1. Introduction
Hypertension is the leading risk factor for cardiovascular morbidity and mortality [
1]. Several key mechanisms, including inflammation oxidative stress and endothelial dysfunction, play an important role in cardiovascular risk in late-life hypertension [
1,
2]. Over the past 25 years, a large body of clinical evidence has indicated a close relationship between the degree of endothelial dysfunction and clinical cardiovascular events in patients with cardiovascular risk factors, coronary heart disease, or both [
3,
4,
5,
6,
7,
8,
9]. As a consequence, it has also been hypothesized that the reversal of endothelial dysfunction might slow down atherogenesis and improve individual cardiovascular prognosis [
6,
7,
8,
9].
Moreover, recent studies have shown that risk factors for vascular diseases are associated with a blunted capacity for repair of the endothelial damage evidenced by a dysfunction of bone marrow-derived circulating endothelial progenitor cells (EPCs) [
10]. Indeed, a reduced ability of EPCs to proliferate ex vivo and to express an endothelial phenotype is associated with risk factors for coronary artery disease as well as endothelial dysfunction [
10,
11]. Cells grown under these conditions were formerly termed “early EPCs”, but are currently referred to as “circulating angiogenic cells” (CACs) [
12,
13]. CAC quantity and function are robust biomarkers of vascular risk for a multitude of diseases, particularly cardiovascular disease. Importantly, infused ex vivo-expanded CACs have shown a potential for improved endothelial function, either reducing the risk of events or enhancing recovery from ischemia [
10,
11,
12,
13,
14].
Among various risk factors, hypertension is shown to be the strongest predictor of CAC migratory impairment [
14]. Indeed, CACs serve as a cellular reservoir to replace dysfunctional endothelium and to form a cellular patch at the site of denuding injury [
10,
11,
12,
13,
14].
Epidemiological studies have shown an inverse correlation between flavonoid-rich diets and cardiovascular disease [
15,
16]. Tea accounts for a major proportion of total flavonoid intake in a number of Western countries [
15,
16,
17]. Increasing attention is currently being paid to the link between tea ingestion and a suggested lower incidence of cardiovascular events [
16,
17].
Some dietary intervention studies reported that both acute and chronic [
18,
19,
20] black tea consumption increases NO-mediated flow-mediated dilation (FMD) in healthy volunteers as well as in patients with cardiovascular disease. In response to this, we performed a dose-finding study with black tea, showing that the daily consumption of even a single cup of tea (100 mg tea flavonoids) per day increased the FMD of healthy volunteers, and improving further with escalating dose [
20].
In contrast, previous studies have shown that a meal rich in fat decreases FMD and negatively modifies vascular function [
21,
22]. However, the effects of tea on endothelial function under these challenging conditions has not been completely clarified. Indeed, Hodgson et al. [
23], aiming to evaluate only the acute effects of tea compared with hot water in pharmacologically treated patients with coronary artery disease aged between 45 and 70 years, suggested that a mixed meal with tea was able to improve endothelium-dependent dilatation, but tea alone was not able to positively affect this parameter. We recently reported that black tea consumption lowered wave reflections and blood pressure in the fasting state and, during the challenging hemodynamic conditions after a fat load, in hypertensives [
24]. The effects of high doses of green tea (1 L per day) on CACs have been reported for chronic heavy smokers [
25], but could not be confirmed in patients with chronic renal failure [
26]. However, the effects of black tea when consumed at a moderate dose on the number of CACs have not been examined. Therefore, the aim of the present study was to investigate the effects of black tea on endothelial function before and after an oral fat load and the number of functionally active CACs in a group of never-treated grade I essential hypertensives without additional cardiovascular risk factors.
4. Discussion
Our study showed that, in grade I hypertensive patients, one-week consumption of black tea resulted in a significant improvement in endothelium-dependent FMD, with maximal response two hours after acute intake. This effect was observed with a moderate dose, the equivalent of two cups of tea per day. Moreover, the consumption of tea counteracted a fat challenge-induced impairment of FMD. The protective properties of tea were also reflected by a significantly increased number of CACs, circulating cells capable of repairing the vessel wall. These results suggest that the consumption of tea may improve or even protect endothelial function under challenge conditions.
