DISPOSICIONES CONSTITUCIONALES REFERIDAS A LA CULTURA

The skin is the human body’s largest organ and represents an easily accessible vascular bed to assess

peripheral microvascular reactivity, also providing a model to investigate underlying mechanisms in

various diseased states (Levy et al., 2001; Sokolnicki et al., 2007; Levy et al., 2008). The findings from

Chapters 4, 5 and 6 support this notion, but further demonstrate the value of using multiple local heating protocols simultaneously. Interrogating the microvessels using a combination of local heating

protocols provides a means of discovering the underlying mechanisms that are responsible for

changes in function following intervention studies. Specifically, use of rapid and gradual local heating

protocols exhibited different cutaneous responses to local heating in Chapters 5 and 6 that

investigated the effects of acute and regular tea consumption, respectively. The data presented in

Chapter 5 revealed that middle-aged healthy individuals exhibited increased cutaneous microvascular function 2-hours following black tea ingestion compared to placebo, as demonstrated following

gradual local heating that is largely NO-mediated. Given that rapid local heating did not exhibit any

such differences between tea and placebo, these findings highlight the importance of interrogating

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supports this approach, as gradual local heating again demonstrated tea-induced increases in

cutaneous vasodilation, whereas rapid local heating observed no changes between pre- and post-tea,

compared to a reduction in vasodilation following placebo. It is possible that the studies in Chapters 5

and 6 were statistically underpowered to detect differences pre- vs post-tea with rapid local heating,

according to the findings and sample size estimations contained within Chapter 4. However, the

observed differences between rapid and gradual local heating protocols may also be due to essential

differences in the underlying mechanisms. Despite rapid local heating protocols also being NO-

mediated, but to a lesser extent than gradual heating, they also activate axon-reflexes and are partly

mediated by EDHFs (Johnson et al., 2014). Consequently, rapid local heating provides complementary

mechanistic insight when used in conjunction with gradual heating.

Assessing cutaneous microvascular function is logical when examining the cardiovascular impact of

interventions, since the skin represents a valid model of generalised microvascular (dys)function

(Holowatz et al., 2008) and is regarded as an earlier sentinel of CVD than the macrovasculature

(Roustit & Cracowski, 2012). Chapter 5 demonstrated acute (2-hour) improvements in cutaneous

microvascular function in response to black tea consumption that is encouraging, given that this

vascular bed is associated with some of the earliest manifestations of CVD and diabetes in multiple

organ systems (Cade, 2008). Similarly, the lifestyle intervention adopted in Chapter 6 demonstrated a

deleterious impact in the microvasculature after only 7-days that was not present in all of the

examined conduit arteries, again suggesting that detrimental changes first manifest in the cutaneous

microvasculature which is, therefore, a crucial component of the vascular tree.

Chapter 6 demonstrated that a short lifestyle intervention had a significant deleterious impact on several markers of cardiovascular health. Previously, no research has examined the impact of such a

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comprehensive vascular assessment and observations in Chapter 6 are indicative of lifestyle-induced

systemic changes in vascular function. Many individuals adopt such a lifestyle for much more

prolonged durations than in Chapter 6 and others who are generally active and consume eucaloric

diets, adopt short-term unhealthy lifestyles during periods of celebration or festivities. It is, therefore,

both surprising and concerning that such detrimental changes in vascular function and insulin

sensitivity were observed in healthy adult males after only 7-days. The data presented in Chapter 6

are, therefore, an important indication of the negative consequences such lifestyles have on

cardiovascular health and furthermore, are suggestive of the early manifestation of changes

throughout the vascular tree that may subsequently progress to overt CVD.

Given that Chapter 6 observed impaired insulin signalling and altered glucose metabolism that are

characteristic of insulin resistance, these findings demonstrate the development of insulin resistant

mechanisms after only 7-days of an altered lifestyle. Under healthy conditions, NO production in the

endothelial cells is stimulated by insulin activating NOS via the PI-3K pathway (Sena et al., 2013), but

this process is impaired in an insulin resistant state, thereby diminishing NO production and

subsequently impairing the ability of vessels to dilate (Kim et al., 2006). That changes in insulin

sensitivity were detectable after only 7-days suggests that the inflammatory cascade leading to

atherosclerosis manifests early following changes in lifestyle. However, it is unknown whether such

rapid changes translate to long-term changes in the vasculature and metabolic pathways.

