Reports on the health benefits of flavonoid-rich foods and beverages have been promising, especially in the areas of cardiovascular and cerebrovascular function. The growing excitement has stemmed largely from epidemiologic reports linking increased flavonoid ingestion to a reduction in cardiovascular events1 and to the development of dementia.2,3 To date, most of the world's attention has been focused on tea and red wine. Like these 2 drinks, cocoa is a source of flavonoids, but in its unprocessed state contains much larger quantities.
Our interest in the cardiovascular effects of dietary flavonoids grew from observations surrounding the very low incidence of hypertension in the Kuna Indians, living off the coast of Panama.4 The responsible factors are clearly environmental, not genetic, as protection is lost on migration to the urban centers. Intensive dietary investigation uncovered consumption of vast amounts of cocoa by these indigenous peoples. Cocoa, in its unprocessed state, is known to contain particularly high quantities of the flavonoid subclass known as flavanols. Coupled with in vitro and animal reports identifying activation of nitric oxide (NO) synthase by flavanol compounds,5–10 these findings provided our stimulus to study the vascular effects of cocoa in healthy people. After our positive finding that flavanol-rich cocoa induced NO-dependent vasodilation in healthy humans, we followed with investigations into aging, a territory where endothelial dysfunction is nearly universal. Our studies in healthy aging demonstrated clear efficacy of cocoa on endothelial function in the elderly.
In addition to protection against coronary disease, flavonoids have been proposed in the prevention of cerebrovascular disease, both of stroke and dementia. Several reports have described a decrease in cerebral blood flow (CBF) in patients with dementia, demonstrating further the clinical value of low CBF in predicting cognitive decline. The final segment of this review will highlight the reported connections between blood flow and cognitive decline, and will report on our early observations with flavanol-rich cocoa and cerebral perfusion.
Cocoa Induced NO-dependent Vasodilation
Intrigued with the possibility of cocoa enhancing endothelial function, we studied 27 healthy individuals before and after ingestion of flavanol-rich cocoa over a 5-day period.11 Each individual consumed 920 mL of cocoa (Cocoapro; Mars Inc, Hackettstown, NJ) daily, in 4 equal doses. The cocoa contained 821 mg total flavanols daily. Pulse wave amplitude (PWA) readings in the fingertip were taken by peripheral arterial tonometry using a plethysmographic device (PAT1000RD; Itamar Medical Inc, Caesarea, Israel). At baseline, there was a typical variability in digital PWA (Fig. 1A). Mean basal PWA on day 1, before cocoa ingestion, was 7.3+0.7 μL (Table 1). After 4 days of cocoa, there was a 29% increase in amplitude, measured in the morning at least 12 hours after the last dose of cocoa (Fig. 1B). On the fifth day, exposure to an additional “dose” of cocoa led to an even further increase (33%) in amplitude 90 minutes later (Fig. 1C; P=0.01). After 4 days of cocoa, the NO synthase inhibitor NG-nitro-L-arginine methyl ester had a significant effect in completely reversing dilation (Fig. 1D). This dramatic response to the specific inhibitor L-NAME clearly implicated inhibition of NO synthase in the reversal of response. Our report provided further compelling evidence for NO activation being the responsible mechanism underlying cocoa-induced vasodilation. Ingestion of a control drink, flavanol-poor cocoa, led to vasodilator responses that were significantly smaller. Therefore, these experiments implicate flavanols as the primary polyphenols responsible for cocoa-induced vasodilation.
A positive vasodilator response to flavanol-rich cocoa, dependent upon NO, can be considered a measure of enhancement of endothelial function. Endothelial dysfunction has been shown to be a significant cardiovascular risk factor and marker for future cardiovascular events.12 Evidence is accumulating that flavonoids have beneficial effects on endothelial function. Until recently, these effects were studied mainly in animals and in vitro.5–9 In both the rabbit and rat aorta, red wine evoked relaxation related to NO activity.6,10 In human umbilical vein endothelial cells, red wine polyphenol extract led to increased endothelial NO synthase expression and NO release.7 The effects of specific flavanol fractions extracted from cocoa were studied by Karim et al,5 who reported endothelium-dependent relaxation and activation of endothelial NO synthase in the rabbit aorta, abolished by L-NAME. Others have found the same enhancement of endothelial function in healthy subjects with flavanol-rich dark chocolate.13,14 The response has also been demonstrated in patients with increased cardiovascular risk.15,16
Endothelial Dysfunction and Aging
Among the myriad of risk factors recognized to decrease endothelial function,17 aging represents the most common and most inevitable cause. Aging also represents the single strongest risk factor for future vascular events. Despite the universality of the aging phenomenon, and the profound connections between endothelial dysfunction and cardiovascular disease,18 targeted therapies are not available.
