The caffeine in your coffee has an unexpected effect on the principal blood vessels, according to new research by a team at Trinity College, Dublin.
It causes the vessels to stiffen, pushing up pressure inside them even when ordinary blood pressure apparently remains low.
This is thought to be the first time that such an effect of caffeine has been seen. It should make people with borderline high blood pressure think twice about that extra cup of coffee in the morning, according to John Feely, professor of pharmacology and therapeutics at Trinity and a physician at St James's Hospital, in Dublin.
"We run a hypertension clinic, so we are interested in how high blood pressure makes arteries stiffer," says Feely. "As people are getting older, this is becoming more of a problem."
Feely is interested in anything that might elevate blood pressure, and caffeine has this tendency. In particular, there was evidence that it would push people with borderline hypertension into a higher range.
He and a colleague, Dr Azra Mahmud, decided to study the effect of caffeine not just on standard blood pressure, at the brachial artery in the arm, but also on the major arteries, including the carotid, which carries blood to the brain, and the aorta, the main vessel carrying blood from the heart into the body. Their results were published in a recent edition of the journal Hypertension.
They found that these large vessels can respond more strongly to caffeine than can peripheral blood vessels. "These results show a significant effect of caffeine intake on arterial tone and function and suggest that caffeine acutely increases arterial stiffness," they write.
The researchers gave test subjects two cups of either caffeinated or decaffeinated coffee in a double-blind study, meaning neither they nor the volunteers knew who had been given caffeine.
An automated unit took the standard blood-pressure measure; Feely and Mahmud measured pressure in the aorta and carotid arteries using a technique called applanation tonometry.
This involves measuring the time it takes for the pulse of blood, or "wave", leaving the aorta to reach the wrist. It is not the blood itself but the timing of the wave's arrival that is important. They took a similar measurement between the carotid and the femoral artery, in the leg.
The wave moves along the vessels at a good clip, making six to seven metres per second, says Feely. The researchers found that the wave moves even faster if the aorta has been made stiffer after taking caffeinated coffee. "It is like an elastic band," he says. "When it is taut, the wave goes down more quickly."
The effects of caffeine are transient, so the two-cup dose might last between two and three hours, he says. Yet if the person is a heavy coffee drinker - four or more cups a day is considered chronic consumption - then the tensing of the large vessels lasts much longer.
Feely found that caffeine is not the only substance to trigger this reaction. "Excess alcohol has the same effect," he says.
Details of this work will be published shortly.
The findings are important because they suggest that caffeine has a greater impact than expected, by influencing the large vessels. This links with another study that involved giving coffee to five groups of men at varying risk of hypertension. It found that the effects of caffeine were particularly strong in those prone to high blood pressure.
The message, says Feely, is that people who know they are borderline hypertensive should consider drinking less coffee.