Bone is made of calcium and other minerals, which make it hard. Bone density refers to the mineral content of the bones. It is related to how hard and strong bones are. Low bone density is a trait of osteoporosis.
Like other tissues in the body, bone constantly repairs and renews itself. This process is called remodeling. Two kinds of cells carry out remodeling in bone:
A balance between the bone-building osteoblasts and the bone-dissolving osteoclasts keeps bones healthy.
In young people, bones lengthen and increase in density. But after about age 35, your bones start to lose density and strength. Most cases of osteoporosis result from the quicker bone loss. This can happen for any of these reasons:
Lower levels of the hormones estrogen and testosterone in the body
Lack of physical activity
Too little calcium, or vitamin D, or both
Excessive alcohol use
Certain medicines, such as steroids, or those used to treat breast cancer
Osteoporosis is more common in women than in men. During menopause, the amount of estrogen made in a woman's ovaries greatly slows down. This hormone keeps the bone-dissolving activity of the osteoclasts in check. After menopause, the osteoblasts continue to build bone. But they can't keep up with the speed at which the osteoclasts break it down. If you don't take measures to prevent or slow bone loss, osteoporosis can occur.
In men, bone loss generally starts later. And it advances more slowly than it does in women. Men tend to have larger and stronger bones than women do. They also don't go through the sudden hormonal changes that occur with menopause. Still, as men age, they do lose bone density. In part, this is because of a natural decrease in testosterone. By age 65 or 70, men and women lose bone mass at similar rates. Calcium absorption, which is needed to keep bones healthy, also decreases in men and women.