Maintaining a Strong Skeleton
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.
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.
Early-Life Steps To Prevent Osteoporosis
Building bone mass in early life may be the most effective way to prevent osteoporosis in later life. If this opportunity is missed, it probably cannot be made up. Even small increases in bone mass can have a great impact on the risk of fracture. For instance, a 5-percent increase in bone mass can reduce the risk of osteoporotic fracture by 40 percent.
A lifelong habit of drinking milk is associated with increased bone mass. Researchers at the Indiana University School of Medicine in Indianapolis have shown that calcium supplements increase the bone mass in preadolescent children, compared to that in their identical twins who received placebos during a 3-year study.
Getting Enough Calcium
In the American diet, almost 75 percent of dietary calcium comes from dairy products. Few other foods are concentrated sources of absorbable calcium. At Purdue University (West Lafayette, IN) and Creighton University (Omaha, NE), plant foods are being screened for calcium absorption. These include broccoli, bok choy, kale, and tofu made with calcium salts. Calcium is well absorbed from these vegetables and from all dairy products—that is, milk, yogurt, cheese, processed cheese, and their low-fat counterparts. Spinach is a concentrated source of calcium, but this calcium is poorly absorbed because it is complexed with oxalic acid and is therefore indigestible.
Depending on their stage of growth, people need 2 to 5 cups of milk or the calcium equivalent each day. American females more than 12 years old typically consume less calcium than this recommended amount. Calcium intake in American women is 40 to 50 percent below that in men. A 1984 National Institutes of Health consensus-development conference recommended 1,000 mg of calcium per day for premenopausal women and 1,500 mg per day for postmenopausal women. However, 25 percent of American women have an intake below 300 mg per day, which is the amount of calcium in one glass of milk. Calcium supplements are recommended for individuals who cannot get adequate calcium through diet. However, supplements do not contain all the nutrients necessary for building bones, and people often forget to take pills. An alternative source of calcium is the fortified beverages now on the market.
Weight-bearing exercise has a positive impact on bone density. An effective exercise program applies weight loading to all parts of the skeleton. For example, the right arm of a right-handed tennis player has a higher bone density than does the left arm. Activities that are exclusively aerobic seem to be the least effective in building peak hone mass. Thus, weight lifters have higher bone density than do swimmers. We do not know if the positive effects of exercise on bone mass are retained when exercise is discontinued.
A partial explanation for bone loss in the elderly is the reduction in physical activity with age. The physical work of the average sedentary elderly adult is 30 percent less than that of the average younger adult. If immobilization occurs, bone loss is accelerated; but bone mass can increase when the individual again becomes ambulatory.
The Known and Unknown
Obtaining adequate dietary calcium, exercise, and estrogen-replacement therapy following menopause are three lifestyle choices for maintaining a strong skeleton. The interaction of these factors is not well understood. Nor do we know the residual positive effect after cessation of treatment. Research to determine the best food sources of absorbable calcium and the most effective exercise programs, in combination with education programs on behavior modification, can help reduce the suffering and the healthcare costs associated with bone loss.