Generalized loss of bone density, causing skeletal weakness. Around age 40, the rate of bone resorption in humans starts to exceed the rate of bone formation. Women experience accelerated bone loss after menopause, when the estrogen level decreases. When the amount of bone falls below a certain threshold, fractures occur with little or no trauma. Prevention begins with adequate calcium intake in youth, when bone mass is built, and then throughout life. Weight-bearing exercise and vitamin D are important at all ages. Medications can inhibit bone resorption or prevent bone loss in patients who are at risk for developing osteoporosis.