Aims Experimental and clinical studies indicate that tea exerts protection against cardiovascular diseases. However, a question of much debate is whether addition of milk modifies the biological activities of tea. We studied the vascular effects of tea, with or without milk, in humans and elucidated the impact of individual milk proteins in cell culture experiments, with isolated rat aortic rings and by HPLC analysis.Methods and results A total of 16 healthy female volunteers consumed either 500 mL of freshly brewed black tea, black tea with 10% skimmed milk, or boiled water as control. Flow-mediated dilation (FMD) was measured by high-resolution vascular ultrasound before and 2 h after consumption. Black tea significantly improved FMD in humans compared with water, whereas addition of milk completely blunted the effects of tea. To support these findings, similar experiments were performed in isolated rat aortic rings and endothelial cells. Tea induced vasorelaxation in rat aortic rings and increased the activity of endothelial nitric oxide synthase by phosphorylation of the enzyme in endothelial cells. All effects were completely inhibited by the addition of milk to tea. Of the various kinds of milk proteins, the caseins accounted for these inhibiting effects of milk, probably by formation of complexes with tea catechins. Conclusion Milk counteracts the favourable health effects of tea on vascular function. This finding indicates the need for particular awareness in the interpretation and design of studies comprising nutritional flavonoids.
Consumption of tea has been shown to improve endothelial function. It is assumed that catechins are the tea components responsible for these beneficial effects. In black tea, catechin concentrations are significantly lower than in green tea. The present study was designed to compare green and black tea with regard to amelioration of endothelial function. Endothelial function in response to both teas was assessed in bovine aortic endothelial cells (BAEC) and rat aortic rings. To elucidate whether these findings are also applicable to humans, flow-mediated dilation (FMD) and nitro-mediated dilation (NMD) were assessed by ultrasound in twenty-one healthy women before and 2 h after consumption of green and black tea (2 h of FMD and NMD), in comparison with water (control). In BAEC, green and black tea significantly increased endothelial NO synthase activity to the same extent. Similarly, both teas induced comparable endothelial-dependent vasodilation in rat aortic rings. In human subjects, ingestion of green and black tea led to significant increases in FMD: from 5·4 (SD 2·3) to 10·2 (SD 3) % (baseline-adjusted difference (BAD) for 2 h of FMD, green tea v. water: 5·0 (95 % CI 3·0, 7·0) %; P,0·001) and from 5 (SD 2·6) to 9·1 (SD 3·6) % (BAD for 2 h of FMD, black tea v. water: 4·4 (95 % CI 2·3, 6·5) %; P,0·001), respectively. The increase in FMD was not significantly different between the two tea preparations (BAD for 2 h of FMD, green tea v. black tea: 0·66 (95 % CI 20·76, 2·09) %; P¼ 0·36). NMD did not vary between any of the groups. In conclusion, green and black tea are equally effective in improving endothelial function. Tea: Endothelial function: Flow-mediated dilation: Nitric oxideCompelling evidence from epidemiological and clinical studies has established a positive correlation between the consumption of green and black tea and protection against atherosclerosis and CVD 1 -4 . In addition to the antioxidative, anti-inflammatory, anti-proliferative and anti-thrombotic properties of polyphenols contained in tea, favourable effects on endothelial function are the main underlying mechanisms suggested as being involved in the prevention of CHD by tea consumption 5,6 . Evidence is accumulating that catechins, the main polyphenolic compounds in green tea, are the substances responsible for these beneficial effects. Accordingly, we and others have found that catechins, particularly epigallocatechin-3-gallate (EGCG), evoke endothelial-dependent vasodilation via rapid activation of endothelial NO synthase (eNOS) 7,8 .Both green and black teas contain catechins. In black tea, however, catechin concentrations are significantly lower than in green tea. This is ascribed to the manufacturing process, which either prevents or allows tea polyphenols in the leaves to be oxidised. Whereas in green tea the intention is to avoid oxidation of polyphenols, black tea is manufactured by promoting enzymic oxidation (fermentation). During this process, catechins are oxidised to theaflavins and thearubigins. As a result, catechins const...
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