Perhaps the most important outcomes in intervention studies are changes in risk factors or biomarkers of disease, especially for cardiovascular disease. Lipoprotein cholesterol (HDL and LDL) concentrations are considered important biomarkers for risk of cardiovascular disease. In a large (N = 65) study of men and women consuming six cups of black tea per day, there was no change in plasma LDL-cholesterol, HDL-cholesterol, or triglycerides compared to a control beverage. However, in a controlled diet study of 12 individuals with slightly elevated LDL-cholesterol, consumption of five cups of black tea per day compared to a control, caffeine-containing beverage resulted in a 6.5% decrease in total cholesterol and an 11.1% decrease in LDL-cholesterol after 3 weeks of consumption. There were no concomitant changes in HDL-cholesterol or triglycerides. A cholesterol-lowering effect of a theaflavin-enriched green tea extract has also been observed in a large (N = 240), double-blind, randomized outpatient study of individuals with mild to moderate hypercholesterolemia. In this study, individuals consumed either a capsule containing 375 mg of theaflavin-enriched green tea extract or a placebo capsule. After 12 weeks, the individuals taking the theaflavin-enriched green tea extract capsule had an 11.3% decrease in plasma total cholesterol and a 16.4% decrease in LDL-cholesterol from their pretreatment concentrations. The were no significant changes in total cholesterol or LDL-cholesterol in the placebo group and no change in HDL-cholesterol or triglyceride concentration in either group. In an average US diet supplemented with cocoa powder and dark chocolate, no change in LDL-cholesterol or the ratio of total cholesterol to HDL-cholesterol was found but HDL-cholesterol increased.
Although black tea was found to decrease platelet aggregation in vitro, similar to water, it had no acute or chronic effect on platelet aggregation in patients with coronary artery disease or in healthy individuals. Chocolate procyanidins increase plasma prostacyclin and decrease plasma leukotrienes, a possible mechanism by which chocolate flavonoids can decrease platelet activation. These changes occur rapidly (within 2 h) after consumption of chocolate and quickly disappear. However, in a well-controlled intervention, there was no effect of chocolate and cocoa powder on thromboxane and 6-keto-prostaglandin F1q, urinary excretion.
In addition to traditional biomarkers of cardiovascular disease, there are newer markers associated with cardiovascular disease that are related to endothelial function. One measure is brachial artery reactivity. In response to acute and chronic tea consumption, black tea consumption improves arterial reactivity in individuals with coronary artery disease. Tea consumption also improves arterial reactivity in mildly hypercholesterolemic individuals. Similar improvement in endothelial function occurred in individuals consuming purple grape juice.
Changes in whole body concentration of cyto-kines (especially those that control synthesis of acute phase proteins such as IL-6 and TNF-a), acute phase proteins (especially C-reactive protein), and soluble adhesion molecules (especially eSelectin, ICAM, and VCAM) are important indicators of changes in cell signaling pathways, subclinical inflammation, and interactions of circulating cells with the endothelium. Some of these proteins have been identified as independent risk factors of cardiovascular disease and they remain important, emerging markers of disease. Despite the improvement in vascular reactivity associated with tea consumption, cell adhesion molecules do not appear to be altered by tea consumption in smokers and nonsmokers. These adhesion molecules are also related to endothelial function and are involved in the early etiology of atherosclerosis. Similar to tea, cocoa powder and chocolate consumption also do not alter proinflammatory cytokines, adhesion molecules, and acute phase proteins.
In summary, the observation that total circulating antioxidant capacity increases with consumption of foods high in flavonoids appears to be consistent in most studies, and tea is an important source of dietary flavonoids. However, improved circulating anti-oxidant status is not always consistent with a decrease in oxidative damage to DNA, protein, or lipid. Endothelial function, an important risk factor for cardiovascular disease, also appears to improve in response to catechins. A potential limitation of many studies is the lack of a controlled diet, which may account for the variability in results observed.
See also: Antioxidants: Diet and Antioxidant Defense; Observational Studies; Intervention Studies. Cholesterol: Sources, Absorption, Function and Metabolism. Coronary Heart Disease: Prevention. Lipoproteins.
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