4.3.3 Bone markers: Change in bone metabolism in response to 25(OH)D will be assessed by a marker of osteoblast activity related to mineralization, plasma osteocalcin (20 ul; Diasorin). Osteoclast activity will be assessed by measuring urinary N-telopeptide (ELISA, Osteomark) corrected to creatinine. The assay is specific to Type 1 collagen with a CV% <8%. N-telopeptide measurements urine selected over blood to reserve the small blood sample for other measurements. Urinary N-telopeptide in spot urine samples is a validated measurement in infancy (42). The PI is experienced in all of these measurements (6, 48).