In order to better understand how to optimize vitamin D status in infants, newer biomarkers of bone formation and resorption should be included to more comprehensively assess bone metabolism in association with 25(OH)D, PTH and whole body BMC. Suggested biomarkers include plasma osteocalcin and urinary N-telopeptide. Osteocalcin increases with growth and bone mineralization in the first year of life, as does alkaline phosphatase (40). Osteocalcin is the superior marker, since it also reflects seasonal changes in 25(OH)D and PTH in infancy (41). For bone resorption, urinary N-telopeptide is a practical marker, as it can be measured in urine and is also a specific index for bone resorption. Excretion of N-telopeptide in a spot urine sample in infants over the first year of life can be used with confidence for comparing groups of patients (42).