It's repeating what the GP said, but: exercise. Your bones are strengthened by use -- put them under mild stress by doing load-bearing exercise and they'll get

It's repeating what the GP said, but: exercise. Your bones are strengthened by use -- put them under mild stress by doing load-bearing exercise and they'll get stronger. (Getting plenty of calcium and vitamin D probably helps, but there's a lot of argument about whether supplements actually work there.)