Acute Cardiovascular Effects of an L-Theanine and L-Glycine Beverage in Postmenopausal Women at Rest

The study is a randomized, double-blind, placebo-controlled, parallel trial consisting of two visits. The first visit for baseline assessments and the second for the supplementation intervention. At the first visit, participants will fast overnight for 10 hours before capillary blood (~100 µL) is collected to measure cardiometabolic risk biomarkers, including fasting glucose, HbA1c, total cholesterol, triglycerides, and HDL-C. Following blood collection, participants will receive breakfast, and baseline anthropometric measurements (height, weight, waist, and hip circumferences) and physical performance tests (grip strength, gait speed, chair-rise ability, and submaximal exercise test) will be conducted. The second visit, occurring within one week, involves a 15-minute supine rest followed by baseline measurements of resting heart rate (HR), heart rate variability (HRV), blood pressure (BP), and skin microvascular function. The aim of the skin microvascular test is to measure the reactive hyperaemia response, providing insights into vascular reactivity and endothelial function of the microcirculation. Participants will then consume either the active supplement containing 5 g glycine, 200 mg L-theanine amino acids, 3.3 g sweetener, 10mg watermelon flavour, and 1.5 g citric acid, or a colour, flavour, and Odor matched placebo dissolved in 300 mL of water and consumed within 5 minutes. Adherence to the intervention was ensured by direct observation. After the supplement or placebo was provided, research team monitored participants as they drank the entire beverage, confirming that the full dose was consumed as intended. Any issues during intake were recorded immediately. Post-intervention measurements of HR, HRV, BP, and skin microvascular function will be repeated at 60 and 120 minutes. Dietary intake will be assessed using a 24-hour recall for the day prior to the second visit, along with a brief recall of breakfast on the day of supplementation.