The best explanation I have read is one that says to think of bone as a bank account where you “deposit” and “withdraw” bone tissue. During childhood and the teenage years, new bone is added to the skeleton faster than old bone is removed. As a result, bones become larger, heavier, and denser. For most people, bone formation continues at a faster pace than removal until bone mass peaks during the third decade of life.
After age 20, bone “withdrawals” can begin to exceed “deposits.” For many people, this bone loss can be prevented by continuing to get calcium, vitamin D, and exercise and by avoiding tobacco and excessive alcohol use. Osteoporosis develops when bone removal occurs too quickly, replacement occurs too slowly, or both. You are more likely to develop osteoporosis if you did not reach your maximum peak bone mass during your bone-building years.
I have to take high doses of cortico-steroids periodically. I know they have a major effects on how the body uses calcium and vitamin D which can lead to bone loss, osteoporosis, and broken bones so my doctor prescribes calcium and Vitamin D supplements to counteract that but has never mentioned HRT as a preventative.