Osteoporosis: Causes, Risk Factors, and Diet's Role

Introduction to Osteoporosis

Osteoporosis is a disease in which bones become fragile and are therefore more likely to break (NOF, 2004). The common perception that bones are inert structures, like wooden beams, is incorrect. A bone is actually a living tissue that is constantly being remodeled. After age 35, however, on average a little more bone is lost each year than is gained during this remodeling. Between ages 40 and 50, men characteristically lose 0.5 to 0.75% of bone mass yearly, while women lose it at more than twice that rate. Bones that were once sturdy may become lighter and fragile, with their interiors resembling lacy honeycombs. The rate of natural loss increases substantially after age 50. If severe enough, the thinned-out bones become porous and osteoporosis develops — a term that literally means "bone porosity" (Walford, 2000, p. 131).

Consequences and Implications

Osteoporosis is often called the "silent disease" because bone loss occurs without any symptoms. People may not know they have osteoporosis until their bones become so weak that a sudden strain, bump, or fall causes a fracture or a vertebra to collapse. If one or more vertebrae collapse, the result can be severe back pain, loss of height, or spinal deformities such as kyphosis or stooped posture. Typically, osteoporosis results in hip, spine, wrist, and rib fractures, although any bone can be affected (NOF, 2004).

Although any fracture is painful and slow to heal, fractures caused by osteoporosis can be particularly debilitating. Because older people are more prone to the disease, recovery is further slowed by age-related complications. For instance, an average of 24% of hip fracture patients aged 50 and over die in the year following their fracture. Additionally, an osteoporosis patient can fracture a bone very easily through minor incidents such as slipping on a rug, lifting a bag of groceries, or even a friendly hug (Walford, 2000, p. 132).

Causal Factors and Hormonal Mechanisms

One major cause of osteoporosis is the natural aging process, though the actual onset and progression of the disease is complex. Under normal conditions, a biologically active form of vitamin D regulates the absorption of calcium from the gut. With age, the body develops a deficiency in its ability to convert regular vitamin D to this active form. This leads to decreased calcium absorption and, consequently, lowered blood calcium levels. The small drop in blood calcium triggers an increased release of parathyroid hormone (PTH) from the parathyroid glands, located on either side of the neck. PTH raises blood calcium by drawing it from the bones, since blood calcium must be maintained at normal levels for the body's mechanisms to function. The kidneys then remove excess PTH (Walford, 2000, p. 132).

One reason osteoporosis tends to develop late in life is that blood levels of PTH may rise secondary to a decline in kidney function, which normally occurs with age. The situation is further worsened in menopausal women, since estrogen levels in the blood also decrease — and estrogen makes bone less sensitive to dissolution by PTH. Another hormone, calcitonin, secreted by the thyroid gland, serves to inhibit bone resorption; however, its level also decreases with age. Together, these hormonal changes contribute to the development of bone porosity and osteoporosis (Walford, 2000, p. 132).