Nitrates and Bone Turnover; Nitrates and Bone Turnover Bisphosphonate Sub-Study

Inclusion Criteria: NABT: * Women aged 50 years and older whose last menstrual period occurred at least 3 years ago * Women without a uterus will be eligible after age 55 NABT-B: * Women aged 50 years and older whose last menstrual period occurred at least 3 years ago * Women without a uterus will be eligible after age 55 * Previously treated with alendronate but stopped within 2 years of study commencement; or previously treated with risedronate but stopped within 1 year of study commencement; or previously treated with zoledronate. Exclusion Criteria: NABT: * A previous fracture of the hip, wrist, spine or ankle within the last 3 months; or a diagnosis of osteoporosis; * A history of bone disorders such as hyperparathyroidism or Paget's disease; * Treatment within 12 months of study entry with any agent that may influence bone metabolism including any hormone, anti-estrogen or raloxifene and prednisone (equivalent to 5 mg/d for 12 months or greater); * Treatment with any antiresorptive agent, including alendronate, risedronate, etidronate or denosumab use for at least four weeks within the last three years; * Previous treatment with intravenous zoledronate or parathyroid hormone; * Current treatment with nitrates; * A history of migraine headaches; * A history of angina or cardiovascular disease; * Inability to give informed consent; * Hypersensitivity to nitroglycerin. NABT-B: * A previous fracture of the hip, wrist, spine or ankle; or a self-reported diagnosis of osteoporosis; * A history of bone disorders such as hyperparathyroidism or Paget's disease; * Treatment within 12 months of study entry with any agent that may influence bone metabolism including any hormone, anti-estrogen or raloxifene and prednisone (equivalent to 5 mg/d for 12 months or greater); * Treatment with etidronate or denosumab use for at least four weeks within the last three years and any previous treatment with parathyroid hormone; * Current treatment with nitrates; * A history of migraine headaches; * A history of angina or cardiovascular disease; * Inability to give informed consent; * Hypersensitivity to nitroglycerin.