Enhancing Skeletal Adaptations to PTH and Exercise

Inclusion Criteria: Female and male Veterans aged 25 to 45 y and 55 to 75 y will be enrolled. Eligible volunteers will be normally active (e.g., recreational cycling or walking) but will not participate in regular moderate-to-vigorous exercise. Women will be premenopausal with regular menstrual cycles or postmenopausal, defined as absence of menses for at least 12 mo or, in those who underwent a hysterectomy, a serum follicle stimulating hormone (FSH) \>30 mIU/mL. Exclusion Criteria: * Initiation or change in dose in the past 6 months of medications that affect bone metabolism * e.g., osteoporosis medications, thiazide/loop diuretics, systemic glucocorticoids * BMD T-score \<-2.5 at the total hip, femoral neck, or lumbar spine * Impaired renal function, defined as an estimated glomerular filtration rate (eGFR) \<60 mL/min/1.73m2 * Abnormal alkaline phosphatase * Untreated thyroid dysfunction, defined as an ultrasensitive thyroid stimulating hormone (TSH) \<0.5 or \>5.0 mU/L * Serum Ca \<8.5 or \>10.3 mg/dL * Serum 25(OH)D \<20 ng/mL * Uncontrolled hypertension (resting systolic blood pressure (BP) \>150 mmHg or diastolic BP \>90 mmHg) * Type 1 diabetes * Type 2 diabetes if on insulin or sulfonylurea therapy * hemoglobin A1c \>7% * Cardiovascular disease; defined as subjective or objective indicators of ischemic heart disease (e.g., angina, ST segment depression) or serious arrhythmias at rest or during the graded exercise test (GXT) * volunteers who have a positive G…