These micronutrients are discussed elsewhere (see 00033 and 00051). Childhood is an important time for deposition of bone mineral and the development of peak bone mass (PBM). Seventy-five per cent of bone mineral is deposited in childhood. Low PBM in late adolescence is a significant precursor of later osteoporosis. Much of the population variation in PBM is genetically determined, but low calcium and vitamin D together with a relatively sedentary lifestyle seem factors likely to contribute to low PBM and risks of osteoporosis later in life. Data from The Gambia show that in preadolescent children adequate calcium deposition takes place despite very low intakes of calcium. In some studies where milk-derived calcium phosphate was fed to children there was accelerated growth and maturation in the supplemented children. The Gambian studies showed no change in growth in the supplemented children although there was increased bone mineralization.
Vigorous calcium deposition in children in the tropics and subtropics despite very low calcium intakes may relate to higher circulating 25 hydroxy vitamin D derived from the action of sunlight on 7-dehydrocholesterol in the skin and to high levels of physical activity amongst these children. Deposition of calcium in bone is significantly affected by the weight-bearing activity of the individual. Where children are less active and vitamin D levels lower, dietary calcium intakes may have a greater determining effect on bone mineralization. In northerly temperate climates, such as the UK, children's diets should provide a good source of calcium and there should be reasonable exposure to summer sunshine (a slightly controversial area in view of current concerns about increased skin cancer risk from UVL). An active lifestyle is also important for optimal bone mineralization and high PBM, particularly in those where there is reason to suspect genetic predisposition to low PBM.
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