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Abstract
We determined the effect of changes in vitamin D metabolites on muscle in 323 children (9-13 y), from US latitudes 34N and 40N, and randomly assigned to either: 0, 400, 1,000, 2,000, or 4,000 IU/d vitamin D3 for 12 weeks (in winter). Muscle parameters [fat-free mass, muscle cross-sectional area (MCSA), muscle density (MD), intermuscular adipose tissue (IMAT), and forearm strength] and serum [25-hydroxyvitamin D (25(OH)D), 1,25-diydroxyvitamin D (1,25(OH)2D), and intact parathyroid hormone (iPTH)] were measured at baseline and 12 weeks. 25(OH)D and 1,25(OH2D, but not iPTH, increased significantly over 12 weeks. Significant correlations existed between 1,25(OH)2D and arm MCSA, and between iPTH and leg MCSA, leg MD, and leg IMAT. These data suggest that 25(OH)D is not a sensitive biomarker of muscle development, and iPTH may play a key role in adolescent muscle growth. Whether these observations with respect to iPTH and muscle are clinically relevant should be further explored.