We thank Dr Sugiyama and colleagues for their comments  on our recent review on the role of vitamin D status in bone health during growth , and read with great interest their hypothesis relating to potential mechanisms.
As Sugiyama et al. suggest, there are complex relationships among bone mineral density (BMD), bone size, bone strength and fracture risk in children. Indeed, physical activity has been positively associated with both measures of bone size and density in children [3, 4], yet is also associated with increased fracture risk in childhood . The mechanostat theory recognises that muscular strains stimulate bone modelling and mineralisation ; however, more active children are also likely to have greater exposure to potential fracture-causing events. It is possible that a similar explanation also confounds associations between childhood vitamin D status and fracture occurrence: children following sedentary indoor lifestyles are more likely to have...
KeywordsBone Mineral Density Fracture Risk Bone Size Volumetric Bone Mineral Density Maternal Educational Attainment
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