Calcium, physical activity and bone health--building bones for a stronger future.

Adequate provision of nutrients composing the bone matrix and regulating bone metabolism should be provided from birth in order to achieve maximal bone mass, compatible with individual genetic background, and to prevent osteoporosis later in life. Low calcium intake (<250 mg day(-1)) in children is associated with both a reduced bone mineral content and hyperparathyroidism. Optimal calcium intake is, however, still a matter of controversy. The minimisation of fracture risk would be the ideal functional outcome on which to evaluate lifetime calcium intakes, but proxy indicators, such as bone mass measurements or maximal calcium retention, are used instead. Calcium recommendations in Europe and the United States are based on different concepts as to requirements, leading to somewhat different interpretations of dietary adequacy. Minerals and trace elements other than calcium are involved in skeletal growth, some of them as matrix constituents, such as magnesium and fluoride, others as components of enzymatic systems involved in matrix turnover, such as zinc, copper and manganese. Vitamins also play a role in calcium metabolism (e.g. vitamin D) or as co-factors of key enzymes for skeletal metabolism (e.g. vitamins C and K). Physical activity has different effects on bone depending on its intensity, frequency, duration and the age at which it is started. The anabolic effect on bone is greater in adolescence and as a result of weight-bearing exercise. Adequate intakes of calcium appear necessary for exercise to have its bone stimulating action.