Safety & Efficacy of Nicotinamide Riboside Supplementation for Improving Physiological Function in Middle-Aged and Older Adults

Inclusion Criteria: * Ability to provide informed consent * Baseline brachial flow-mediated dilation (FMD) \< 6%Δ (rationale: non-invasive screening to ensure exclusion of subjects with exceptionally high baseline endothelial function. * Ability to perform motor and cognitive tests (e.g., can rise from a chair, walk for 2 min, climb 10 stairs) * Women will be confirmed as postmenopausal (either natural or surgical) based on cessation of menses for \>1 year. Exclusion Criteria: * ages \<55 years or ≥80 years * body mass index (BMI) \>40 kg/m2 (rationale: vascular function and blood pressure measurements can be inaccurate in severely obese subjects and these subjects may differ in many ways from normal weight, overweight or less obese subjects) * not weight stable in the prior 3 months (\>2 kg weight change) or unwilling to remain weight stable throughout study (rationale: recent weight change or weight loss can influence vascular function) * having unstable angina, acute myocardial infarction, coronary angioplasty, or aorto-coronary bypass surgery as defined by the occurrence of an event, symptom, surgery, or change in medication and/or dosage within 3 months prior to enrollment * having thyroid disease that is not controlled by medications or \<3 month's use of a particular medication and/or dosage (rationale: uncontrolled thyroid diseases are associated with alterations in vascular function) * having past or present alcohol dependence or abuse, as defined by the American Psychiatry Association, Diagnostic and Statistical Manual of Mental Disorders * abnormal blood chemistries for renal and liver function (\>1 standard deviation outside the normal range) * Moderate or severe peripheral artery disease (ankle-brachial index \<0.7). * Insufficient health to participate in a VO2max test based on a physical examination and/or graded exercise test (this will be determined in accordance with stated contraindications for exercise testing provided by the American Heart Association).