What are the causes of osteoporosis? Early in life, more bone is laid down than is removed, and an individual’s peak bone mass is typically achieved by around age 30. After peak bone mass is reached, the remodeling process (the process of laying down new bone and removing old bone) takes away more bone than is replaced. Thus, the process of bone loss typically begins between one’s early to late thirties. Starting from about that age to the onset of menopause, women lose a certain amount of bone steadily every year, as follows: Trabecular bone (the spongy bone inside the hard, cortical bone) is lost at a rate of about 1% per year Cortical bone (the hard bone) is lost at a rate of about 0.5% per year Estrogen plays an important part in maintaining bone strength because it helps keep bone remodeling (which is now taking away more bone than is added) rates low. There are two lines of cells for bone remodeling, the bone-eating cells (osteoclasts) and the bone-forming cells (osteoblasts). Without estrogen, the osteoclasts are favored and more bone is resorped (removed) than laid down, resulting in thinning of the bone. Therefore, when women reach menopause and their estrogen levels decrease, the rate of bone loss increases to 2% to 3% per year. After 8 to 10 years, the rate of bone loss returns to the previous rate of 1% and 0.5% per year, respectively.