Bone becomes stronger when subjected to mechanical stress, such as walking, running, weight-lifting, or hard physical labor. When under stress, bone tissue deposits more of the mineral salts that lend strength to bone. When the same stress is removed, bone-resorbing cells (called osteoclasts) go to work and tear down the unnecessary bone. This is why a bone seems to shrink in size when it has been in a cast for some time.
Why bones thicken during exercise is quite interesting. Evidently, stress on bone causes the calcium phosphate crystals to produce tiny currents of electricity. This is referred to as the piezoelectric effect. This tiny amount of electricity stimulates bone-building cells (osteoblasts) to go to work, and bones become thicker, particularly in the areas receiving stress.
Currents of electricity are sometimes applied to bones that fail to heal properly. Electric coils are wrapped around the cast, and very small amounts of electricity are run through the coils. These weak electrical currents circling around the fracture are called pulsating electromagnetic fields. Like the piezoelectric effect, this stimulates bone-building cells.
Exercise also helps to increase the level of a thyroid hormone, called calcitonin, in the blood. Calcitonin aids bone growth by inhibiting the actions of the osteoclasts, those bone-resorbing cells.
Osteoporosis, the advanced demineralization of bones that occurs with age, is a major problem of older American women. While this may not interest the younger female students at the outset, it should, because eating behavior during the teen years and early- to mid-twenties can greatly affect the health of bones later in life. Impress upon your students the need to include enough calcium and protein in their diets to ensure bone health throughout life.
Post-menopausal women are at the greatest risk of developing osteoporosis because the levels of estrogen hormones, abundant in younger women, have declined. Estrogens stimulate bone-building cells to build new bone. Factors implicated in the likelihood of developing osteoporosis are:
Women begin losing bone mass in their thirties. The rate of loss accelerates with age until 30% or more of bone mass is lost by age 70. This can lead to spontaneous fractures, shortening of the vertebral column, and intense pain.
There are several ways to help prevent osteoporosis: (1) Exercise. Any weight-bearing exercise is best, such as walking, jogging, dancing, skating, or skiing. (2) Eat an adequate diet with sufficient calcium and vitamin D. Newest findings are suggesting three to four 8-oz glasses of milk per day, or their equivalent, are needed to stave off osteoporosis. If your bones are thick in your youth, even as you lose bone mass with aging, your bones will still be able to support your body. Anorexia also predisposes young women to osteoporosis later in life. (3) Reduce or eliminate soda pop from the diet, especially if calcium intake is insufficient. Something about the phosphoric acid in soda pop interferes with the metabolism of calcium phosphate in bones, and demineralization or lack of calcium deposition, occurs. (4) Don't smoke.