What factors are involved in building and maintaining skeletal health throughout life?

Growth in bone size and strength occurs during childhood, but bone accumulation is not completed until the third decade of life, after the cessation of linear growth. The bone mass attained early in life is perhaps the most important determinant of life-long skeletal health. Individuals with the highest peak bone mass after adolescence have the greatest protective advantage when the inexorable declines in bone density associated with increasing age, illness, and diminished sex-steroid production take their toll. Bone mass may be related not only to osteoporosis and fragility later in life but also to fractures in childhood and adolescence. Genetic factors exert a strong and perhaps predominant influence on peak bone mass, but physiological, environmental, and modifiable lifestyle factors can also play a significant role. Among these are adequate nutrition and body weight, exposure to sex hormones at puberty, and physical activity. Thus, maximizing bone mass early in life presents a critical opportunity to reduce the impact of bone loss related to aging. Childhood is also a critical time for the development of lifestyle habits conducive to maintaining good bone health throughout life. Cigarette smoking, which usually starts in adolescence, may have a deleterious effect on achieving bone mass.

Nutrition

Good nutrition is essential for normal growth. A balanced diet, adequate calories, and appropriate nutrients are the foundation for development of all tissues, including bone. Adequate and appropriate nutrition is important for all individuals, but not all follow a diet that is optimal for bone health. Supplementation of calcium and vitamin D may be necessary. In particular, excessive pursuit of thinness may affect adequate nutrition and bone health.

Calcium is the specific nutrient most important for attaining peak bone mass and for preventing and treating osteoporosis. Sufficient data exist to recommend specific dietary calcium intakes at various stages of life. Although the Institute of Medicine recommends calcium intakes of 800 mg/day for children ages 3 to 8 and 1,300 mg/day for children and adolescents ages 9 to 17, only about 25 percent of boys and 10 percent of girls ages 9 to 17 are estimated to meet these recommendations. Factors contributing to low calcium intakes are restriction of dairy products, a generally low level of fruit and vegetable consumption, and a high intake of low calcium beverages such as sodas. For older adults, calcium intake should be maintained at 1,000 to 1,500 mg/day, yet only about 50 to 60 percent of this population meets this recommendation.

Vitamin D is required for optimal calcium absorption and thus is also important for bone health. Most infants and young children in the United States have adequate vitamin D intake because of supplementation and fortification of milk. During adolescence, when consumption of dairy products decreases, vitamin D intake is less likely to be adequate, and this may adversely affect calcium absorption. A recommended vitamin D intake of 400 to 600 IU/day has been established for adults.

Other nutrients have been evaluated for their relation to bone health. High dietary protein, caffeine, phosphorus, and sodium can adversely affect calcium balance, but their effects appear not to be important in individuals with adequate calcium intakes.

Exercise

Regular physical activity has numerous health benefits for individuals of all ages. The specific effects of physical activity on bone health have been investigated in randomized clinical trials and observational studies. There is strong evidence that physical activity early in life contributes to higher peak bone mass. Some evidence indicates that resistance and high impact exercise are likely the most beneficial. Exercise during the middle years of life has numerous health benefits, but there are few studies on the effects of exercise on BMD. Exercise during the later years, in the presence of adequate calcium and vitamin D intake, probably has a modest effect on slowing the decline in BMD. It is clear that exercise late in life, even beyond 90 years of age, can increase muscle mass and strength twofold or more in frail individuals. There is convincing evidence that exercise in elderly persons also improves function and delays loss of independence and thus contributes to quality of life. Randomized clinical trials of exercise have been shown to reduce the risk of falls by approximately 25 percent, but there is no experimental evidence that exercise affects fracture rates. It also is possible that regular exercisers might fall differently and thereby reduce the risk of fracture due to falls, but this hypothesis requires testing.