In a study of 19 habitual coffee drinkers who wore "ambulatory" blood-pressure monitors throughout their daily jobs, the researcher found that the equivalent of four to five cups raised blood pressure an average of five points, compared to days when they consumed only one cup. The effect occurred within an hour of consumption, and the subjects' blood pressure remained elevated throughout the day, the study found.

The volunteers also reported higher levels of stress during the day when they received the higher, 500-milligram dose of caffeine, and they showed a corresponding increase in heart rate, said James Lane, associate research professor of psychiatry at Duke and lead author of the study. Results of the study, funded by the National Heart, Lung and Blood Institute, are published in the May issue of the journal Psychosomatic Medicine.

While a 5-point increase in blood pressure is not excessive, it can have significant clinical implications over time, Lane said. A review of nine major studies of blood-pressure and cardiovascular-disease risk showed that a 5-point difference in diastolic blood pressure -- the lower number used to assess health risk -- was associated with at least a 34 percent increase in the incidence of stroke and a 21 percent increase in the incidence of coronary heart disease.

In another unrelated study, called the Hypertension Detection and Follow-up Program, researchers reported that reducing blood pressure by five points through medication was associated with a 20 percent reduction in 5-year mortality, Lane said.

"The relevant message here is that the more caffeine you consume during the day in coffee, tea or soft drinks, the higher your blood pressure is likely to be," Lane said. "Over many years, this increase in blood pressure may heighten your risk of suffering a heart attack or stroke, even if you don't have high blood pressure now."

While researchers have long known that caffeine boosts blood pressure, nearly all the studies have been conducted in a laboratory setting under tightly controlled circumstances where a single dose of caffeine is compared to none in a short time span. Lane said his study is among the first to analyze blood pressure levels at 15-minute intervals during normal working conditions, while subjects were exposed to a range of moods and activities, from sitting to standing to walking.

"You can measure how caffeine affects people in the laboratory, but that doesn't tell you what effects the drug has in the real world when people are exposed to normal stressors and activities," he said.

In previous studies conducted over the past 15 years, Lane has shown that caffeine raises levels of adrenalin, the body's major stress chemical, during everyday work activities, indicating that caffeine made the day more stressful. The question he wanted to answer in the current study was whether or not caffeine acted directly on blood pressure, or whether it elevated blood pressure through its effect on stress and activity levels.

By correlating the blood pressure readings with the subjects' self-reports of their activities and moods throughout the day, the Duke researchers were able to rule out the possibility that caffeine was simply acting through a stress response or burst of physical activity.

"Essentially, we have demonstrated that the effects of caffeine on ambulatory blood pressure cannot be accounted for by changes in activity, posture (sitting, standing or walking) or perceived stress," Lane said. "This strengthens the evidence that caffeine is working directly on blood pressure rather than through other mediating factors."

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