People who enjoy the occasional decaf latte may be getting more of a fix than they realise, report Swiss scientists in the journal of the American Heart Association, Circulation this week.
The scientists reported that when occasional coffee drinkers drank a triple espresso, they saw markedly raised blood pressure and increased nervous system activity, regardless of whether or not the espresso contained caffeine.
Surprisingly, people who drank coffee on a regular basis showed increased stimulation of sympathetic nerve pathways - but no increase in blood pressure.
This is the first time such disparities in reactions to coffee have been reported, said lead researcher Dr Roberto Corti, a cardiologist at University Hospital in Zurich.
The results suggest that some unknown ingredient or ingredients in coffee - not caffeine - is responsible for cardiovascular activation, he explained. Coffee contains several hundred different substances.
"Until now we have attributed the cardiovascular effects of coffee to caffeine, but we found non-coffee drinkers given decaffeinated coffee also display these effects," Corti said. "This demonstrates how little we know about the effects of one of our most popular beverages and the most abundantly consumed stimulant worldwide."
"Coffee's cardiovascular safety remains controversial," he added. Until now, possible health hazards have been related to its main ingredient - caffeine.
The researchers measured blood pressure, heart rate and muscle sympathetic nervous system activity (MSA) in 15 healthy volunteers (aged 27 to 38) - six habitual coffee drinkers and nine who either abstained or drank coffee only occasionally. Measurements were recorded before, during and after participants consumed a triple espresso, a decaf triple espresso or intravenous administration of the equivalent amount of caffeine, or a placebo. None of the subjects knew whether they were receiving caffeine.
Sympathetic nervous system activity plays an important role in the regulation of blood pressure and over-activation has been linked with high blood pressure.
The non-habitual or occasional coffee drinkers had systolic blood pressure (the top number in a blood pressure reading) increases of 12 millimeters of mercury (mm Hg) after 60 minutes. No significant change was observed in habitual drinkers' blood pressure. MSA increased in both caffeine and decaffeinated coffee groups by 29 per cent after 30 minutes and 53 per cent after 60 minutes, with almost identical activation times.
In non-habitual coffee drinkers given decaffeinated espresso, systolic blood pressure increased despite no increase in blood concentrations of caffeine. MSA activity was only marginally increased, and heart rate and diastolic blood pressure remained unchanged.
"Recent epidemiological studies have revealed a possible beneficial effect on cardiovascular disease and deaths in habitual coffee drinkers," Corti said. "But our study strongly supports the hypothesis that ingredients other than caffeine are responsible for the stimulating effects of coffee on the cardiovascular system."
The lack of blood pressure elevation in coffee drinkers suggests the effects may be mediated through increased tolerance, the researcher noted however, sympathetic nerve activation occurred in both groups when caffeine was administered intravenously, and habitual drinkers' MSA increased after drinking caffeinated espresso, both of which suggest tolerance to coffee does not appear to be related to caffeine.
Dr Corti concluded that the potential adverse effects attributed to coffee could be less hazardous in regular consumers with normal blood pressure. In such people, especially those without a hereditary predisposition to hypertension, coffee drinking cannot be considered a risk factor for hypertension.
What remains to be seen is whether people with hypertension should be advised to avoid decaffeinated coffee as well, Corti said.
The American Heart Association commented that studies investigating a direct link between caffeine, coffee drinking and coronary heart disease have so far produced conflicting results. Moderate coffee drinking (one - two cups per day) is not regarded as harmful.
The study is published in today's rapid access issue of Circulation.