, Shapiro says. And considering a round-trip journey to Mars could take more than two years, bone loss is a significant barrier to extended time in space.
"The fact that they've lost this bone is a continuing hazard to their health after they return to earth," Shapiro says.
With astronauts, it takes about twice as long to regain the bone as it took to lose it, Shapiro says. And, even with intensive physical therapy, some never regain it fully.
In the one-year study, people who have recently had a spinal-cord injury will receive Zoledronate or a placebo once intravenously.
"This particular medicine is many times stronger than the treatments in common use," Shapiro says. "Current intravenous treatments must be taken every two to three months, and oral medications of this type are taken daily."
Baldwin says other research has shown bisphosphonates to be powerful bone-protectors.
"A pharmacological strategy would certainly be beneficial," Baldwin says. "The promise is certainly there, based on early trials and studies."
What To Do: For more information on spinal-cord injuries, visit the National Spinal Cord Injury Association. Or read more about space travel and research at Space Daily.
