Effects of Dark Chocolate on Insulin Sensitivity in People With High Blood Pressure

Dark chocolate and other cocoa products contain antioxidants including the polyphenol epicatechin that have beneficial effects on vascular function. Oral consumption of dark chocolate lowers blood pressure in elderly subjects with isolated systolic hypertension while drinking cocoa acutely improves flow-mediated vasodilation in subjects with cardiovascular risk factors. Work from our lab and elsewhere has shown that regulation of hemodynamic and metabolic homeostasis are coupled such that subjects with essential hypertension are also insulin resistant. Therefore, we hypothesize that cocoa consumption will improve both blood pressure and insulin sensitivity in subjects with essential hypertension. To test this hypothesis, we will conduct a randomized double-blind, crossover study in subjects with essential hypertension to evaluate the effects of cocoa versus placebo on blood pressure, insulin sensitivity, and insulin-mediated capillary recruitment in skeletal muscle. After a 7 day cocoa-free run-in period, subjects will be randomized to receive 15 consecutive daily doses of either cocoa drink (150 ml twice a day with a total of 900 mg of polyphenols and 237 kcal) or polyphenol poor placebo drink (150 ml twice a day with a total of 36 mg of polyphenols and 234 kcal). After the 15 day treatment period, the subjects will enter a 1 week cocoa-free washout period followed by cross-over to the other treatment. Subjects will be counseled to maintain an isocaloric diet during the study. Blood pressure, insulin sensitivity, and insulin-mediated capillary recruitment will be assessed in each subject after the run-in period, after 15 days of treatment, and after completion of the cross-over. Blood pressure will also be measured every Monday, Wednesday, and Friday for the duration of the study. Insulin sensitivity will be measured using the hyperinsulinemic euglycemic glucose clamp technique. Insulin-stimulated capillary recruitment in forearm skeletal muscle will be measured by ultrasound with "microbubble" contrast during the glucose clamp studies. Peak and trough plasma epicatechin levels will be measured prior to each glucose clamp study using an HPLC assay. This study will determine if short-term oral administration of cocoa is effective at lowering blood pressure, improving insulin sensitivity, and restoring vascular function in subjects with essential hypertension. This is highly relevant to major public health problems such as diabetes, obesity, hypertension, and cardiovascular diseases where insulin resistance and vascular dysfunction are important pathophysiological components.