Stearic Acid and Endothelial Function

The goal of this clinical trial is to evaluate the postprandial and long-term effects of dietary stearic acid on vascular function in overweight or obese adult males and females aged 40-70 years. The main questions it aims to answer are: 1. Does consuming a high stearic acid-rich meal improve postprandial vascular function compared to a high oleic acid-rich meal? 2. Does a 14-day stearic acid-rich diet improve fasting vascular function compared to an oleic acid-rich diet? 3. Does a 14-day stearic acid-rich diet reduce the impairment of postprandial vascular function following a high palmitic-acid challenge meal? Participants will: * Complete screening questionnaires, a 3-day food log, body composition measurements, and fasting blood and urine tests. * Consume a provided, 14-day controlled diet high in palmitic acid while avoiding outside food and beverages other than water. * Be randomly assigned to consume a provided 14-day diet high in either stearic acid (cocoa butter blend) or oleic acid (plant oil blend). * Attend two 5-hour testing visits at the clinic after overnight fasting. * Undergo intravenous (IV) catheter placement for repeated blood sampling and non-invasive vascular health assessments (ultrasound artery checks and pulse measurements). * Consume assigned high-fat test meals within 15 minutes and undergo hourly vascular and blood assessments over a 4-hour period.