Inclusion Criteria:  Lean: BMI < 25 kg/m Normal insulin sensitivity: (calculation based on fasting glucose and insulin concentrations in blood) Middle-aged: 18-79 years Blood pressure: < 140 (systolic) and < 90 mmHg (diastolic Weight stable (+/- 5 lbs) for the previous 3 months Participants must be willing and able to discontinue taking any vitamin C or E supplements or omega-3 fatty acids beginning 2 weeks prior. Able and willing to provide written informed consent  Exclusion Criteria:  Diabetes mellitus Type I or Type II: fasting glucose < 126 mg/dL and/or HbA1c < 6.5% Current smoker or History of smoking in the past 3 months. Hyperlipidemia: Fasting triglycerides < 250 mg/dL Hypertension: >130/80 mmHg History of heart disease (e.g., myocardial infarction, stent, heart attack, stroke, heart failure, valvular heart disease, cardiomyopathy) History of vascular disease (e.g., bypass, stroke)