How does osteoporosis occur?
Osteoporosis, or bone loss, occurs when the process of bone breakdown and bone formation gets out of balance. The cells that cause bone breakdown (osteoclasts) start to make canals and holes in the
Figure 3 Microscopic view of osteoporotic bone. Courtesy of the National Association of Nurse Practitioners in Women's Health (NPWH).
bone faster than the cells that cause bone formation (osteoblasts) can make new bone to fill in the holes. The bone becomes fragile and more likely to break. Figure 3 shows a microscopic view of weak bone and surrounding holes.
When bone has to give up some of its calcium to ensure that blood levels of calcium stay normal, bone is weakened by the loss of calcium. The weakening of bone by its loss of calcium also leads to osteopenia and osteoporosis.
Taking in extra calcium and vitamin D alone will not prevent osteoporosis. Because of the way bone develops, the mechanical stress on bone caused by exercise is also important for preventing osteoporosis. The less you exercise, the less the osteoblasts work to make new bone. You need both weight-bearing and resistive exercise to promote strong bones (see Questions 42 and 43).
When the body has not formed adequate bone during childhood and young adulthood, the lack of bone mass is also termed osteoporosis or osteopenia, depending on how frail the bones are. Even if your bones are not so frail that you have osteopenia or osteoporosis, not reaching peak bone mass in your youth makes osteoporosis more likely to occur. The reason that building strong bones in childhood is so important is that if a young adult does not have peak bone mass, osteoporosis is more likely to develop despite preventive measures taken later in life.
How is bone made? Do we stop making bone when we become adults?
The 206 bones of the human body are important for their ability to support surrounding tissue and muscles, protect the body's organs, allow movement, manufacture blood cells, and provide storage for calcium and phosphorus, minerals that are released from the bone when needed. Given bone's many functions, it's not surprising that bone development is a complicated process.
Each bone is made up of collagen, which is a protein substance, and minerals such as calcium and phosphorus. This matrix of soft and hard elements gives bone its versatility in being able to support the weight and movement of the body, while also storing vital calcium for the normal functioning of muscles and nerves.
In infancy, the process of hardening the cartilage that we're born with begins. Bone modeling is a process that takes place in childhood and adolescence. It is the development of new bone at one site and the destruction of old bone at another site within the same bone at the same time. The amount of new bone that is developed exceeds the amount of old bone that is broken down. This process is the way in which peak bone mass (the maximum amount of bone) is achieved, usually in our early 20s to 30s, and also allows the bones of childhood to develop at different rates and shift in space, until the adult skeleton is fully formed.
Yes, we continue to make bone as adults in a continuous process called bone remodeling. Bone-resorbing cells (osteoclasts) secrete enzymes, which digest bone and create holes. Bone-forming cells (osteoblasts) migrate to the bone surface and, by secreting a particular type of collagen, fill in the holes. Unlike the process in childhood, the osteoblasts do not function independently. Instead, they act in response to the activity of the osteoclasts and stresses on bone, such as exercise. Figure 4 shows the process of remodeling. In response to the hole-making activity of the osteoclasts, the osteoblasts help to form new bone to fill in the holes.
Figure 4 The bone remodeling process. Courtesy of Eli Lilly and Company.
Calcium, phosphate, and other substances mix with water, somewhat like cement, and harden the collagen, which has been laid down in a matrix-type pattern. As we age, the process of bone remodeling does not stop, but it does slow down. Our bones are very busy—we recycle our entire skeleton about every 10 years!
Osteoclasts and osteoblasts are found in all bone, but are found in greater numbers in the hip, spine, and "long" bones of the thigh (femur), upper arms (humerus), lower legs (tibia and fibula), and lower arms (radius and ulna). This is why the most accurate measurements of bone density are taken from the hip and spine, rather than the smaller bones of the hands and feet.
We recycle our entire skeleton about every 10 years!