Subjects will include 15 newly diagnosed NS patients, 60 patients with pre-existing NS, 90 patients with newly diagnosed CD, 45 patients diagnosed within the last two years and 200 healthy controls of similar age, gender and ethnicity. Participants will visit the Children's Hospital of Philadelphia (CHOP) three times over a 12-month period for assessment of bone mineralization and turnover, fracture history, dietary calcium intake, physical activity, growth, body composition, muscle strength and glucocorticoid exposure.