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 bone faster than the cells that cause bone formation (osteoblasts) can make new bone to fill in the holes. The bone becomes

Figure 3 Microscopic view of osteoporotic bone. Courtesy of the National Association of Nurse Practitioners in Women's Health (NPWH).

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 43 and 44).

When the body has not formed adequate bone during childhood and young adulthood, the lack of bone mass

Resistive exercise

Type of activity that pushes and pulls muscles to strengthen them: examples are swimming, biking, and weight-lifting.

Bone mass

The volume, density, or quantity of bone.

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.


A protein substance used by osteoblasts to make new bone and keep teeth strong. Also found in connective tissue such as skin, tendons, and ligaments throughout the body.


Rubbery connective tissue that is found in joints and the outer ear.

Bone modeling

A process that takes place in childhood and adolescence where new bone is developed at one site and old bone at another site within the same bone is destroyed at the same time.

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