Why do astronauts lose bone mass in space? Here’s an explanation by John Uri, who was mission scientist for the shuttle-Mir cooperation program and will serve a similar role for the International Space Station.
Bone is a living tissue, like your heart, your liver or anything else. We tend to think of bones as something inert, but that’s not the case. They’re metabolically active. Bone tissue is constantly being broken down by certain cells, and built up by other cells to maintain the functional rigidity of the bone. Much of that activity comes in response to the stresses we put on the bones, during walking or exercising. Even when you’re in bed, there are still some muscle forces acting on the bone, providing the stimulus for the remodeling of the bone.
When you go into space — and let’s say you don’t do any kind of physical exercise — that stimulus is gone because of the near absence of gravity. So the balance between the cells that break down the bone and the cells that make up the bone is upset. You have more breakdown of the bone, and not so much rebuilding. As a result, the calcium and other minerals that make up the bone tend to leach out, making the bone somewhat weaker.
The effect is not universal across the whole body. The lower body — the leg, the hips and the spine — tend to lose more bone mass.
From what we’ve seen so far, the process continues at a level rate throughout a six-month or longer space flight. We don’t understand why the bone loss doesn’t plateau. Perhaps it plateaus after, say, two years. We just don’t know that yet.
We know that exercise does counteract that effect to some degree — walking or running, using a treadmill or a stationary bicycle.
Bone loss in space isn’t identical to osteoporosis on Earth, since we see a hormonal component in osteoporosis. But patients who are debilitated and bedridden experience very similar kinds of bone losses to what we see in space. So what we’re learning about how astronauts can recover from bone loss could help those patients on Earth, and what we learn about bone loss on Earth could help astronauts in space.