“A single espresso a day can damage heart.” “Just one espresso can put your heart at risk.” “One caffeine-packed cup can slow blood flow to the heart by 22%.” Whoa! Those are headlines that can make your heart skip a beat. They come from British newspapers reporting on an Italian study about the effects of a cup of espresso on blood flow and blood pressure. As is so often the case, the headlines don’t reflect what the study actually found, and anyone reading only the headlines may end up unnecessarily depriving themselves of the delights of an espresso.
Coffee is obviously of interest to researchers because it is so widely consumed and because it has a number of biologically active compounds. Caffeine is the best known of these, but recently we have heard a great deal about the antioxidants that are also present in coffee. Part of the reason for the interest in coffee is that acute effects, such as a rise in blood pressure, are deemed to be detrimental, whereas long term consumption has been associated with a lower risk of cardiovascular disease and type 2 diabetes. This has actually been termed the coffee paradox. The current thinking is that the long term benefits are due to coffee’s antioxidants while the unfavourable acute effects are due to caffeine.
Whether the acute effects present a risk is essentially unknown and this is what the Italian researchers decided to explore. They enlisted ten healthy males and ten healthy females to drink either a cup of espresso or a cup of decaf and have their blood pressure and arterial response be monitored. In the latter case, the technique is to cut off the blood supply in an arm and then using ultrasound, measure how effectively the brachial artery, the main artery in the arm, dilates when blood flow is restored. If the artery doesn’t dilate effectively, it means that blood flow is impaired, which is a possible sign of atherosclerosis in the making. The dilation of arteries is triggered by the release of nitric oxide, which is also an inhibitor of blood clot formation and inflammation. Basically, impaired endothelial function, as the lack of arterial dilation is called, means a reduction in the bioavailability of nitric oxide, which may be a harbinger of future trouble.
What the researchers found in this case was a poorer dilation response after drinking a cup of espresso than after drinking a cup of decaffeinated coffee. There was actually a 22% greater dilation after the decaf, inspiring headlines about damaging the heart and reducing blood flow to the heart by 22%. As far as blood pressure goes, there was a small increase, but essentially within the scope of experimental error. And of course the study did not show any heart damage at all, no attempts were even made to study any such effect. And the endothelial response was measure in an artery in the arm, not in the coronary arteries that supply the heart with oxygen. The study was quite well carried out in the sense that neither the subjects nor the researchers who evaluated blood flow and blood pressure knew who was drinking which beverage. Furthermore, the study was of a cross-over design meaning that all subjects were investigated after drinking both types of coffee. On the other hand, twenty subjects makes for a pretty small sample. And measurements were taken only thirty and sixty minutes after drinking the coffee, so the longevity of the vasoconstriction effect is unknown. Most importantly, the short term constriction of the arteries may have no clinical significance since arteries in any case constrict and dilate throught the day in response to activity.
A large body of evidence indicates that up to four or five cups of coffee a day is safe for the general population. Contrary to the implication inherent in the headlines, this Italian study does not show that a single cup of espresso damages the heart. The overwhelming evidence shows that when consumed in moderation - 400-500mg of caffeine per day, the equivalent of approximately four to five cups, coffee is safe for the general population.
Joe Schwarcz
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