Vitamin D (25(OH)D), osteocalcin (OC) and PTH concentration will be measured the same day (or batched monthly) using a Liaison autoanalyzer (DiaSorin): Total Vitamin D (sensitivity: 4 ng/ml); bioactive PTH kit (sensitivity <4 pg/ml) and OC (sensitivity 0.3 ng/mL) kits. In batches, manual ELISA will be used for CTX. Ionized calcium will be measured on-site using a radiometer blood gas tube within 4 h of collection; this will also provide us with hemoglobin and hematocrit concentrations. Total serum plus urinary calcium and creatinine and lipid assessments including serum HDL, LDL and total cholesterol (LDL) and triacylglycerols will be measured through the MUHC clinical laboratory fee for service system.