Furthermore, the precise mechanisms responsible for tea-induced changes in glucose control and

insulin sensitivity are uncertain. Animal models suggest several mechanisms, including a reduction in

glucose production in the intestinal tract due to inhibition of carbohydrate digestive enzymes

(Kobayashi et al., 2000; Oh et al., 2015). Enhanced insulin binding to adipocytes and improved glucose

uptake by myocytes are also suggested to be responsible for improved insulin sensitivity following tea

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2015). However, the metabolic pathways in humans have yet to be determined and Chapter 6 did not

investigate the gut microbiota so the mechanisms underlying the improved glucose handling and

enhanced insulin sensitivity remain uncertain. It is perhaps interesting to consider the possibility that

improved vascular function (as outlined in Chapter 6) could have also contributed to the improved

glucose control and insulin sensitivity through improved delivery/disposal for example (Wagenmakers

et al., 2016).

The observations of previous research suggesting a cardioprotective role of tea on the vasculature

were both supported and further extended by the findings of Chapter 6, in that daily green tea

consumption attenuated the negative consequences of the short-term lifestyle intervention in

previously healthy male adults. Previous studies have generally assessed the acute impact of tea on

conduit artery endothelium-dependent vasodilation (Hodgson et al., 2005; Schreuder et al., 2014),

with no prior work examining the vascular effects of tea on lifestyle changes. The ability of green tea

to not only attenuate, but improve vascular function throughout the vascular tree in the presence of

an unhealthy lifestyle, suggests that tea exerts a potent effect at the level of the endothelium, likely

related to activation/upregulation of endothelium-derived vasodilators, such as NO. The observed

metabolic changes are also consistent with this theory, given that green tea enhanced insulin

sensitivity which is partly responsible for stimulating NO production in endothelial cells via the

catalytic conversion of L-arginine to L-citrulline. Whilst the precise mechanisms underlying the tea-

induced improvements in vascular and metabolic function remain unclear, the cumulative findings of

Chapters 5 and 6 are consistent with a NO-mediated mechanism.

Despite the beneficial effects of tea consumption observed in the peripheral vasculature in Chapters

5 and 6, based on current work contained within this thesis, tea and the tea-derived catechin (-)- epicatechin, do not appear to exert any effect upon cerebrovascular function in healthy adult males.

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Such findings are largely consistent with the few studies previously undertaken concerning the

cerebrovascular effects of tea consumption (Vidyasagar et al., 2013). Given that the brain is a large

organ and requires a large stimulus to induce functional variations, it is feasible that the duration

and/or dosage of tea/(-)-epicatechin were insufficient to influence the cerebrovasculature. Similarly,

the lifestyle intervention may not have been severe enough to generate an impairment in

cerebrovascular function, particularly since the control mechanisms of the cerebrovasculature are

different to those in the peripheral vessels. Furthermore, it is acknowledged that the sample sizes for

the cerebral data of Chapters 6 and 7 were small and unlikely to detect any subtle changes in function

that may have been present. Likewise, current findings were observed in healthy adult males with

optimal cerebral endothelial function. The impact in individuals with compromised endothelial

function may exhibit different changes, particularly an ageing population. Further research is,

therefore, required to determine the effects of tea in such populations who are at greater risk of

cerebrovascular disease.

The work contained within this thesis provides compelling evidence for a cardioprotective role of tea

on peripheral vascular function. The experimental studies detailed herein have contributed to the

collective understanding of the role of tea in mitigating CVD risk and being a useful therapeutic

approach in improving overall cardiovascular health. Furthermore, the experimental work of this

thesis has contributed to the understanding of underlying mechanisms likely responsible for the

observed vascular improvements and this thesis highlights the value of performing simultaneous local

thermal hyperaemic assessment to interrogate the microvasculature for mechanistic insight.