We studied peripheral arterial responses to several days of flavanol-rich cocoa in healthy subjects over the age of 50. Flow-mediated vasodilation, as measured by tonometry in the finger, was enhanced with cocoa in these older subjects. A typical response is seen in a healthy 72-year old (Fig. 2). The responses among the elderly were seen in both reactive hyperemia and basal PWA measurements. As with the young, L-NAME reversed the peripheral vasodilation.
Vascular Cognitive Disorders
In addition to peripheral vascular disease, aging is also the most common risk factor for cerebrovascular disease, including stroke and dementia. Dementias afflict 1 in 10 adults over the age of 65, and half over the age of 85. Alzheimer disease (AD) and vascular dementia (VD) are the 2 most common causes of dementia among the elderly population. In the United States and Europe, AD accounts for the majority of cases, with VD the second most common cause.19,20 This relationship is reversed in Asia and many developing countries, where VD is the most common cause of dementia.21 However, recent advances have highlighted the frequent overlap of these 2 syndromes.
Vascular pathology, which has traditionally been thought as a diagnostic demarcation in the differential diagnosis between AD and VD, may, in fact, be part of the spectrum of changes underlying a common pathogenesis. The connection of vascular disease with AD rests on 3 lines of evidence. First, on the basis of epidemiologic studies, a remarkable number of risk factors are apparently shared between AD and VD.22,23 These risk factors include increasing age, ApoE4 genotype, hypertension, arteriosclerosis, diabetes mellitus, smoking, and atrial fibrillation.22 Second, clinical manifestations of pathologically diagnosed AD appear to be significantly modulated by cerebral infarction. In the first year after a cerebral infarct, the incidence of dementia is 9 times greater than expected24 and after the first year there is a 50% increase in the likelihood of developing AD.25 Moreover, stroke shortens the duration of preclinical AD.25 Finally, neuropathologic findings in these disorders are overlapping.26 There are neurofibrillary plaques and tangles in VD and vascular pathology in AD. In addition to the commonly seen vascular lesions such as cerebral amyloid angiopathy, microvascular degeneration, and periventricular white matter lesions, there are numerous structural and functional cerebromicrovascular abnormalities in AD.19,27 These vascular abnormalities in AD include: impairment of BBB, decreased microvascular density, vascular distortions, functional alteration and derangement of cerebral endothelial function, and arteriolar changes such as lipohyalinosis.19
Flavanols and Cerebral Blood Flow
A mostly animal and epidemiologic literature suggests that flavonoids may play a role in reducing the risk of dementia.28 Galli et al29 reported that fruit polyphenols decreased the susceptibility of rat brains to the damages of oxidative stress seen with aging. In a cohort of over 1300 French elderly, the intake of flavonoids was found to be inversely related to the risk of incident dementia.3 One intriguing possibility is that the benefits of flavonoids involve improving vascular function and increasing CBF. Several studies have recently implicated a decline in CBF in the development of dementias. From the community-based Rotterdam Study, cognitive function and CBF were measured in 1730 participants over the age of 55 over a 6-year period. Their data suggest that cerebral hypoperfusion precedes and possibly contributes to the onset of clinical dementia.30 Using single-photon emission computed tomography, other groups have also found that that CBF in certain brain regions, including prefrontal and inferior parietal cortices, was significantly lower in those who progressed rapidly to AD.31,32
In our earliest studies, CBF was measured by transcranial Doppler ultrasound (TCD), in healthy elderly subjects without evidence of hypertension, smoking, or other known risk factors for endothelial dysfunction apart from aging. The protocol involved ingestion of 900 mg flavanols daily for 1 week, with CBF measurements before and after cocoa ingestion. Results at baseline and follow-up postcocoa in 1 representative subject, showing increase in CBF through the middle cerebral artery (MCA), are depicted in Figure 3. Hypercapnia is known to be a potent cerebral vasodilator, probably achieving maximal cerebral vascular response. For comparison, our subjects also participated in CO2 inhalation. The 2 subjects in Figure 4 show the range of responses to carbon dioxide seen in our healthy volunteers, one large and one small. The vasodilator response to cocoa in our volunteers was similar to the response to carbon dioxide, although somewhat smaller. Importantly, the response to cocoa was sustained. More sophisticated measures, using gadolinium perfusion magnetic resonance imaging in our recent protocol before and after 1 week of flavanol-rich cocoa, have revealed changes in CBF that may well parallel those seen with TCD (Fig. 5).
Evidence of the effects of flavanol-rich cocoa on enhancing endothelial function have been demonstrated in multiple studies of the peripheral vasculature, employing techniques ranging from brachial artery ultrasound, reactive hyperemia, and peripheral arterial tonometry. In healthy humans, as well as smokers and those with coronary disease, cocoa improved endothelial function. Among the elderly, vascular cognitive decline is a growing public health issue with enormous health and financial costs. The prospect of a targeted treatment to reverse the decline in CBF that accompanies dementias is extremely promising.
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