Why is osteoporosis more commom in women than in men?

If I am remembering correctly osteoporosis is an auto immune disorder. It is somewhat related to rhuematiod arthritis. The immune system causes inflammation in and around the bone and chemical cytokines released by the immune cells (white blood cells) cause the degredation of the bone over time. This is more prevelent in post menopausal women. It could be due to the change in hormones that occurs during menopause or it may be that they have less calcium due to the amount that is lost during pregnancy. Men don't get pregnant therefore don't lose calcium to a foetus (babies are very parasitic!).

Mostly it is because of low hormone levels after menopause. It affects both sexes as such.
Possibly men being more active physically are less prone.
In fact, the word "osteoporosis" means "porous bone." I wonder if it can be put under the category- autoimmiune disorders. If a joint is diseased and that part of the body is not used for some time even at an young age, the bones around the joint will get porous called disuse atrophy. It won't have anything to do with autoimmune etiology. Just as new hair formation is affected in the old age and and some people get bald, new bone formation (that is a continuous process throughout life) is affected in the old age and people get porous, fragile bones.

Prevention is by eating healthy, calcium and vitamin D3 supplements, exercises and avoiding smoking.

The loss of Estrogen at the onset of menopause..one cause makes women more at risk than men. There are many factors..smoking..giving birth..a persons ability to absorb calcium and Vit D..