Healthy bones – Activity and nutrition

Osteoporosis has been defined as a systemic skeletal disease characterized by reduced bone mass and altered microarchitecture, resulting in increased bone fragility and susceptibility to fractures. It has been estimated that 30% to 40% of adults over 60 years of age have osteoporosis. The significance of this disease lies in the extraordinarily high personal health and economic costs, estimated to be $14 billion per annum in the United States (1). The primary determinant of osteoporosis risk is peak bone mass (PBM), the maximum amount of whole body bone mineral content that is dependent, in turn, on net bone acquisition during childhood and adolescence (2). Bone comprises a collagen matrix into which calcium and phosphate are deposited as a hydroxyapatite. The establishment and ongoing maintenance of bone is a dynamic equilibrium between formation (directed by osteoblasts) and resorption (controlled by osteoclasts). Total skeletal calcium increases from approximately 25 g at birth to 900 g and 1200 g in women and men, respectively. Ninety per cent of PBM is acquired by the age of 18 years (3). During childhood and adolescence, bone formation predominates, leading to a net gain of bone. Within this period, the highest acquisition rates take place during the rapid growth phases of early infancy and peak height velocity of puberty, during which bone size and bone mass increase rapidly. Thus, childhood and adolescence are the major periods during which bone health is established or compromised. In early adulthood, PBM remains stable until the end of the third decade of life, after which age-related bone loss begins (1). Approximately 50% to 80% of PBM is determined by genetic factors, while nutrition, physical activity, endocrine and other lifestyle factors such as smoking account for the remaining 20% (1). Nutrition and physical exercise are key determinants of bone mass that are potentially and readily modifiable. As will be shown, increasing calcium intake and exercise has a relatively small impact on bone mineral content. However, even a small positive change in bone mass density (BMD) translates into a significant reduction in the risk of osteoporosis and fractures in the adult population. It has been estimated that a 7% increase in the population average for BMD will reduce the risk of hip fracture by nearly 50% (3). A sufficient calcium intake is needed to achieve and maintain PBM. During the phases of especially rapid growth in infancy and adolescence, there is an increased requirement for calcium. The relationship between calcium intake and bone mass has been examined in several studies in children and adolescents (4,5). Many studies have shown a clear relationship between increased calcium intake and increased bone mass. Some studies indicate that the benefits of calcium may be regional – that is, they may be restricted to the radial and femoral sites, or confined to specific groups such as prepubertal girls and individuals with low basal calcium intakes (4). Furthermore, sustained benefit appears to be achieved only with ongoing calcium supplementation (6). Recommendations for calcium intake have been established based on the need of the rapidly growing skeleton and the relationship of calcium intake to PBM. The recommended daily intake (RDI) of calcium is not achieved frequently in the general population. Some studies suggest that most adolescents may take in only about 50% of the RDI of calcium for their age group (1). Calcium is plentiful in dietary sources (Tables 1,2), and these sources should provide the primary means of achieving the RDI. Dietary assessment often reveals a calcium-poor diet, which should result in the recommendation of an improved intake of calcium-rich foods and/or supplementation. The recommendations of the National Academy of Science for daily calcium intake are provided in the Table 3. Dietary sources of calcium (where one serving contains approximately 300 mg of calcium) Calcium is listed immediately after “soy milk” or “soybeans and water”. Data from Sunnybrook & Women's College Health Science Centre Multidisciplinary Osteoporosis Program; Bowes & Church's Food Values of Portions Commonly Consumed, 6th Edition, 1994; Bon Vivant! Jan Main, 1997; Osteoporosis Society of Canada: Building Better Bones: A Guide to Active Living Dietary sources of calcium (where one serving contains approximately 300 mg of calcium) Calcium is listed immediately after “soy milk” or “soybeans and water”. Data from Sunnybrook & Women's College Health Science Centre Multidisciplinary Osteoporosis Program; Bowes & Church's Food Values of Portions Commonly Consumed, 6th Edition, 1994; Bon Vivant! Jan Main, 1997; Osteoporosis Society of Canada: Building Better Bones: A Guide to Active Living Dietary sources of calcium (where one serving contains approximately 150 mg of calcium) Data from Sunnybrook & Women's College Health Science Centre Multidisciplinary Osteoporosis Program; Bowes & Church's Food Values of Portions Commonly Consumed, 6th Edition, 1994; Bon Vivant! Jan Main, 1997; Osteoporosis Society of Canada: Building Better Bones: A Guide to Active Living Dietary sources of calcium (where one serving contains approximately 150 mg of calcium) Data from Sunnybrook & Women's College Health Science Centre Multidisciplinary Osteoporosis Program; Bowes & Church's Food Values of Portions Commonly Consumed, 6th Edition, 1994; Bon Vivant! Jan Main, 1997; Osteoporosis Society of Canada: Building Better Bones: A Guide to Active Living National Academy of Science adequate calcium intake guidelines National Academy of Science adequate calcium intake guidelines Calcium intake is ineffective unless it is coupled with a sufficient intake of vitamin D. Unlike calcium, most dietary sources are deficient in vitamin D. People who receive adequate sunlight exposure are protected by endogenous skin production of vitamin D from cholesterol metabolites. However, in northern climates, there may be a need for supplementation with vitamin D in the winter months in those people whose diet contains little of this vitamin. Severe vitamin D deficiency results in rickets. While the effects of milder degrees of vitamin D deficiency on the skeleton are not well documented, a 12-month study of bone mineral content has shown that baseline 1,25-dihydroxyvitamin D predicts gains in total BMD accretion in children and adolescents (7). The current recommendation for vitamin D is 400 IU/day, although many authorities have suggested that higher amounts may be required. In healthy individuals, calcium and vitamin D are necessary, but are not sufficient for optimum bone health. For this, adequate weight-bearing physical activity is needed. Several studies have shown that physical activity is a determinant of bone mass (8). In children and adolescents there is good evidence that increased activity is associated with increased bone mass. High impact loading in sports such as gymnastics and racquet sports produces a significant increase in regional bone mass. However, most of this benefit seems to be achieved in individuals during early puberty. The feasibility of implementing physical activities that will benefit bone mass in the school curriculum has also been demonstrated (9). It is important to note that it is during growth, rather than in adulthood, that exercise produces its most beneficial effects. Alcohol and smoking can negatively impact bone mass and may be important issues to address with the adolescent. All the major hormones affect bone in one way or another. In general, most endocrine disorders have an impact on bone health and none more than Cushing's Syndrome, with exogenous glucocorticoids being the most important cause of bone loss. Furthermore, most chronic diseases of childhood may have very negative effects on bone health.

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