bone is constantly being remodeled. this is the natural healthy state of continuous uptake of old bone (resorption) followed by the deposit of new bone. this turnover is important in keeping bones healthy and in repairing any minor damage that may occur with wear and tear. the cells that lay new bone down are called osteoblasts and the cells responsible for resorption of old bone are called osteoclasts. osteoporosis occurs as a result of a mismatch between osteoclast and osteoblast activity. this mismatch can be caused by many different disease states or hormonal changes. it is also commonly a result of aging the change in normal hormones that occurs after menopause and from diets low in calcium and vitamin d. in osteoporosis osteoclasts outperform osteoblasts so that more bone is taken up than is laid down. the result is a thinning of the bone with an accompanying loss in bone strength and a greater risk of fracture. a thinning bone results in a lower bone density or bone mass. there are two major types of bone. cancellous bone (also known as trabecular bone) is the inner softer portion of the bone and cortical bone is the outer harder layer of bone. cancellous bone undergoes turnover at a faster rate than cortical bone. as a result if osteoclast and osteoblast activity becomes mismatched cancellous bone is affected more rapidly than cortical bone. certain areas in the body have a higher ratio of cancellous bone to cortical bone such as the spine (vertebrae) the wrist (distal radius) and the hips (femoral neck). most of a person's bone mass is achieved by early adulthood. after that time the bone mass gradually declines throughout the rest of a person's life. there is a normal rate of decline in bone mass with age in both men and women. for women in addition to age the menopause transition itself causes an extra degree of bone loss. this bone loss is greatest in the first three to six years after menopause. women can lose up to 20% of their total bone mass during this time. since women generally have a lower bone mass to begin with in comparison with men the ultimate result is a higher risk of fracture in postmenopausal women as compared to men of the same age. nevertheless it is important to remember that men may also be at risk for osteoporosis especially if they have certain illnesses a low testosterone level are smokers take certain medications or are sedentary. the best method to prevent osteoporosis is to achieve as high a bone mass by early adulthood with a proper diet and regular exercise. unfortunately osteoporosis is not often considered during this time in a person's life.
how does osteoporosis occur?