Approximately 95 percent of our skeleton is developed during the first 18 years of life. Periods of rapid growth occur during the first year of life and during the adolescent growth spurt. After adult height is achieved, our bones continue to become more dense as minerals are deposited. This is the consolidation phase. An additional 5-percent increase in bone mass is accumulated by age 30 to 35. At this age, our bones are the most dense and we are in a period of peak bone mass. After age 40, we experience an age-related phase of slow bone loss. The most rapid loss of bone mass for women occurs during the first 4 to 8 years after menopause. This chapter discusses the consequences of bone loss and also the lifestyle factors that protect the skeleton.
Osteoporosis
When enough bone mass is lost that bones become vulnerable to fracture, the individual has developed osteoporosis. Osteoporosis is a debilitating disease that affects over 24 million Americans. Each year in the United States, 1.3 million fractures are attributable to osteoporosis. The most common fractures occur at the wrist, the spine, and the hip. Hip fractures alone result in annual health-care costs of $10 billion. This figure will continue to increase with the increase of the elderly population. Between 15 and 25 percent of persons with a hip fracture enter long-term-care institutions. Hip fractures are associated with a high mortality rate due to surgical deaths and to complications such as thromboembolism, fat embolism, and pneumonia.
Treatment of Osteoporosis
A number of drugs are being investigated for their efficacy in the treatment of osteoporosis. These include calcitonin, bisphosphonates, and 1,25-dihydroxyvitamin D3. These drugs slow bone resorption but have little effect on the stimulation of bone formation. Other agents being researched are fluoride and parathyroid hormone; these may stimulate bone formation but are not proven to reduce the rates of fracture.
Because of the lack of a cure for osteoporosis, the prevention of excessive bone loss is the current focus. Approximately 80 percent of bone mass is genetically determined. The other 20 percent can be modified by lifestyle factors. Adequate calcium intake, weight-bearing exercise, and estrogen-replacement therapy for women who have entered menopause are the primary lifestyle factors associated with reducing the risk of osteoporosis. Factors associated with increased risk of osteoporosis include smoking and abuse of alcohol and caffeine. Thin, small-framed women are more vulnerable to osteoporosis, and Caucasians and Asians are at higher risk than African-Americans. Women are at greater risk than men by a ratio of 4 to 1. Women have less hone mass, experience accelerated loss of bone mass following menopause, and ingest less calcium than do men.
A researcher at the USDA Human Nutrition Research Center at Tufts University (Boston, MA), Bess Dawson-Hughes, has shown that calcium supplements can prevent the usual bone loss associated with aging in women who consume less than 400 milligrams (mg) of calcium per day. Furthermore, two studies have reported that the risk of hip fracture is reduced by as much as 60 percent on higher calcium intakes.
Other nutrients that are important to the skeleton are protein, vitamins C and D, phosphorus, magnesium, manganese, copper, zinc, and boron.
Estrogen-replacement therapy can also prevent or retard bone loss in perimenopausal and postmenopausal women as long as the therapy is continued and the dietary calcium intake is sufficient. Calcium supplementation in combination with estrogen replacement has synergistic positive effects on bone loss; that is, the effectiveness of each treatment is enhanced.