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, proton pump inhibitors 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  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 GXT can be re-considered after follow-up evaluation by a cardiologist Anemia (hemoglobin <12.1 g/dL for women, <14.3 g/dL for men) Fracture in the past 6 months  Body mass index >39 kg/m2  In the event of abnormal eGFR, alkaline phosphatase, TSH, BP, 25(OH)D, or hemoglobin values, volunteers can be reassessed, including after appropriate follow-up evaluation and treatment by their health care provider