Bone is living, growing tissue made mostly of collagen. Collagen is a protein that provides a soft "framework", and calcium is a mineral that adds strength and hardens the framework. The combination of collagen and calcium makes bone the strongest material in the body. More than 99 percent of the body’s calcium is contained in the bones and teeth. Most of the remaining 1 percent is found in the blood.
Throughout life, bone is constantly renewed through a three-part process called remodeling. This process consists of reabsorption, collection, and formation. During reabsorption, old bone tissue is broken down and removed by special cells called osteoclasts. During collection, red blood cells bring proteins to the osteoblasts. During bone formation, new bone tissue is laid down to replace the old. Osteoclast and osteoblast function is regulated by several hormones including calcitonin, parathyroid hormone, vitamin D, estrogen (in women) and testosterone (in men), among others.
Women are more likely than men to develop osteoporosis. This is because women generally have smaller, thinner bones, and because they can lose bone tissue rapidly in the first 4 to 8 years after menopause due to the sharp decline in production of the hormone estrogen. Produced by the ovaries, estrogen has been shown to have a protective effect on bone. Women usually go through menopause between ages 45 and 55. After menopause, bone loss in women greatly exceeds that in men. However, by age 65, women and men tend to lose bone tissue at the same rate. While men do not undergo the equivalent of menopause, production of the male hormone testosterone may decrease, and this can lead to increased bone loss and a greater risk of developing osteoporosis.