Inclusion Criteria:  Healthy older (60+ y) Veteran women and men will be eligible to participate All volunteers must be accustomed to performing 60 minutes of vigorous cycling and treadmill exercise  Exclusion Criteria:  impaired renal function, defined as an eGRF of <60 mL/min/1.73m2;(Florkowski and Chew-Harris 2011) hepatobiliary disease, defined as liver function tests (AST, ALT) >1.5 times the upper limit of normal 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; 5) serum 25(OH)D <20 ng/mL uncontrolled hypertension, defined as resting systolic blood pressure (BP) >150 mmHg or diastolic BP >90 mmHg history of type 1 or type 2 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, which must include diagnostic testing (e.g., stress echocardiogram or thallium stress test) with interpretation by a cardiologist anemia, defined as a serum hemoglobin <12.1 g/dL for women and <14.3 g/dL for men fracture in the past 6 months  current diagnosis or symptoms of COVID-19  In the event of abnormal BP, live function, TSH, 25(OH)D, or hemoglobin values, volunteers can be reassessed, including after appropriate follow-up evaluation and treatment by a primary care provider Those who have experienced symptoms of COVID-19 or have been formally diagnosed will be allowed to participate once symptoms have resolved and they are approved to return to exercise by their primary care provider