Why do astronauts have to work out on the International Space Station?

The Importance of Fitness on the ISS

Astronaut G. David Low using a treadmill aboard the space shuttle Columbia as fellow crewmembers Daniel C. Brandenstein and James D. Wetherbee look on.
Astronaut G. David Low using a treadmill aboard the space shuttle Columbia as fellow crewmembers Daniel C. Brandenstein and James D. Wetherbee look on.
Time Life Pictures/NASA/Getty Images

When an astronaut spends a prolonged amount of time in space, he or she experiences the effects of microgravity and remains weightless during the whole trip. Instead of staying anchored to the floor like we do on Earth, astronauts float around much like swimmers do underwater, and they have to hold on to something if they want to stay stable.

Researchers have found that after spending weeks or months in a weightless environment, astronauts lose a significant amount of bone mineral density (BMD). The loss of BMD in the spine, neck and pelvis is about 1.0 to 1.6 percent per month, while cortical bone, the heavy, outer part of the bone found around the whole body and legs, experiences a loss of about 0.3 to 0.4 percent per month. For comparison, a healthy adult on Earth loses 3 percent of cortical bone structure over the course of a decade -- an astronaut could lose that much in less than a year in space.

The result of this bone loss is weakened bones that are more prone to fracturing upon returning to Earth. What's more, even after several years, the astronaut won't have recovered the same bone density he or she had before launch.

So why does something like this happen in space? Astronauts experience bone loss for the same reason chronically bedridden patients do: Their entire skeletons don't bear any weight. They go through a period called skeletal unloading, in which bones lose the ability to make new bone cells and replace old ones. The movement of important minerals such as calcium and phosphorus also slows down.

Although experts aren't sure exactly why this happens in microgravity, Dr. Roger K. Long, an endocrinology research fellow performing research for the National Space Biomedical Research Institute (NSBRI) is currently looking for this specific answer. He and his mentor, Dr. Daniel B. Bikle, believe there are three substances at play when astronauts undergo bone loss: insulin-like growth factor (IGF-1), a chemical produced in the bones that causes bones and cartilage to grow; IGF-1 receptor, which is found inside bone cells and allows them to react to IGF-1; and beta-3 intergrin, a protein that helps the IGF-1 receptor function. The researchers believe that during weightlessness, the body produces less beta-3 integrin, which makes it harder for the IGF-1 receptor to relay any messages from IGF-1 to the bone cells and tell them what to do. The result should be a decrease in bone production and an increase in bone loss.

What exercises do astronauts perform to reduce the risk of bone loss? And can they take any medicine to help? To learn more about the techniques and equipment used in space, read the next page.