Question: Why do bones go weak when you come back down from Space?

Kirsty Lindsay answered on 12 Jun 2016:

The science behind bones getting weaker in space is pretty complex, and there are lots of things going on. Some of them we know about and study, and some of them we haven’t discovered or understood yet.

On Earth moving against gravity sends signals to our bones that they have to work hard to keep us moving-some of the signals say ‘build’ and some say ‘breakdown’ (so that new bones can be built). As adults we get a whole new skeleton after about 7 years of breaking down and rebuilding bones.

In space, without the need to work against the pull of gravity, some of the signals are lost. We know from experiments that the ‘breakdown’ signals get stronger and the ‘build’ signals get weaker- the overall result of this is more bone is destroyed and less bone is rebuilt in space- making the bones weaker- especially in the spine and legs.

At the moment we don’t really know how totally stop the bones becoming weaker in space- we know exercise important, and we know food is important (having plenty of calcium and vitamin D), but we aren’t sure what else is needed.