Coffee, Tea, and Cocoa and Risk of Stroke

Larsson S
Stroke. 2013 Dec 10. [Epub ahead of print]

Background

Coffee, tea and cocoa are common sources of polyphenols. They have recently received attention due to their potential beneficial effects on cardiovascular (CV) health. Antihypertensive, hypocholesterolemic, antioxidant, and anti-inflammatory effects as well as improvements of vascular endothelial function and insulin sensitivity have been proposed as possible mechanisms through which polyphenols may exert a protective effect on stroke.
This literature review summarises the available evidence from experimental studies, prospective studies and meta-analyses of the potential effect of coffee, tea and cocoa in the prevention of stroke.

Coffee

In addition to polyphenols, caffeine and diterpenes are the most abundant bioactive substances in coffee. Their effect on the CV system depends on the preparation method and individual characteristics of the drinker.
Caffeine induces a transient increase in blood pressure (BP). A meta-analysis found a mean rise of 8.1 mmHg in systolic BP and of 5.8 mmHg in diastolic BP after 200-300 mg caffeine (~1.5-2 cups of coffee) in hypertensives. This effect was observed within the first hour, and up to 3 hours after consumption. Another meta-analysis of studies in healthy, normotensive individuals on the long-term effects of coffee found no significant BP changes. Tolerance to the effect of caffeine on BP may partly explain the differences seen between short- and long-term effects.
Diterpenes cafestol and kahweol have cholesterol-raising properties. In filtered coffee, diterpenes are retained in the filter paper. A meta-analysis found that consumption of unfiltered coffee significantly increased total cholesterol, LDL-c and triglyceride concentrations, while hardly any change was seen after consumption of filtered coffee. Those with hyperlipidemia appeared more sensitive to the cholesterol-raising effect of coffee.
The polyphenols have antioxidant activity in vitro, and also in vivo antioxidant activity has been shown. Both protective and neutral effects have observed on whether polyphenols in coffee could suppress the oxidative modification of LDL particles in humans.

Coffee and stroke

Recent studies have questioned the long-held view that coffee is a risk factor for CV disease, as moderate consumption may reduce stroke risk. In a large meta-analysis of prospective studies, a nonlinear relationship between coffee consumption and stroke risk was seen: with the most protective effect around 3-4 four cups of coffee per day, and less risk reduction with fewer or more cups. However, some heterogeneity among studies in different populations has been seen, which may be the result of a different relative composition of bioactive compounds due to different preparation methods, or incomplete correction for dietary and lifestyle factors.

Green and black tea

All types of tea are rich in various flavonoids, which have a hypocholesterolemic effect and reduced the development of atherosclerosis in animal models. Via improvement on nitric oxide status it can improve endothelial function.
Studies in humans have also shown potential beneficial effects on cardiometabolic risk factors, including endothelial function, BP, and cholesterol and blood glucose concentrations. A meta-analysis revealed a substantial improvement of flow-mediated dilation in individuals with a median consumption of 2-3 cups of tea per day, as compared to placebo. Short-term RCTs (<3 months) have shown that green tea consumption lowered total and LDL cholesterol levels, but left HDL-c unchanged. A meta-analysis of RCTs longer than 3 months showed that both green and black tea reduced LDL-c and BP. Two meta-analyses have found a positive effect of green tea on fasting blood glucose concentrations, while effects on insulin and hemoglobin A1c levels are less clear.

Tea and stroke

A meta-analysis of prospective studies yielded an overall RR of total stroke for a 3-cup/day increment in consumption of green or black tea of 0.87 (95%CI: 0.81-0.94). Heterogeneity among studies was seen, possibly due to differences in types of tea, preparation method, stroke measures and analysis methods. Two recent large prospective studies in Japanese and Swedish men and women showed that consumption of at least 4 cups of green tea per day was associated with about 20% reduction of stroke risk, both ischemic stroke and intracerebral hemorrhage.

Cocoa products

Cocoa products, including chocolate, are also rich sources of flavonoids (mainly flavan-3-ols), which are antioxidant and anti-inflammatory compounds. Indeed, the flavan-3-ol content and the total antioxidant capacity in plasma increase after cocoa consumption, and flavan-3-ol in cocoa have been shown to increase endothelial nitric oxide formation, promoting vasodilation. There is controversy about whether these effects are reduced when cocoa is consumed with milk.
Meta-analyses of short-term RCTs suggest that cocoa or chocolate intake may modestly reduce systolic and diastolic BP, although findings from individual trials are inconsistent. Another meta-analysis found that cocoa or chocolate consumption reduced fasting insulin concentrations, insulin resistance and mean arterial pressure and improved flow-mediated dilation.

Chocolate and stroke

Few studies have been performed to investigate the relationship between chocolate consumption and stroke, reporting statistically significant or non-significant inverse relationships. A meta-analyses of these studies yielded a significant 19% stroke risk reduction with 50g/week increment in chocolate consumption.

In summary, current evidence from both experimental studies in animals and humans, and from prospective studies indicates that green and black tea and chocolate have beneficial effects on cardiovascular health, and that it may reduce the risk of stroke. These effects are most likely mediated via effects on endothelial function, and cholesterol in the case of tea, and insulin sensitivity with cocoa. Although a weak inverse association between moderate coffee consumption and risk of stroke was seen in most studies, possible underlying biological mechanisms for a cardiovascular effect remain unclear.

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