Effect of EGCG on the Body's Response to Insulin

Overview

This study will examine whether epigallocatechin gallate (EGCG), a major component of green tea, affects how the body responds to insulin in healthy and obese people. Insulin is not as effective in people who are overweight, have high blood pressure or diabetes. This condition is known as insulin resistance. Laboratory studies suggest that green tea or EGCG treatment lowers blood pressure, lowers blood sugar and increases blood flow. This study will see if EGCG improves insulin resistance or insulin's effects on blood flow in people with insulin resistance. Healthy normal weight or overweight people between 21 and 65 years of age may be eligible for this study. Participants are randomly assigned to take EGCG or a placebo ( inactive dummy pill ) in two 4-week treatment phases with a 2-week period of no study medication before each treatment phase. After the first 4-week treatment, patients on placebo are switched to EGCG and those on EGCG are switched to placebo. In addition to treatment, participants undergo the following procedures during the study period: * Screening, including medical history, physical examination and blood and urine tests, and finger-stick blood sugar measurement for patients with diabetes * Complete a dietary and physical activity questionnaire and consult with a dietitian * Blood and urine tests * At-home and clinic blood pressure monitoring * Glucose clamp test to measure how the body responds to insulin. This test is done three times during the study. A needle is placed in a vein in each of the subject's arms, one for sampling blood and the other for infusing insulin, glucose and potassium. Glucose and insulin levels, electrolytes, lipids, fatty acids, cytokines and epicatechin are measured. * Forearm blood flow measurement with microbubbles and ultrasound. Before beginning the glucose clamp test, a test of how well the blood vessels relax is done. A device that measures the size of the artery in the upper arm is placed above the elbow. Blood flow in the muscle of the forearm is measured by ultrasound using a small infusion through a vein of microbubble contrast agent consisting of gas-filled bubbles the size of red blood cells. The contrast agent is infused over a 7- to 9-minute period at the beginning of the glucose clamp test and again 2 hours after the beginning of the test.

Green tea is a functional food whose consumption is associated with improved cardiovascular morbidity and mortality in several large epidemiological studies. One third of the solids in green tea are composed of the bioactive polyphenol epigallocatechin 3-gallate (EGCG). Studies in both cell- and animal-based models (from our lab and elsewhere) suggest that EGCG may mimic and/or augment beneficial metabolic, vascular, and anti-inflammatory actions of insulin. Indeed, we have recently shown that 3-week EGCG therapy of SHR rats (genetic model of hypertension with features of human metabolic syndrome including insulin resistance, hyperinsulinemia, endothelial dysfunction, and overweight) lowers blood pressure, improves endothelial dysfunction, increases insulin sensitivity, and raises adiponectin levels nearly as effectively as treatment with the conventional ACE-inhibitor enalapril. Obesity, type 2 diabetes, and hypertension are all important interrelated public health problems that are characterized by reciprocal relationships between insulin resistance and endothelial dysfunction. Thus, therapies for these diseases that improve insulin resistance often simultaneously improve endothelial function and vice versa. Based on results from cellular, physiological, and epidemiological studies, we hypothesize that oral EGCG administration will simultaneously ameliorate insulin resistance and lower blood pressure in human subjects with obesity. To test these hypotheses, we will conduct a randomized, placebo-controlled, double-blind, cross-over study to evaluate potential beneficial effects of EGCG to modulate insulin sensitivity, blood pressure, vascular function, and inflammatory markers in two groups of subjects (lean healthy controls, obesity). After a 2-week EGCG-free run-in period, each subject will be randomized to receive EGCG or placebo capsules (400 mg p.o. B.I.D.) for 4 weeks. This will be followed by a 2-week EGCG-free washout period after which subjects will cross-over to the other treatment arm. At baseline, and after each 4-week treatment period, we will assess insulin sensitivity (hyperinsulinemic isoglycemic glucose clamp technique) and vascular function. Regarding vascular function, we will measure basal and insulin-stimulated brachial artery blood flow (large conduit artery assessed by Doppler ultrasound) as well as capillary recruitment in forearm skeletal muscle (small nutritive arterioles assessed by ultrasound with microbubble contrast). Blood pressure will be measured weekly in the UMB GCRC throughout the duration of the study. EGCG pharmacokinetics will be measured at the beginning of each glucose clamp study day after oral administration of a single dose of EGCG or placebo. Finally, various plasma markers of inflammation will be measured at baseline and at the end of each treatment arm to evaluate potential changes that may be related to improvements in metabolic and/or vascular function. This study will explore whether EGCG, a single compound thought to be a major bioactive component of green tea, is effective at improving insulin resistance and lowering blood pressure in subjects with obesity. Results from this study may have important implications for understanding potential health benefits of functional foods that contain bioactive polyphenols including green tea, dark chocolate, and red wine.