system
Given the following post, respond as if you're commenting on Reddit:
human
Why do astronauts lose bone mass in space?
gpt
Bone tissue is constantly being broken down and rebuilt by osteoclasts and osteoblasts. The balance between the rate of ossification (making new bone tissue) and resorption (breaking down existing bone tissue) is normally carefully balanced.

It turns out that one of the signals that stimulate ossification is microscopic damage to the bone tissue, which is created all the time as you move around. Except when you go to space, that damage is reduced dramatically because you're no longer working against gravity. And thus, the careful balance is upset because not enough bone is being made to replace what is broken down as normal.