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Med J Aust 2000; 173 Suppl 7 August: S12-S13

Normal bone development
Peak bone mineral density (BMD) is the maximal lifetime amount of bone tissue accrued in the skeleton during growth; it may be a more important determinant of low BMD in old age than age-related bone loss. Therefore, maximising the attainment of peak bone mass is now considered to be an important component of osteoporosis prevention. There is a large variation in the normal range for peak BMD that is influenced by both genetic and environmental factors.⁸⁰ Physical activity and diet may be the most important modifiable environmental factors that can increase peak BMD for both children and adults.⁸⁰

Physical activity
For physical activity to have a role in preventing osteoporosis, the osteotrophic response must be large enough to be considered clinically important and the benefits must be maintained into later life when fractures occur. Large increases in BMD (up to 30%) have been reported in children involved in exercise training, but it is not known if these gains are maintained into adulthood.⁸⁰ The site-specific higher bone density related to unique loading patterns reported in retired athletes supports the notion that large osteotrophic benefits (enough to halve the risk of fracture) from exercise during childhood can be maintained into adulthood.

These data provide information about what is possible in young elite athletes, but not what is probable in healthy non-athletic children. However, the results of many retrospective studies do support the notion that weight-bearing physical activity in healthy non-athletic children is associated with higher BMD in adulthood.⁸⁰ The osteotrophic increases reported in non-athletic children are more modest (1%-10%) compared with athletes, although they are still large enough to reduce the risk of fracture. A 5%-10% increase in BMD theoretically reduces fracture rates by 25% to 50%. Exercise prescription details at this stage are limited to recommending exercise regimens that include moderate to high impact loading on the skeleton (hopping, skipping, and jumping). Typical sports that involve moderate to high impact loading include basketball, netball and gymnastics. Other prescription details, such as how much or how often children need to exercise to elicit a clinically important increase in bone density, are unknown.^80,81 It is also not known how long children need to exercise before residual benefits will be maintained into adulthood, or if there is an optimal time during growth (prepuberty or peripuberty) when exercise results in the greatest osteotrophic response.^80,81

Calcium
The calcium requirement during childhood is based on the amount of dietary calcium needed to maintain calcium balance and optimal bone accrual. In the first 20 years of life, approximately 1500 g of calcium is accrued in the skeleton -- this is nearly three times the amount of calcium that is lost during 40 years of ageing. Of this 1500 g of calcium, approximately 50% is accrued in the prepubertal years and 50% is accrued during puberty.^82,83 Consequently, calcium requirements are greatest during growth, and in particular during puberty.

The results of the National Nutrition Survey showed that the diets of 77% of girls and 64% of boys aged 12-15 years (the time of peak mineral accrual in both sexes) did not contain the recommended daily intake for calcium.² Further, girls of all ages (4-18 years) were not consuming adequate calcium.²

Calcium is well absorbed from dairy products, but poorly absorbed from foods rich in oxalic acid, such as spinach and rhubarb. The National Nutrition Survey found that the major sources of calcium for Australian children were dairy foods.² Milk alone accounted for the total calcium intake in 38% of boys and 32% of girls aged 12-18 years.²

Shona L Bass and Deborah A Kerr

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