Vascular endothelial dysfunction is determined by both genetic and environmental factors that cause decreased bioavailability of the vasodilator nitric oxide (NO). Under physiologic conditions, the endothelium regulates vascular tone via the balanced production of vasodilating substances such as NO. Impaired endothelium-dependent vasodilation has been reported after a fat load, probably by an increased production of oxygen-derived free radicals and a quenching of NO [
22,
34]. In particular, it has been observed [
34] that acute fat load administered orally or intravenously significantly increased BP, impaired endothelial function, and activated the sympathetic nervous system via mechanisms not likely depending on changes in leptin, glucose, and insulin levels in obese healthy subjects. Thus, fat load has deleterious hemodynamic effects on obese subjects as well as on other conditions of cardiovascular risk, which may be involved in endothelial function [
34,
35]. Nevertheless, the effect of a high fat meal on endothelial function is not completely clear cut; indeed, Hodgson et al. [
23] showed that a mixed meal with tea was able to improve endothelium-dependent dilatation, but tea alone was not able to positively affect this parameter. However, that study only evaluated the acute effects of tea compared with hot water, with a mixed meal (not specifically a fat load) in pharmacologically treated patients (with a number of putative confounding treatments: the use of aspirin, statins, or specific antihypertensive medications including angiotensin-converting enzyme inhibitors, angiotensin II receptor blockers, beta-blockers, calcium channel entry blockers, and diuretics) at very high cardiovascular risk (with coronary artery disease) aged between 45 and 70 years.
The increased FMD in fasted conditions and preserved postprandial FMD after tea intake may explain our observed findings on BP and arterial hemodynamics as described previously [
23]. Taken together, our findings imply that the oral fat load may lead to a transient loss of NO bioavailability and transient vascular damage reflected in endothelial dysfunction and increased peripheral vascular tone and arterial stiffness.
The severity of endothelial dysfunction correlates with the development of coronary artery disease and predicts future cardiovascular events [
3,
4,
5,
6,
7,
8]. Thus, endothelial dysfunction may be considered a strategic target in the treatment of hypertension. The flavonoids in tea may increase or preserve the bioavailability of NO by decreasing the formation or scavenging of reactive oxygen and nitrogen species, increasing NO synthase activity, or both [
16]. Besides the functionality of the endothelium, the morphologic integrity of the monolayer constituted by resident vascular endothelial cells plays a pivotal role in the maintenance of a number of vascular functions [
36]. Anatomical or functional interruption continuity of the endothelial barrier is considered a fundamental step during the atherogenetic process [
36]. The integrity of the vascular endothelial barrier is continuously maintained by the rapid migration of resident endothelial cells toward wounded areas of the endothelium [
35]. In addition to the migratory capability of resident vascular endothelial cells neighboring a damaged area, a growing body of evidence suggests that CACs also play a critical role in restoring the integrity of the endothelial monolayer [
36]. Cardiovascular risk factors are associated with a reduced number of CACs [
36], whereas, in turn, statins, angiotensin-converting enzyme inhibitors, and angiotensin II type 1 receptor blockers have been reported to increase the number of CACs [
36]. Low levels of CACs in patients with increasing cardiovascular risk, such as hypertension, could have several mechanistic causes [
10,
11,
14]. The most likely mechanism affected by tea flavonoids is the counteraction of oxidative stress, increasing NO bioavailability and FMD and decreasing BP levels in grade I hypertensive patients [
24]. This hypothesis is supported by a study of Hill et al. [
10] reporting a strong correlation between the number of CACs and the subjects’ combined Framingham risk factor score (
r = −0.47). Further, measurement of FMD also revealed a significant relationship between endothelial function and the number of EPCs (
r = 0.59,
p < 0.001), where the level of CACs was a better predictor of vascular reactivity than the presence or absence of conventional cardiovascular risk factors [
10]. In addition, CACs from subjects at high risk for cardiovascular events had higher rates of in vitro senescence than cells from subjects at low risk. A study by Bocchio et al. [
30] showed that the number of CACs was significantly reduced in patients with cardiovascular risk compared with controls (
p < 0.0001). The percentage variation of CACs and of FMD after treatment was significantly associated with the presence of endothelial dysfunction at baseline. The close relationship between FMD and the number of CACs may be explained by the important role of eNOS in the mobilization of progenitor cells from bone marrow [
37].
A number of biomarkers were analyzed in this study in an attempt to provide more mechanistic insight; however, as blood lipids and markers of endothelial dysfunction and low-grade inflammation were not modified by the intervention, these results do not provide links to potential underlying mechanisms.
The finding that tea improves FMD and increases the number of CACs has previously been reported by Kim et al. [
25]. They observed in smokers that circulating and cultured angiogenic cells increased rapidly two weeks after green tea consumption. FMD correlated with CAC counts (
r = 0.67) before and after treatment (
r = 0.60). Of note, the flavonoid dose used by Kim et al. [
24] was about three times higher than in the present study, and their study did not have a placebo group. Our hypothesis is that flavonoids in black tea increase the functionally active CACs numbers by the activation of eNOS, increasing NO bioavailability and concomitantly increasing FMD.